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101-500: According to the standard model of cosmology, the scale factor of the universe is accelerating , and, in the future era of cosmological constant dominance, will increase exponentially. However, this expansion is similar for every moment of time (hence the exponential law – the expansion of a local volume is the same number of times over the same time interval), and is characterized by an unchanging, small Hubble constant , effectively ignored by any bound material structures. By contrast, in

202-414: A ˙ ( t ) {\displaystyle {\dot {d}}(t)=d_{0}{\dot {a}}(t)} , and also that d 0 = d ( t ) a ( t ) {\displaystyle d_{0}={\frac {d(t)}{a(t)}}} , so combining these gives d ˙ ( t ) = d ( t ) a ˙ ( t )

303-466: A ( t 0 ) {\displaystyle a(t_{0})} or 1 {\displaystyle 1} . The evolution of the scale factor is a dynamical question, determined by the equations of general relativity , which are presented in the case of a locally isotropic, locally homogeneous universe by the Friedmann equations . The Hubble parameter is defined as: where the dot represents

404-452: A ( t ) {\displaystyle {\dot {d}}(t)={\frac {d(t){\dot {a}}(t)}{a(t)}}} , and substituting the above definition of the Hubble parameter gives d ˙ ( t ) = H ( t ) d ( t ) {\displaystyle {\dot {d}}(t)=H(t)d(t)} which is just Hubble's law . Current evidence suggests that the expansion of

505-410: A deficit or a surplus of electrons are called ions . Electrons that are farthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism, atoms are able to bond into molecules and other types of chemical compounds like ionic and covalent network crystals . By definition, any two atoms with an identical number of protons in their nuclei belong to

606-422: A different way, is internal conversion —a process that produces high-speed electrons that are not beta rays, followed by production of high-energy photons that are not gamma rays. A few large nuclei explode into two or more charged fragments of varying masses plus several neutrons, in a decay called spontaneous nuclear fission . Each radioactive isotope has a characteristic decay time period—the half-life —that

707-450: A distant object with a redshift of z , then the scale factor at the time the object originally emitted that light is a ( t ) = 1 1 + z {\displaystyle a(t)={\frac {1}{1+z}}} . After Inflation , and until about 47,000 years after the Big Bang , the dynamics of the early universe were set by radiation (referring generally to

808-456: A finite set of orbits, and could jump between these orbits only in discrete changes of energy corresponding to absorption or radiation of a photon. This quantization was used to explain why the electrons' orbits are stable and why elements absorb and emit electromagnetic radiation in discrete spectra. Bohr's model could only predict the emission spectra of hydrogen, not atoms with more than one electron. Back in 1815, William Prout observed that

909-529: A form of light but made of negatively charged particles because they can be deflected by electric and magnetic fields. He measured these particles to be at least a thousand times lighter than hydrogen (the lightest atom). He called these new particles corpuscles but they were later renamed electrons since these are the particles that carry electricity. Thomson also showed that electrons were identical to particles given off by photoelectric and radioactive materials. Thomson explained that an electric current

1010-419: A fractional electric charge. Protons are composed of two up quarks (each with charge + ⁠ 2 / 3 ⁠ ) and one down quark (with a charge of − ⁠ 1 / 3 ⁠ ). Neutrons consist of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles. The quarks are held together by the strong interaction (or strong force), which

1111-484: A given accuracy in measuring a position one could only obtain a range of probable values for momentum, and vice versa. Thus, the planetary model of the atom was discarded in favor of one that described atomic orbital zones around the nucleus where a given electron is most likely to be found. This model was able to explain observations of atomic behavior that previous models could not, such as certain structural and spectral patterns of atoms larger than hydrogen. Though

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1212-451: A mathematical function that characterises the probability that an electron appears to be at a particular location when its position is measured. Only a discrete (or quantized ) set of these orbitals exist around the nucleus, as other possible wave patterns rapidly decay into a more stable form. Orbitals can have one or more ring or node structures, and differ from each other in size, shape and orientation. Each atomic orbital corresponds to

1313-415: A particular energy level of the electron. The electron can change its state to a higher energy level by absorbing a photon with sufficient energy to boost it into the new quantum state. Likewise, through spontaneous emission , an electron in a higher energy state can drop to a lower energy state while radiating the excess energy as a photon. These characteristic energy values, defined by the differences in

1414-547: A series of experiments in which they bombarded thin foils of metal with a beam of alpha particles . They did this to measure the scattering patterns of the alpha particles. They spotted a small number of alpha particles being deflected by angles greater than 90°. This shouldn't have been possible according to the Thomson model of the atom, whose charges were too diffuse to produce a sufficiently strong electric field. The deflections should have all been negligible. Rutherford proposed that

1515-519: A set of atomic numbers, from the single-proton element hydrogen up to the 118-proton element oganesson . All known isotopes of elements with atomic numbers greater than 82 are radioactive, although the radioactivity of element 83 ( bismuth ) is so slight as to be practically negligible. About 339 nuclides occur naturally on Earth , of which 251 (about 74%) have not been observed to decay, and are referred to as " stable isotopes ". Only 90 nuclides are stable theoretically , while another 161 (bringing

1616-472: A short-ranged attractive potential called the residual strong force . At distances smaller than 2.5 fm this force is much more powerful than the electrostatic force that causes positively charged protons to repel each other. Atoms of the same element have the same number of protons, called the atomic number . Within a single element, the number of neutrons may vary, determining the isotope of that element. The total number of protons and neutrons determine

1717-440: A size that is too small to be measured using available techniques. It was the lightest particle with a positive rest mass measured, until the discovery of neutrino mass. Under ordinary conditions, electrons are bound to the positively charged nucleus by the attraction created from opposite electric charges. If an atom has more or fewer electrons than its atomic number, then it becomes respectively negatively or positively charged as

1818-460: A source term in the Einstein field equation, can be viewed as equivalent to a "mass" of empty space, or dark energy . Since this increases with the volume of the universe, the expansion pressure is effectively constant, independent of the scale of the universe, while the other terms decrease with time. Thus, as the density of other forms of matter – dust and radiation – drops to very low concentrations,

1919-507: A subsequent dark-energy-dominated era . Some insight into the expansion can be obtained from a Newtonian expansion model which leads to a simplified version of the Friedmann equation. It relates the proper distance (which can change over time, unlike the comoving distance d C {\displaystyle d_{C}} which is constant and set to today's distance) between a pair of objects, e.g. two galaxy clusters, moving with

2020-423: A time derivative . The Hubble parameter varies with time, not with space, with the Hubble constant H 0 {\displaystyle H_{0}} being its current value. From the previous equation d ( t ) = d 0 a ( t ) {\displaystyle d(t)=d_{0}a(t)} one can see that d ˙ ( t ) = d 0

2121-432: A tiny atomic nucleus , and are collectively called nucleons . The radius of a nucleus is approximately equal to 1.07 A 3 {\displaystyle 1.07{\sqrt[{3}]{A}}}   femtometres , where A {\displaystyle A} is the total number of nucleons. This is much smaller than the radius of the atom, which is on the order of 10  fm. The nucleons are bound together by

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2222-434: A whole. If an atom has more electrons than protons, then it has an overall negative charge, and is called a negative ion (or anion). Conversely, if it has more protons than electrons, it has a positive charge, and is called a positive ion (or cation). The electrons of an atom are attracted to the protons in an atomic nucleus by the electromagnetic force . The protons and neutrons in the nucleus are attracted to each other by

2323-470: A whole; a charged atom is called an ion . Electrons have been known since the late 19th century, mostly thanks to J.J. Thomson ; see history of subatomic physics for details. Protons have a positive charge and a mass of 1.6726 × 10  kg . The number of protons in an atom is called its atomic number . Ernest Rutherford (1919) observed that nitrogen under alpha-particle bombardment ejects what appeared to be hydrogen nuclei. By 1920 he had accepted that

2424-499: Is 29.5% nitrogen and 70.5% oxygen. Adjusting these figures, in nitrous oxide there is 80 g of oxygen for every 140 g of nitrogen, in nitric oxide there is about 160 g of oxygen for every 140 g of nitrogen, and in nitrogen dioxide there is 320 g of oxygen for every 140 g of nitrogen. 80, 160, and 320 form a ratio of 1:2:4. The respective formulas for these oxides are N 2 O , NO , and NO 2 . In 1897, J. J. Thomson discovered that cathode rays are not

2525-427: Is 88.1% tin and 11.9% oxygen, and the other is a white powder that is 78.7% tin and 21.3% oxygen. Adjusting these figures, in the grey powder there is about 13.5 g of oxygen for every 100 g of tin, and in the white powder there is about 27 g of oxygen for every 100 g of tin. 13.5 and 27 form a ratio of 1:2. Dalton concluded that in the grey oxide there is one atom of oxygen for every atom of tin, and in

2626-555: Is a key parameter of the Friedmann equations . In the early stages of the Big Bang , most of the energy was in the form of radiation, and that radiation was the dominant influence on the expansion of the universe. Later, with cooling from the expansion the roles of matter and radiation changed and the universe entered a matter-dominated era. Recent results suggest that we have already entered an era dominated by dark energy , but examination of

2727-408: Is a measure of the distance out to which the electron cloud extends from the nucleus. This assumes the atom to exhibit a spherical shape, which is only obeyed for atoms in vacuum or free space. Atomic radii may be derived from the distances between two nuclei when the two atoms are joined in a chemical bond . The radius varies with the location of an atom on the atomic chart, the type of chemical bond,

2828-573: Is affected by the ratio of protons to neutrons, and also by the presence of certain "magic numbers" of neutrons or protons that represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the shell model of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. Of the 251 known stable nuclides, only four have both an odd number of protons and odd number of neutrons: hydrogen-2 ( deuterium ), lithium-6 , boron-10 , and nitrogen-14 . ( Tantalum-180m

2929-418: Is dimensionless, with t {\displaystyle t} counted from the birth of the universe and t 0 {\displaystyle t_{0}} set to the present age of the universe : 13.799 ± 0.021 G y r {\displaystyle 13.799\pm 0.021\,\mathrm {Gyr} } giving the current value of a {\displaystyle a} as

3030-467: Is easily obtained solving the Friedmann equations : Here, the coefficient H 0 {\displaystyle H_{0}} in the exponential, the Hubble constant , is This exponential dependence on time makes the spacetime geometry identical to the de Sitter universe , and only holds for a positive sign of the cosmological constant, which is the case according to the currently accepted value of

3131-438: Is higher than its proton number, so Rutherford hypothesized that the surplus weight was carried by unknown particles with no electric charge and a mass equal to that of the proton. In 1928, Walter Bothe observed that beryllium emitted a highly penetrating, electrically neutral radiation when bombarded with alpha particles. It was later discovered that this radiation could knock hydrogen atoms out of paraffin wax . Initially it

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3232-521: Is increasing with time. In contrast, the Hubble parameter seems to be decreasing with time, meaning that if we were to look at some fixed distance d and watch a series of different galaxies pass that distance, later galaxies would pass that distance at a smaller velocity than earlier ones. According to the Friedmann–Lemaître–Robertson–Walker metric which is used to model the expanding universe, if at present time we receive light from

3333-429: Is mediated by gluons . The protons and neutrons, in turn, are held to each other in the nucleus by the nuclear force , which is a residuum of the strong force that has somewhat different range-properties (see the article on the nuclear force for more). The gluon is a member of the family of gauge bosons , which are elementary particles that mediate physical forces. All the bound protons and neutrons in an atom make up

3434-481: Is not based on these old concepts. In the early 19th century, the scientist John Dalton found evidence that matter really is composed of discrete units, and so applied the word atom to those units. In the early 1800s, John Dalton compiled experimental data gathered by him and other scientists and discovered a pattern now known as the " law of multiple proportions ". He noticed that in any group of chemical compounds which all contain two particular chemical elements,

3535-425: Is not possible due to quantum effects . More than 99.9994% of an atom's mass is in the nucleus. Protons have a positive electric charge and neutrons have no charge, so the nucleus is positively charged. The electrons are negatively charged, and this opposing charge is what binds them to the nucleus. If the numbers of protons and electrons are equal, as they normally are, then the atom is electrically neutral as

3636-502: Is odd-odd and observationally stable, but is predicted to decay with a very long half-life.) Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: potassium-40 , vanadium-50 , lanthanum-138 , and lutetium-176 . Most odd-odd nuclei are highly unstable with respect to beta decay , because the decay products are even-even, and are therefore more strongly bound, due to nuclear pairing effects . The large majority of an atom's mass comes from

3737-413: Is often mistaken as marking the end of the radiation era. For a matter-dominated universe the evolution of the scale factor in the Friedmann–Lemaître–Robertson–Walker metric is easily obtained solving the Friedmann equations : In physical cosmology , the dark-energy-dominated era is proposed as the last of the three phases of the known universe, the other two being the radiation-dominated era and

3838-477: Is required to bring them together. It is this energy-releasing process that makes nuclear fusion in stars a self-sustaining reaction. For heavier nuclei, the binding energy per nucleon begins to decrease. That means that a fusion process producing a nucleus that has an atomic number higher than about 26, and a mass number higher than about 60, is an endothermic process . Thus, more massive nuclei cannot undergo an energy-producing fusion reaction that can sustain

3939-455: Is responsible for most of the physical changes observed in nature. Chemistry is the science that studies these changes. The basic idea that matter is made up of tiny indivisible particles is an old idea that appeared in many ancient cultures. The word atom is derived from the ancient Greek word atomos , which means "uncuttable". But this ancient idea was based in philosophical reasoning rather than scientific reasoning. Modern atomic theory

4040-421: Is that an accelerating charged particle radiates electromagnetic radiation, causing the particle to lose kinetic energy. Circular motion counts as acceleration, which means that an electron orbiting a central charge should spiral down into that nucleus as it loses speed. In 1913, the physicist Niels Bohr proposed a new model in which the electrons of an atom were assumed to orbit the nucleus but could only do so in

4141-486: Is the distance at the reference time t 0 {\displaystyle t_{0}} , usually also referred to as comoving distance, and a ( t ) {\displaystyle a(t)} is the scale factor. Thus, by definition, d 0 = d ( t 0 ) {\displaystyle d_{0}=d(t_{0})} and a ( t 0 ) = 1 {\displaystyle a(t_{0})=1} . The scale factor

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4242-470: Is the mass loss and c is the speed of light . This deficit is part of the binding energy of the new nucleus, and it is the non-recoverable loss of the energy that causes the fused particles to remain together in a state that requires this energy to separate. The fusion of two nuclei that create larger nuclei with lower atomic numbers than iron and nickel —a total nucleon number of about 60—is usually an exothermic process that releases more energy than

4343-460: Is the passing of electrons from one atom to the next, and when there was no current the electrons embedded themselves in the atoms. This in turn meant that atoms were not indivisible as scientists thought. The atom was composed of electrons whose negative charge was balanced out by some source of positive charge to create an electrically neutral atom. Ions, Thomson explained, must be atoms which have an excess or shortage of electrons. The electrons in

4444-485: The Schroedinger equation , which describes electrons as three-dimensional waveforms rather than points in space. A consequence of using waveforms to describe particles is that it is mathematically impossible to obtain precise values for both the position and momentum of a particle at a given point in time. This became known as the uncertainty principle , formulated by Werner Heisenberg in 1927. In this concept, for

4545-520: The Solar System would become gravitationally unbound about three months before the Big Rip, and planets would fly off into the rapidly expanding universe. In the last minutes, stars and planets would be torn apart, and the now-dispersed atoms would be destroyed about 10 seconds before the end (the atoms will first be ionized as electrons fly off, followed by the dissociation of the atomic nuclei ). At

4646-438: The cosmological constant , Λ, that is approximately 2 · 10 s . The current density of the observable universe is of the order of 9.44 · 10 kg m and the age of the universe is of the order of 13.8 billion years, or 4.358 · 10 s . The Hubble constant, H 0 {\displaystyle H_{0}} , is ≈70.88 km s Mpc (The Hubble time is 13.79 billion years). Atom Atoms are

4747-401: The cosmological event horizon is continually shrinking – the distance at which objects can influence an observer becomes ever closer, and the distance over which interactions can propagate becomes ever shorter. When the size of the horizon becomes smaller than any particular structure, no interaction by any of the fundamental forces can occur between the most remote parts of the structure, and

4848-438: The hydrostatic equilibrium of a star. The electrons in an atom are attracted to the protons in the nucleus by the electromagnetic force . This force binds the electrons inside an electrostatic potential well surrounding the smaller nucleus, which means that an external source of energy is needed for the electron to escape. The closer an electron is to the nucleus, the greater the attractive force. Hence electrons bound near

4949-473: The matter-dominated era . The dark-energy-dominated era began after the matter-dominated era, i.e. when the Universe was about 9.8 billion years old. In the era of cosmic inflation , the Hubble parameter is also thought to be constant, so the expansion law of the dark-energy-dominated era also holds for the inflationary prequel of the big bang. The cosmological constant is given the symbol Λ, and, considered as

5050-547: The nuclear force . This force is usually stronger than the electromagnetic force that repels the positively charged protons from one another. Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force. In this case, the nucleus splits and leaves behind different elements . This is a form of nuclear decay . Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules or crystals . The ability of atoms to attach and detach from each other

5151-468: The nuclide . The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing radioactive decay . The proton, the electron, and the neutron are classified as fermions . Fermions obey the Pauli exclusion principle which prohibits identical fermions, such as multiple protons, from occupying the same quantum state at the same time. Thus, every proton in

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5252-505: The 'surface' of these particles is not sharply defined. The neutron was discovered in 1932 by the English physicist James Chadwick . In the Standard Model of physics, electrons are truly elementary particles with no internal structure, whereas protons and neutrons are composite particles composed of elementary particles called quarks . There are two types of quarks in atoms, each having

5353-411: The Big Rip scenario the Hubble constant increases to infinity in a finite time. According to recent studies, the universe is currently set for a constant expansion and heat death , because the equation of state parameter w = -1. The possibility of sudden rip singularity occurs only for hypothetical matter ( phantom energy ) with implausible physical properties. The truth of the hypothesis relies on

5454-470: The Big Rip to be where w is defined above, H 0 is Hubble's constant and Ω m is the present value of the density of all the matter in the universe. Observations of galaxy cluster speeds by the Chandra X-ray Observatory seem to suggest the value of w is between approximately −0.907 and −1.075, meaning the Big Rip cannot be definitively ruled out. Based on the above equation, if

5555-466: The Hubble flow in an expanding or contracting FLRW universe at any arbitrary time t {\displaystyle t} to their distance at some reference time t 0 {\displaystyle t_{0}} . The formula for this is: where d ( t ) {\displaystyle d(t)} is the proper distance at epoch t {\displaystyle t} , d 0 {\displaystyle d_{0}}

5656-399: The amount of Element A per measure of Element B will differ across these compounds by ratios of small whole numbers. This pattern suggested that each element combines with other elements in multiples of a basic unit of weight, with each element having a unit of unique weight. Dalton decided to call these units "atoms". For example, there are two types of tin oxide : one is a grey powder that

5757-444: The atom logically had to be balanced out by a commensurate amount of positive charge, but Thomson had no idea where this positive charge came from, so he tentatively proposed that it was everywhere in the atom, the atom being in the shape of a sphere. This was the mathematically simplest hypothesis to fit the available evidence, or lack thereof. Following from this, Thomson imagined that the balance of electrostatic forces would distribute

5858-422: The atomic mass unit (for example the mass of a nitrogen-14 is roughly 14 Da), but this number will not be exactly an integer except (by definition) in the case of carbon-12. The heaviest stable atom is lead-208, with a mass of 207.976 6521  Da . As even the most massive atoms are far too light to work with directly, chemists instead use the unit of moles . One mole of atoms of any element always has

5959-491: The atomic weights of many elements were multiples of hydrogen's atomic weight, which is in fact true for all of them if one takes isotopes into account. In 1898, J. J. Thomson found that the positive charge of a hydrogen ion is equal to the negative charge of an electron, and these were then the smallest known charged particles. Thomson later found that the positive charge in an atom is a positive multiple of an electron's negative charge. In 1913, Henry Moseley discovered that

6060-497: The authors consider a hypothetical example with w  = −1.5, H 0  = 70 km/s/Mpc, and Ω m  = 0.3, in which case the Big Rip would happen approximately 22 billion years from the present. In this scenario, galaxies would first be separated from each other about 200 million years before the Big Rip. About 60 million years before the Big Rip, galaxies would begin to disintegrate as gravity becomes too weak to hold them together. Planetary systems like

6161-412: The basic particles of the chemical elements . An atom consists of a nucleus of protons and generally neutrons , surrounded by an electromagnetically bound swarm of electrons . The chemical elements are distinguished from each other by the number of protons that are in their atoms. For example, any atom that contains 11 protons is sodium , and any atom that contains 29 protons is copper . Atoms with

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6262-413: The center of the potential well require more energy to escape than those at greater separations. Electrons, like other particles, have properties of both a particle and a wave . The electron cloud is a region inside the potential well where each electron forms a type of three-dimensional standing wave —a wave form that does not move relative to the nucleus. This behavior is defined by an atomic orbital ,

6363-478: The chemical elements, at least one stable isotope exists. As a rule, there is only a handful of stable isotopes for each of these elements, the average being 3.1 stable isotopes per element. Twenty-six " monoisotopic elements " have only a single stable isotope, while the largest number of stable isotopes observed for any element is ten, for the element tin . Elements 43 , 61 , and all elements numbered 83 or higher have no stable isotopes. Stability of isotopes

6464-430: The constituents of the universe which moved relativistically , principally photons and neutrinos ). For a radiation-dominated universe the evolution of the scale factor in the Friedmann–Lemaître–Robertson–Walker metric is obtained solving the Friedmann equations : Between about 47,000 years and 9.8 billion years after the Big Bang , the energy density of matter exceeded both the energy density of radiation and

6565-450: The core of the Sun protons require energies of 3 to 10 keV to overcome their mutual repulsion—the coulomb barrier —and fuse together into a single nucleus. Nuclear fission is the opposite process, causing a nucleus to split into two smaller nuclei—usually through radioactive decay. The nucleus can also be modified through bombardment by high energy subatomic particles or photons. If this modifies

6666-435: The cosmological constant (or "dark energy") term will eventually dominate the energy density of the Universe. Recent measurements of the change in Hubble constant with time, based on observations of distant supernovae , show this acceleration in expansion rate, indicating the presence of such dark energy. For a dark-energy-dominated universe, the evolution of the scale factor in the Friedmann–Lemaître–Robertson–Walker metric

6767-480: The earliest possible date of the Big Rip can be pushed back further with more accurate measurements but the Big Rip is very difficult to completely rule out. Scale factor (cosmology) The expansion of the universe is parametrized by a dimensionless scale factor a {\displaystyle a} . Also known as the cosmic scale factor or sometimes the Robertson–Walker scale factor , this

6868-512: The electrons throughout the sphere in a more or less even manner. Thomson's model is popularly known as the plum pudding model , though neither Thomson nor his colleagues used this analogy. Thomson's model was incomplete, it was unable to predict any other properties of the elements such as emission spectra and valencies . It was soon rendered obsolete by the discovery of the atomic nucleus . Between 1908 and 1913, Ernest Rutherford and his colleagues Hans Geiger and Ernest Marsden performed

6969-506: The energies of the quantum states, are responsible for atomic spectral lines . The amount of energy needed to remove or add an electron—the electron binding energy —is far less than the binding energy of nucleons . For example, it requires only 13.6 eV to strip a ground-state electron from a hydrogen atom, compared to 2.23  million eV for splitting a deuterium nucleus. Atoms are electrically neutral if they have an equal number of protons and electrons. Atoms that have either

7070-665: The energies of the recoiling charged particles, he deduced that the radiation was actually composed of electrically neutral particles which could not be massless like the gamma ray, but instead were required to have a mass similar to that of a proton. Chadwick now claimed these particles as Rutherford's neutrons. In 1925, Werner Heisenberg published the first consistent mathematical formulation of quantum mechanics ( matrix mechanics ). One year earlier, Louis de Broglie had proposed that all particles behave like waves to some extent, and in 1926 Erwin Schroedinger used this idea to develop

7171-418: The expansion of the universe tends to accelerate, but the dark energy tends to dissipate over time, and the Big Rip does not happen. Phantom energy has w  < −1, which means that its density increases as the universe expands. A universe dominated by phantom energy is an accelerating universe , expanding at an ever-increasing rate. However, this implies that the size of the observable universe and

7272-433: The frequencies of X-ray emissions from an excited atom were a mathematical function of its atomic number and hydrogen's nuclear charge. In 1919 Rutherford bombarded nitrogen gas with alpha particles and detected hydrogen ions being emitted from the gas, and concluded that they were produced by alpha particles hitting and splitting the nuclei of the nitrogen atoms. These observations led Rutherford to conclude that

7373-416: The hydrogen nucleus is a distinct particle within the atom and named it proton . Neutrons have no electrical charge and have a mass of 1.6749 × 10  kg . Neutrons are the heaviest of the three constituent particles, but their mass can be reduced by the nuclear binding energy . Neutrons and protons (collectively known as nucleons ) have comparable dimensions—on the order of 2.5 × 10  m —although

7474-445: The hydrogen nucleus is a singular particle with a positive charge equal to the electron's negative charge. He named this particle " proton " in 1920. The number of protons in an atom (which Rutherford called the " atomic number " ) was found to be equal to the element's ordinal number on the periodic table and therefore provided a simple and clear-cut way of distinguishing the elements from each other. The atomic weight of each element

7575-432: The mutual repulsion of the protons requires an increasing proportion of neutrons to maintain the stability of the nucleus. The number of protons and neutrons in the atomic nucleus can be modified, although this can require very high energies because of the strong force. Nuclear fusion occurs when multiple atomic particles join to form a heavier nucleus, such as through the energetic collision of two nuclei. For example, at

7676-509: The nucleus must occupy a quantum state different from all other protons, and the same applies to all neutrons of the nucleus and to all electrons of the electron cloud. A nucleus that has a different number of protons than neutrons can potentially drop to a lower energy state through a radioactive decay that causes the number of protons and neutrons to more closely match. As a result, atoms with matching numbers of protons and neutrons are more stable against decay, but with increasing atomic number,

7777-515: The nucleus to emit particles or electromagnetic radiation. Radioactivity can occur when the radius of a nucleus is large compared with the radius of the strong force, which only acts over distances on the order of 1 fm. The most common forms of radioactive decay are: Other more rare types of radioactive decay include ejection of neutrons or protons or clusters of nucleons from a nucleus, or more than one beta particle . An analog of gamma emission which allows excited nuclei to lose energy in

7878-449: The number of hydrogen atoms. A single carat diamond with a mass of 2 × 10  kg contains about 10 sextillion (10 ) atoms of carbon . If an apple were magnified to the size of the Earth, then the atoms in the apple would be approximately the size of the original apple. Every element has one or more isotopes that have unstable nuclei that are subject to radioactive decay, causing

7979-450: The number of neighboring atoms ( coordination number ) and a quantum mechanical property known as spin . On the periodic table of the elements, atom size tends to increase when moving down columns, but decrease when moving across rows (left to right). Consequently, the smallest atom is helium with a radius of 32  pm , while one of the largest is caesium at 225 pm. When subjected to external forces, like electrical fields ,

8080-451: The number of protons in a nucleus, the atom changes to a different chemical element. If the mass of the nucleus following a fusion reaction is less than the sum of the masses of the separate particles, then the difference between these two values can be emitted as a type of usable energy (such as a gamma ray , or the kinetic energy of a beta particle ), as described by Albert Einstein 's mass–energy equivalence formula, E=mc , where m

8181-399: The observation determines that the value of w is less than −1, but greater than or equal to −1.075, the Big Rip would occur approximately 152 billion years into the future at the earliest. More recent data from Planck mission indicates the value of w to be -1.028 (+/-0.031), pushing the earliest possible time of Big Rip to be approximately 200 billion years into the future. In their paper,

8282-435: The positive charge of the atom is concentrated in a tiny volume at the center of the atom and that the electrons surround this nucleus in a diffuse cloud. This nucleus carried almost all of the atom's mass, the electrons being so very light. Only such an intense concentration of charge, anchored by its high mass, could produce an electric field that could deflect the alpha particles so strongly. A problem in classical mechanics

8383-448: The protons and neutrons that make it up. The total number of these particles (called "nucleons") in a given atom is called the mass number . It is a positive integer and dimensionless (instead of having dimension of mass), because it expresses a count. An example of use of a mass number is "carbon-12," which has 12 nucleons (six protons and six neutrons). The actual mass of an atom at rest is often expressed in daltons (Da), also called

8484-421: The red powder there is about 42 g of oxygen for every 100 g of iron. 28 and 42 form a ratio of 2:3. Dalton concluded that in these oxides, for every two atoms of iron, there are two or three atoms of oxygen respectively ( Fe 2 O 2 and Fe 2 O 3 ). As a final example: nitrous oxide is 63.3% nitrogen and 36.7% oxygen, nitric oxide is 44.05% nitrogen and 55.95% oxygen, and nitrogen dioxide

8585-419: The roles of matter and radiation are most important for understanding the early universe. Using the dimensionless scale factor to characterize the expansion of the universe, the effective energy densities of radiation and matter scale differently. This leads to a radiation-dominated era in the very early universe but a transition to a matter-dominated era at a later time and, since about 4 billion years ago,

8686-412: The same chemical element . Atoms with equal numbers of protons but a different number of neutrons are different isotopes of the same element. For example, all hydrogen atoms admit exactly one proton, but isotopes exist with no neutrons ( hydrogen-1 , by far the most common form, also called protium), one neutron ( deuterium ), two neutrons ( tritium ) and more than two neutrons . The known elements form

8787-498: The same number of atoms (about 6.022 × 10 ). This number was chosen so that if an element has an atomic mass of 1 u, a mole of atoms of that element has a mass close to one gram. Because of the definition of the unified atomic mass unit , each carbon-12 atom has an atomic mass of exactly 12 Da, and so a mole of carbon-12 atoms weighs exactly 0.012 kg. Atoms lack a well-defined outer boundary, so their dimensions are usually described in terms of an atomic radius . This

8888-448: The same number of protons but a different number of neutrons are called isotopes of the same element. Atoms are extremely small, typically around 100  picometers across. A human hair is about a million carbon atoms wide. Atoms are smaller than the shortest wavelength of visible light, which means humans cannot see atoms with conventional microscopes. They are so small that accurately predicting their behavior using classical physics

8989-539: The shape of an atom may deviate from spherical symmetry . The deformation depends on the field magnitude and the orbital type of outer shell electrons, as shown by group-theoretical considerations. Aspherical deviations might be elicited for instance in crystals , where large crystal-electrical fields may occur at low-symmetry lattice sites. Significant ellipsoidal deformations have been shown to occur for sulfur ions and chalcogen ions in pyrite -type compounds. Atomic dimensions are thousands of times smaller than

9090-409: The structure is "ripped apart". The progression of time itself will stop. The model implies that after a finite time there will be a final singularity, called the "Big Rip", in which the observable universe eventually reaches zero size and all distances diverge to infinite values. The authors of this hypothesis, led by Robert R. Caldwell of Dartmouth College , calculate the time from the present to

9191-421: The time the Big Rip occurs, even spacetime itself would be ripped apart and the scale factor would be infinity. Evidence indicates w to be very close to −1 in our universe, which makes w the dominating term in the equation. The closer that w is to −1, the closer the denominator is to zero and the further the Big Rip is in the future. If w were exactly equal to −1, the Big Rip could not happen, regardless of

9292-677: The total to 251) have not been observed to decay, even though in theory it is energetically possible. These are also formally classified as "stable". An additional 35 radioactive nuclides have half-lives longer than 100 million years, and are long-lived enough to have been present since the birth of the Solar System . This collection of 286 nuclides are known as primordial nuclides . Finally, an additional 53 short-lived nuclides are known to occur naturally, as daughter products of primordial nuclide decay (such as radium from uranium ), or as products of natural energetic processes on Earth, such as cosmic ray bombardment (for example, carbon-14). For 80 of

9393-461: The type of dark energy present in our universe . The type that could prove this hypothesis is a constantly increasing form of dark energy, known as phantom energy . If the dark energy in the universe increases without limit, it could overcome all forces that hold the universe together. The key value is the equation of state parameter w , the ratio between the dark energy pressure and its energy density . If −1 <  w  < 0,

9494-445: The unified atomic mass unit (u). This unit is defined as a twelfth of the mass of a free neutral atom of carbon-12 , which is approximately 1.66 × 10  kg . Hydrogen-1 (the lightest isotope of hydrogen which is also the nuclide with the lowest mass) has an atomic weight of 1.007825 Da. The value of this number is called the atomic mass . A given atom has an atomic mass approximately equal (within 1%) to its mass number times

9595-567: The universe is accelerating , which means that the second derivative of the scale factor a ¨ ( t ) {\displaystyle {\ddot {a}}(t)} is positive, or equivalently that the first derivative a ˙ ( t ) {\displaystyle {\dot {a}}(t)} is increasing over time. This also implies that any given galaxy recedes from us with increasing speed over time, i.e. for that galaxy d ˙ ( t ) {\displaystyle {\dot {d}}(t)}

9696-448: The vacuum energy density. When the early universe was about 47,000 years old (redshift 3600), mass–energy density surpassed the radiation energy , although the universe remained optically thick to radiation until the universe was about 378,000 years old (redshift 1100). This second moment in time (close to the time of recombination ), at which the photons which compose the cosmic microwave background radiation were last scattered,

9797-441: The values of H 0 or Ω m . According to the latest cosmological data available, the uncertainties are still too large to discriminate among the three cases w  < −1, w  = −1, and w  > −1. Moreover, it is nearly impossible to measure w to be exactly at −1 due to statistical fluctuations. This means that the measured value of w can be arbitrarily close to −1 but not exactly at −1 hence

9898-406: The wavelengths of light (400–700  nm ) so they cannot be viewed using an optical microscope , although individual atoms can be observed using a scanning tunneling microscope . To visualize the minuteness of the atom, consider that a typical human hair is about 1 million carbon atoms in width. A single drop of water contains about 2  sextillion ( 2 × 10 ) atoms of oxygen, and twice

9999-432: The white oxide there are two atoms of oxygen for every atom of tin ( SnO and SnO 2 ). Dalton also analyzed iron oxides . There is one type of iron oxide that is a black powder which is 78.1% iron and 21.9% oxygen; and there is another iron oxide that is a red powder which is 70.4% iron and 29.6% oxygen. Adjusting these figures, in the black powder there is about 28 g of oxygen for every 100 g of iron, and in

10100-407: The word atom originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various subatomic particles . The constituent particles of an atom are the electron , the proton and the neutron . The electron is the least massive of these particles by four orders of magnitude at 9.11 × 10  kg , with a negative electrical charge and

10201-432: Was thought to be high-energy gamma radiation , since gamma radiation had a similar effect on electrons in metals, but James Chadwick found that the ionization effect was too strong for it to be due to electromagnetic radiation, so long as energy and momentum were conserved in the interaction. In 1932, Chadwick exposed various elements, such as hydrogen and nitrogen, to the mysterious "beryllium radiation", and by measuring

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