atom - intro

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AtomFrom Wikipedia, the free encyclopedia

Jump to: navigation , searchFor other uses, see Atom (disambiguation) .

The atom is a basic unit of matter consisting of a dense, central nucleus surrounded by acloud of negatively charged electrons . The atomic nucleus contains a mix of positivelycharged protons and electrically neutral neutrons (except in the case of hydrogen-1 ,which is the only stable nuclide with no neutron). The electrons of an atom are bound to

Helium atom

An illustration of the helium atom, depicting the nucleus (pink) and the electron cloud distribution (black). Thenucleus (upper right) in helium-4 is in reality sphericallysymmetric and closely resembles the electron cloud,although for more complicated nuclei this is not alwaysthe case. The black bar is one ngstrm , equal to 10 10 m or 100,000 fm.

Classification

Smallest recognized division of a chemicalelement

Properties

Mass range : 1.67 10 27 to 4.52 10 25 kgElectric

charge :zero (neutral), or ion charge

Diameter range:

62 pm (He) to 520 pm ( Cs)(data page )

Components :Electrons and a compactnucleus of protons andneutrons
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the nucleus by the electromagnetic force . Likewise, a group of atoms can remain boundto each other, forming a molecule . An atom containing an equal number of protons andelectrons is electrically neutral, otherwise it has a positive or negative charge and is anion . An atom is classified according to the number of protons and neutrons in its nucleus:the number of protons determines the chemical element , and the number of neutrons

determine the isotope of the element.[1]

The name atom comes from the Greek /tomos, -, which meansuncuttable, or indivisible, something that cannot be divided further. The concept of anatom as an indivisible component of matter was first proposed by early Indian and Greek

philosophers. In the 17th and 18th centuries, chemists provided a physical basis for thisidea by showing that certain substances could not be further broken down by chemicalmethods. During the late 19th and early 20th centuries, physicists discovered subatomiccomponents and structure inside the atom, thereby demonstrating that the 'atom' wasdivisible. The principles of quantum mechanics were used to successfully model theatom. [2][3]

Relative to everyday experience, atoms are minuscule objects with proportionately tinymasses. Atoms can only be observed individually using special instruments such as thescanning tunneling microscope . Over 99.9% of an atom's mass is concentrated in thenucleus, [note 1] with protons and neutrons having roughly equal mass. Each element has atleast one isotope with unstable nuclei that can undergo radioactive decay . This can resultin a transmutation that changes the number of protons or neutrons in a nucleus. [4] Electrons that are bound to atoms possess a set of stable energy levels , or orbitals , andcan undergo transitions between them by absorbing or emitting photons that match theenergy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.

Contents

[hide ] 1 History

o 1.1 Atomismo 1.2 Origin of scientific theoryo 1.3 Subcomponents and quantum theoryo 1.4 Fission, high energy physics and condensed matter

2 Componentso 2.1 Subatomic particleso 2.2 Nucleuso 2.3 Electron cloud

3 Propertieso 3.1 Nuclear propertieso 3.2 Masso 3.3 Sizeo 3.4 Radioactive decay
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o 3.5 Magnetic momento 3.6 Energy levelso 3.7 Valence and bonding behavior o 3.8 States

4 Identification 5 Origin and current state

o 5.1 Nucleosynthesiso 5.2 Eartho 5.3 Rare and theoretical forms

6 See also 7 Notes 8 References

o 8.1 Book references

9 External links

History

Main article: Atomic theory

Various atoms and molecules as depicted in John Dalton 's A New System of Chemical Philosophy (1808), one of the earliest scientific works on atomic theory.

Atomism

Main article: Atomism

The concept that matter is composed of discrete units and cannot be divided intoarbitrarily tiny quantities has been around for millennia , but these ideas were founded in

abstract, philosophical reasoning rather than experimentation and empirical observation .The nature of atoms in philosophy varied considerably over time and between culturesand schools, and often had spiritual elements. Nevertheless, the basic idea of the atomwas adopted by scientists thousands of years later because it elegantly explained newdiscoveries in the field of chemistry. [5]

The earliest references to the concept of atoms date back to ancient India in the 6thcentury BCE ,[6] appearing first in Jainism .[7] The Nyaya and Vaisheshika schools
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developed elaborate theories of how atoms combined into more complex objects .[8] In theWest, the references to atoms emerged a century later from Leucippus , whose student,Democritus , systematized his views. In approximately 450 BCE, Democritus coined theterm tomos (Greek : ), which means "uncuttable" or "the smallest indivisible

particle of matter". Although the Indian and Greek concepts of the atom were based

purely on philosophy, modern science has retained the name coined by Democritus.[5]

Corpuscularianism is the postulate, expounded in the 13th-century by the alchemist Pseudo-Geber (Geber), [9] that all physical bodies possess an inner and outer layer of minute particles or corpuscles .[10] Corpuscularianism is similar to the theory atomism,except that where atoms were supposed to be indivisible, corpuscles could in principle bedivided. In this manner, for example, it was theorized that mercury could penetrate intometals and modify their inner structure. [11] Corpuscularianism stayed a dominant theoryover the next several hundred years and was blended with alchemy by Robert Boyle andIsaac Newton in the 17th century. [10][ 12] It was used by Newton, for instance, in hisdevelopment of the corpuscular theory of light .

Origin of scientific theory

Further progress in the understanding of atoms did not occur until the science of chemistry began to develop. In 1661, natural philosopher Robert Boyle published TheSceptical Chymist in which he argued that matter was composed of various combinationsof different "corpuscules" or atoms, rather than the classical elements of air, earth, fireand water. [13] In 1789 the term element was defined by the French nobleman and scientificresearcher Antoine Lavoisier to mean basic substances that could not be further brokendown by the methods of chemistry. [14]

In 1803, English instructor and natural philosopher John Dalton used the concept of atoms to explain why elements always react in a ratio of small whole numbers the lawof multiple proportions and why certain gases dissolve better in water than others. He

proposed that each element consists of atoms of a single, unique type, and that theseatoms can join together to form chemical compounds .[15][ 16] Dalton is considered theoriginator of modern atomic theory .[17]

Additional validation of particle theory (and by extension atomic theory ) occurred in1827 when botanist Robert Brown used a microscope to look at dust grains floating inwater and discovered that they moved about erraticallya phenomenon that becameknown as " Brownian motion ". J. Desaulx suggested in 1877 that the phenomenon was

caused by the thermal motion of water molecules, and in 1905 Albert Einstein producedthe first mathematical analysis of the motion. [18][ 19][ 20] French physicist Jean Perrin usedEinstein's work to experimentally determine the mass and dimensions of atoms, therebyconclusively verifying Dalton's atomic theory. [21]
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A modern periodic table

In 1869, building upon earlier discoveries by such scientists as Lavoisier, DmitriMendeleev published the first functional periodic table .[22] The table itself is a visualrepresentation of the periodic law which states certain chemical properties of elements repeat periodically when arranged by atomic number .

Subcomponents and quantum theory

The physicist J. J. Thomson , through his work on cathode rays in 1897, discovered theelectron. These subatomic particles had the same properties, regardless of the type of

atom whence they came. This universal component of all atoms destroyed the concept of atoms as being indivisible units. [23] Thomson postulated that the low mass, negatively-charged electrons were distributed throughout the atom, possibly rotating in rings, withtheir charge balanced by the presence of a uniform sea of positive charge. This later

became known as the plum pudding model .

In 1909, Hans Geiger and Ernest Marsden , under the direction of physicist ErnestRutherford , bombarded a sheet of gold foil with alpha raysby then known to be

positively charged helium atomsand discovered that a small percentage of these particles were deflected through much larger angles than was predicted using Thomson's proposal. Rutherford interpreted the gold foil experiment as suggesting that the positivecharge of a heavy gold atom and most of its mass was concentrated in a nucleus at thecenter of the atomthe Rutherford model .[24]

While experimenting with the products of radioactive decay , in 1913 radiochemist Frederick Soddy discovered that there appeared to be more than one type of atom at each

position on the periodic table. [25] The term isotope was coined by Margaret Todd as asuitable name for different atoms that belong to the same element. J.J. Thomson created a
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technique for separating atom types through his work on ionized gases, whichsubsequently led to the discovery of stable isotopes .[26]

A Bohr model of the hydrogen atom, showing an electron jumping between fixed orbitsand emitting a photon of energy with a specific frequency.

Meanwhile, in 1913, physicist Niels Bohr suggested that the electrons were confined intoclearly defined, quantized orbits, and could jump between these, but could not freelyspiral inward or outward in intermediate states. [27] An electron must absorb or emitspecific amounts of energy to transition between these fixed orbits. When the light from aheated material was passed through a prism , it produced a multi-colored spectrum . Theappearance of fixed lines in this spectrum was successfully explained by these orbitaltransitions. [28]

Chemical bonds between atoms were now explained, by Gilbert Newton Lewis in 1916,as the interactions between their constituent electrons .[29] As the chemical properties of the elements were known to largely repeat themselves according to the periodic law ,[30] in1919 the American chemist Irving Langmuir suggested that this could be explained if theelectrons in an atom were connected or clustered in some manner. Groups of electronswere thought to occupy a set of electron shells about the nucleus .[31]

The SternGerlach experiment of 1922 provided further evidence of the quantum natureof the atom. When a beam of silver atoms was passed through a specially shapedmagnetic field, the beam was split based on the direction of an atom's angular momentum, or spin. As this direction is random, the beam could be expected to spreadinto a line. Instead, the beam was split into two parts, depending on whether the atomicspin was oriented up or down. [32]

In 1926, Erwin Schrdinger , using Louis de Broglie 's 1924 proposal that particles behaveto an extent like waves, developed a mathematical model of the atom that described theelectrons as three-dimensional waveforms , rather than point particles. 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 the same time; this became known as the uncertainty principle , formulated by Werner Heisenberg in 1926. Inthis concept, for a given accuracy in measuring a position one could only obtain a rangeof probable values for momentum, and vice versa. This model was able to explain
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observations of atomic behavior that previous models could not, such as certain structuraland spectral patterns of atoms larger than hydrogen. Thus, the planetary model of theatom was discarded in favor of one that described atomic orbital zones around thenucleus where a given electron is most likely to be observed. [33] [34]

Schematic diagram of a simple mass spectrometer.

The development of the mass spectrometer allowed the exact mass of atoms to bemeasured. The device uses a magnet to bend the trajectory of a beam of ions, and theamount of deflection is determined by the ratio of an atom's mass to its charge. Thechemist Francis William Aston used this instrument to show that isotopes had differentmasses. The atomic mass of these isotopes varied by integer amounts, called the whole

number rule .[35] The explanation for these different isotopes awaited the discovery of theneutron , a neutral-charged particle with a mass similar to the proton , by the physicistJames Chadwick in 1932. Isotopes were then explained as elements with the samenumber of protons, but different numbers of neutrons within the nucleus. [36]

Fission, high energy physics and condensed matter

In 1938, the German chemist Otto Hahn , a student of Rutherford, directed neutrons ontouranium atoms expecting to get transuranium elements . Instead, his chemical experimentsshowed barium as a product. [37] A year later, Lise Meitner and her nephew Otto Frisch verified that Hahn's result were the first experimental nuclear fission .[38][ 39] In 1944, Hahnreceived the Nobel prize in chemistry in which, despite the efforts of Hahn, thecontributions of Meitner and Frisch were not recognized. [40]

In the 1950s, the development of improved particle accelerators and particle detectors allowed scientists to study the impacts of atoms moving at high energies .[41] Neutrons and

protons were found to be hadrons , or composites of smaller particles called quarks .Standard models of nuclear physics were developed that successfully explained the
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properties of the nucleus in terms of these sub-atomic particles and the forces that governtheir interactions. [42]

Around 1985, Steven Chu and co-workers at Bell Labs developed a technique for lowering the temperatures of atoms using lasers . In the same year, a team led by William

D. Phillips managed to contain atoms of sodium in a magnetic trap . The combination of these two techniques and a method based on the Doppler effect , developed by ClaudeCohen-Tannoudji and his group, allows small numbers of atoms to be cooled to severalmicrokelvin . This allows the atoms to be studied with great precision, and later led to the

Nobel prize-winning discovery of Bose-Einstein condensation .[43]

Historically, single atoms have been prohibitively small for scientific applications.Recently, devices have been constructed that use a single metal atom connected throughorganic ligands to construct a single electron transistor .[44] Experiments have been carriedout by trapping and slowing single atoms using laser cooling in a cavity to gain a better

physical understanding of matter .[45]

Components

Subatomic particles

Main article: Subatomic particle

Though 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 .However, the hydrogen-1 atom has no neutrons and a positive hydrogen ion has noelectrons.

The electron is by far the least massive of these particles at 9.11 10 31 kg, with anegative electrical charge and a size that is too small to be measured using availabletechniques. [46] Protons have a positive charge and a mass 1,836 times that of the electron,at 1.6726 10 27 kg, although this can be reduced by changes to the energy binding the

proton into an atom. Neutrons have no electrical charge and have a free mass of 1,839times the mass of electrons, [47] or 1.6929 10 27 kg. Neutrons and protons havecomparable dimensionson the order of 2.5 10 15 malthough the 'surface' of these

particles is not sharply defined. [48]

In the Standard Model of physics, both protons and neutrons are composed of elementary particles called quarks . The quark belongs to the fermion group of particles, and is one of the two basic constituents of matterthe other being the lepton , of which the electron isan example. There are six types of quarks, each having a fractional electric charge of either +2/3 or 1/3. Protons are composed of two up quarks and one down quark , while aneutron consists of one up quark and two down quarks. This distinction accounts for thedifference in mass and charge between the two particles. The quarks are held together by
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the strong nuclear force , which is mediated by gluons . The gluon is a member of thefamily of gauge bosons , which are elementary particles that mediate physical forces .[49][50]

Nucleus

Main article: Atomic nucleus

The binding energy needed for a nucleon to escape the nucleus, for various isotopes.

All the bound protons and neutrons in an atom make up a tiny atomic nucleus , and arecollectively called nucleons . The radius of a nucleus is approximately equal tofm, where A is the total number of nucleons. [51] This is much smaller than the radius of

the atom, which is on the order of 10 5 fm. The nucleons are bound together by a short-ranged attractive potential called the residual strong force . At distances smaller than 2.5fm this force is much more powerful than the electrostatic force that causes positively

charged protons to repel each other.[52]

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 the nuclide . Thenumber of neutrons relative to the protons determines the stability of the nucleus, withcertain isotopes undergoing radioactive decay .[53]

The neutron and the proton are different types of fermions . The Pauli exclusion principle is a quantum mechanical effect that prohibits identical fermions, such as multiple

protons, from occupying the same quantum physical state at the same time. Thus every

proton in the nucleus must occupy a different state, with its own energy level, and thesame rule applies to all of the neutrons. This prohibition does not apply to a proton andneutron occupying the same quantum state. [54]

For atoms with low atomic numbers, a nucleus that has a different number of protonsthan neutrons can potentially drop to a lower energy state through a radioactive decaythat causes the number of protons and neutrons to more closely match. As a result, atomswith roughly matching numbers of protons and neutrons are more stable against decay.
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However, with increasing atomic number, the mutual repulsion of the protons requires anincreasing proportion of neutrons to maintain the stability of the nucleus, which modifiesthis trend. Thus, there are no stable nuclei with equal proton and neutron numbers aboveatomic number Z = 20 (calcium); and as Z increases toward the heaviest nuclei, the ratioof neutrons per proton required for stability increases to about 1.5 .[54]

Illustration of a nuclear fusion process that forms a deuterium nucleus, consisting of a proton and a neutron, from two protons. A positron (e+)an antimatter electronisemitted along with an electron neutrino .

The number of protons and neutrons in the atomic nucleus can be modified, although thiscan require very high energies because of the strong force. Nuclear fusion occurs whenmultiple atomic particles join to form a heavier nucleus, such as through the energeticcollision of two nuclei. For example, at the core of the Sun protons require energies of 3

10 keV to overcome their mutual repulsionthe coulomb barrier and fuse together intoa single nucleus. [55] Nuclear fission is the opposite process, causing a nucleus to split intotwo smaller nucleiusually through radioactive decay. The nucleus can also be modifiedthrough bombardment by high energy subatomic particles or photons. If this modifies thenumber of protons in a nucleus, the atom changes to a different chemical element. [56] [57]

If the mass of the nucleus following a fusion reaction is less than the sum of the massesof the separate particles, then the difference between these two values may be emitted asa type of usable energy (such as a gamma ray , or the kinetic energy of a beta particle ), asdescribed by Albert Einstein 's massenergy equivalence formula, E = mc2, where m is themass 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 which causes the fused particlesto remain together in a state which require this energy to separate .[58]

The fusion of two nuclei that create larger nuclei with lower atomic numbers than iron and nickel a total nucleon number of about 60is usually an exothermic process thatreleases more energy than is required to bring them together. [59] It is this energy-releasing

process that makes nuclear fusion in stars a self-sustaining reaction. For heavier nuclei,the binding energy per nucleon in the nucleus begins to decrease. That means fusion
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processes producing nuclei that have atomic numbers higher than about 26, and atomicmasses higher than about 60, is an endothermic process . These more massive nuclei cannot undergo an energy-producing fusion reaction that can sustain the hydrostaticequilibrium of a star. [54]

Electron cloud

Main articles: Atomic orbital and Electron configuration

A potential well, showing the minimum energy V ( x) needed to reach each position x. A particle with energy E is constrained to a range of positions between x1 and x2.

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 thesmaller nucleus, which means that an external source of energy is needed in order for theelectron to escape. The closer an electron is to the nucleus, the greater the attractive force.Hence electrons bound near the center of the potential well require more energy to escapethan those at greater separations.

Electrons, like other particles, have properties of both a particle and a wave . The electroncloud 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 , a mathematical function that characterisesthe probability that an electron will appear to be at a particular location when its positionis measured. [60] Only a discrete (or quantized ) set of these orbitals exist around thenucleus, as other possible wave patterns will rapidly decay into a more stable form. [61] Orbitals can have one or more ring or node structures, and they differ from each other insize, shape and orientation. [62]

Wave functions of the first five atomic orbitals. The three 2p orbitals each display asingle angular node that has an orientation and a minimum at the center.

Each atomic orbital corresponds to a particular energy level of the electron. The electroncan change its state to a higher energy level by absorbing a photon with sufficient energy
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to boost it into the new quantum state. Likewise, through spontaneous emission , anelectron in a higher energy state can drop to a lower energy state while radiating theexcess energy as a photon. These characteristic energy values, defined by the differencesin the energies of the quantum states, are responsible for atomic spectral lines .[61]

The amount of energy needed to remove or add an electronthe electron binding energy is far less than the binding energy of nucleons . For example, it requires only 13.6 eV tostrip a ground-state electron from a hydrogen atom, [63] compared to 2.23 million eV for splitting a deuterium nucleus. [64] Atoms are electrically neutral if they have an equalnumber of protons and electrons. Atoms that have either a deficit or a surplus of electronsare 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 intomolecules and other types of chemical compounds like ionic and covalent network crystals .[65]

The structure of the cloud varies with the number of electrons present in the cloud. There

exist a number of different methods of electron counting , such as the octet rule andeighteen electron rule . These tend to be rules of thumb and are not valid across all atoms.Beginning chemistry students are often told the shell structure is simply 2, 8, 8, 8, 8, 8, 8,[...] to make the teaching process easier. The actual numbers of electrons per shell in thelarger atoms can be considerably different, such as 2, 8, 18, 32, 50, 72, but thiscomplexity is reserved for the more advanced student. [citation needed ]

Properties

Nuclear properties

Main articles: Isotope , Stable isotope , and List of elements by nuclear stability

By definition, any two atoms with an identical number of protons in their nuclei belongto the same chemical element . Atoms with equal numbers of protons but a differentnumber of neutrons are different isotopes of the same element. For example, all hydrogenatoms admit exactly one proton, but isotopes exist with no neutrons hydrogen-1 , oneneutron ( deuterium ), two neutrons ( tritium ) and more than two neutrons . The hydrogen-1is by far the most common form, and is sometimes called protium. [66] The knownelements form a set of atomic numbers from hydrogen with a single proton up to the 118-

proton element ununoctium .[67] All known isotopes of elements with atomic numbersgreater than 82 are radioactive. [68][ 69]

About 339 nuclides occur naturally on Earth , of which 269 (about 79%) have not beenobserved to decay. [70] Of the chemical elements, 80 have one or more stable isotopes .Elements 43, 61, and all elements numbered 83 or higher have no stable isotopes. As arule, there is, for each element, only a handful of stable isotopes, the average being 3.1stable isotopes per element which has any stable isotopes. Twenty-seven elements haveonly a single stable isotope, while the largest number of stable isotopes observed for anyelement is ten, for the element tin.[71]
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Stability of isotopes is affected by the ratio of protons to neutrons, and also by presenceof certain "magic numbers" of neutrons or protons which represent closed and filledquantum shells. These quantum shells correspond to a set of energy levels within theshell model of the nucleus; filled shells, such as the filled shell of 50 protons for tin,confers unusual stability on the nuclide. Of the 250 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 . Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: potassium-40 , vanadium-50 , lanthanum-138 and tantalum-180m . Most odd-odd nuclei are highly unstable with respectto beta decay , because the decay products are even-even, and are therefore more strongly

bound, due to nuclear pairing effects .[71]

Mass

Main article: Atomic mass

Because the large majority of an atom's mass comes from the protons and neutrons, thetotal number of these particles in an atom is called the mass number . The mass of an atom at rest is often expressed using the unified atomic mass unit (u), which is also called aDalton (Da). This unit is defined as a twelfth of the mass of a free neutral atom of carbon-12, which is approximately 1.66 10 27 kg. [72] Hydrogen-1 , the lightest isotope of hydrogen and the atom with the lowest mass, has an atomic weight of 1.007825 u. [73] Anatom has a mass approximately equal to the mass number times the atomic mass unit. [74] The heaviest stable atom is lead-208, [68] with a mass of 207.9766521 u. [75]

As even the most massive atoms are far too light to work with directly, chemists insteaduse the unit of moles . The mole is defined such that one mole of any element will always

have the same number of atoms (about 6.022 1023

). This number was chosen so that if an element has an atomic mass of 1 u, a mole of atoms of that element will have a massvery close to 0.001 kg, or 1 gram. Because of the definition of the unified atomic massunit , carbon has an atomic mass of exactly 12 u, and so a mole of carbon atoms weighsexactly 0.012 kg. [72] Other nuclides have atomic masses and molar masses very close towhole numbers in their usual units, such as hydrogen-1. However, except for carbon-12,they cannot be exactly integer numbers, because the masses of different nuclides are notexact integer ratios of each other, although they do not differ from whole number ratios

by more than 1%, and often much less. [citation needed ]

Size

Main article: Atomic radius

Atoms lack a well-defined outer boundary, so the dimensions are usually described interms of 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, the number of neighboring atoms ( coordination number ) and a quantummechanical property known as spin .[76] On the periodic table of the elements, atom size
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tends to increase when moving down columns, but decrease when moving across rows(left to right). [77] Consequently, the smallest atom is helium with a radius of 32 pm , whileone of the largest is caesium at 225 pm. [78] These dimensions are thousands of timessmaller than the wavelengths of light (400700 nm ) so they can not be viewed using anoptical microscope . However, individual atoms can be observed using a scanning

tunneling microscope .

Some examples will demonstrate the minuteness of the atom. A typical human hair isabout 1 million carbon atoms in width. [79] A single drop of water contains about2 sextillion (2 10 21) atoms of oxygen, and twice the number of hydrogen atoms. [80] Asingle carat diamond with a mass of 2 10 -4 kg contains about 10 sextillion (10 22) atomsof carbon .[note 2] If an apple were magnified to the size of the Earth, then the atoms in theapple would be approximately the size of the original apple. [81]

Radioactive decay

Main article: Radioactive decay

This diagram shows the half-life (T) of various isotopes with Z protons and N neutrons.

Every element has one or more isotopes that have unstable nuclei that are subject toradioactive decay, causing 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. [82]

The most common forms of radioactive decay are: [83] [84]
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Alpha decay is caused when the nucleus emits an alpha particle, which is a heliumnucleus consisting of two protons and two neutrons. The result of the emission isa new element with a lower atomic number .

Beta decay is regulated by the weak force , and results from a transformation of aneutron into a proton, or a proton into a neutron. The first is accompanied by the

emission of an electron and an antineutrino , while the second causes the emissionof a positron and a neutrino . The electron or positron emissions are called beta particles. Beta decay either increases or decreases the atomic number of thenucleus by one.

Gamma decay results from a change in the energy level of the nucleus to a lower state, resulting in the emission of electromagnetic radiation. This can occur following the emission of an alpha or a beta particle from radioactive decay.

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 , or result (throughinternal conversion ) in production of high-speed electrons which are not beta rays, and

high-energy photons which are not gamma rays.

Each radioactive isotope has a characteristic decay time periodthe half-life that isdetermined by the amount of time needed for half of a sample to decay. This is anexponential decay process that steadily decreases the proportion of the remaining isotope

by 50% every half life. Hence after two half-lives have passed only 25% of the isotopewill be present, and so forth. [82]

Magnetic moment

Main articles: Electron magnetic dipole moment and Nuclear magnetic moment

Elementary particles possess an intrinsic quantum mechanical property known as spin .This is analogous to the angular momentum of an object that is spinning around its center of mass , although strictly speaking these particles are believed to be point-like and cannot

be said to be rotating. Spin is measured in units of the reduced Planck constant (), withelectrons, protons and neutrons all having spin , or "spin-". In an atom, electrons inmotion around the nucleus possess orbital angular momentum in addition to their spin,while the nucleus itself possesses angular momentum due to its nuclear spin. [85]

The magnetic field produced by an atomits magnetic moment is determined by thesevarious forms of angular momentum, just as a rotating charged object classically

produces a magnetic field. However, the most dominant contribution comes from spin.Due to the nature of electrons to obey the Pauli exclusion principle , in which no twoelectrons may be found in the same quantum state , bound electrons pair up with eachother, with one member of each pair in a spin up state and the other in the opposite, spindown state. Thus these spins cancel each other out, reducing the total magnetic dipolemoment to zero in some atoms with even number of electrons .[86]
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In ferromagnetic elements such as iron, an odd number of electrons leads to an unpairedelectron and a net overall magnetic moment. The orbitals of neighboring atoms overlapand a lower energy state is achieved when the spins of unpaired electrons are alignedwith each other, a process known as an exchange interaction . When the magneticmoments of ferromagnetic atoms are lined up, the material can produce a measurable

macroscopic field. Paramagnetic materials have atoms with magnetic moments that lineup in random directions when no magnetic field is present, but the magnetic moments of the individual atoms line up in the presence of a field. [86][87]

The nucleus of an atom can also have a net spin. Normally these nuclei are aligned inrandom directions because of thermal equilibrium . However, for certain elements (suchas xenon-129 ) it is possible to polarize a significant proportion of the nuclear spin statesso that they are aligned in the same directiona condition called hyperpolarization . Thishas important applications in magnetic resonance imaging .[88] [89]

Energy levels

Main articles: Energy level and Atomic spectral line

When an electron is bound to an atom, it has a potential energy that is inversely proportional to its distance from the nucleus. This is measured by the amount of energyneeded to unbind the electron from the atom, and is usually given in units of electronvolts(eV). In the quantum mechanical model, a bound electron can only occupy a set of statescentered on the nucleus, and each state corresponds to a specific energy level. The lowestenergy state of a bound electron is called the ground state, while an electron at a higher energy level is in an excited state. [90]

In order for an electron to transition between two different states, it must absorb or emit a photon at an energy matching the difference in the potential energy of those levels. Theenergy of an emitted photon is proportional to its frequency , so these specific energylevels appear as distinct bands in the electromagnetic spectrum .[91] Each element has acharacteristic spectrum that can depend on the nuclear charge, subshells filled byelectrons, the electromagnetic interactions between the electrons and other factors. [92]

An example of absorption lines in a spectrum.

When a continuous spectrum of energy is passed through a gas or plasma, some of the photons are absorbed by atoms, causing electrons to change their energy level. Thoseexcited electrons that remain bound to their atom will spontaneously emit this energy as a

photon, traveling in a random direction, and so drop back to lower energy levels. Thusthe atoms behave like a filter that forms a series of dark absorption bands in the energy
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