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{| border="1" cellspacing="0" align="right" cellpadding="2" style="margin-left:1em" width=300

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! bgcolor=gray | Atom

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| align="center" | Typical depiction of the atomic structure has a darkness of the There was an error working with the wiki: Code[39] which corresponds to the line-of-sight integral over the There was an error working with the wiki: Code[40] is schematic, showing There was an error working with the wiki: Code[75]s in pink and There was an error working with the wiki: Code[76]s in purple. In reality, the nucleus (and the wavefunction of each of the There was an error working with the wiki: Code[77]s) is also spherically symmetric. (For more complex nuclei this is not the case.)

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! bgcolor=gray | Classification

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| Smallest recognised division of a There was an error working with the wiki: Code[78]

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! bgcolor=gray | Properties

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| There was an error working with the wiki: Code[41] : || ? 1.67 × 10There was an error working with the wiki: Code[1] to 4.52 × 10There was an error working with the wiki: Code[2] There was an error working with the wiki: Code[79]

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| Electric charge : || zero(if the number of electrons equal of protons in an atom)

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| There was an error working with the wiki: Code[42])

| 50 There was an error working with the wiki: Code[43](H) to 520 pm(Cs)

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| Number of atoms in the There was an error working with the wiki: Code[80]: || ~1080 Matthew Champion, Re: How many atoms make up the universe?, 1998

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In There was an error working with the wiki: Code[44] ?????? or átomos meaning "indivisible") is the smallest particle of a There was an error working with the wiki: Code[45]) Whereas the word atom originally denoted a particle that cannot be cut into smaller particles, the atoms of modern parlance are composed of There was an error working with the wiki: Code[81]s:

There was an error working with the wiki: Code[46], a size which is so small as to be currently unmeasurable, and which are the least heavy (i.e., massive) of the three

There was an error working with the wiki: Code[82]s, which have a positive charge, and are about 1836 times more massive than electrons and

There was an error working with the wiki: Code[83]s, which have no charge, and are about 1838 times more massive than electrons.

Protons and neutrons make up a dense, massive There was an error working with the wiki: Code[84], and are collectively called There was an error working with the wiki: Code[85]s. The electrons form the much larger There was an error working with the wiki: Code[86] surrounding the nucleus.

Atoms can differ in the number of each of the subatomic particles they contain. Atoms of the same There was an error working with the wiki: Code[47] have the same number of protons (called the There was an error working with the wiki: Code[87]). Within a single element, the number of neutrons may vary, determining the There was an error working with the wiki: Code[88] of that element. The number of electrons associated with an atom is most easily changed, due to the lower energy of binding of electrons. The number of protons (and neutrons) in the atomic nucleus may also change, via There was an error working with the wiki: Code[89], Nuclear fission or There was an error working with the wiki: Code[90], in which case the atom is no longer the same element it was.

Atoms are Electric charge neutral if they have an equal number of protons and electrons. Atoms which have either a deficit or a surplus of electrons are called There was an error working with the wiki: Code[48] into There was an error working with the wiki: Code[49] in There was an error working with the wiki: Code[91]s.

Atoms and molecules

Atoms are the building blocks of matter. They are the basis of all the structures and organisms in the universe. The planets, the sun, grass and trees, the air we breathe, and people are all made up of atoms. There are approximately 100 different types of atom, although many of these exist in very small quantities. Every chemical element is one type of atom, such as hydrogen (H), aluminium (Al) and iron (Fe).

For gases and certain molecular liquids and solids (such as There was an error working with the wiki: Code[50]s, and liquid and solid There was an error working with the wiki: Code[92]s). Thus, while molecules are common on Earth (making up all of the atmosphere and most of the oceans), most of the mass of the Earth (much of the crust, and all of the mantle and core) is not made of identifiable molecules, but rather represents atomic matter in other arrangements, all of which lack the particular type of small-scale order that is associated with molecules.

Most molecules are made up of multiple atoms for example, a molecule of water is a combination of two Hydrogen atoms and one Oxygen atom. The term "molecule" in gases has been used as a synonym for the fundamental particles of the gas, whatever their structure. This definition results in a few types of gases (for example inert elements that do not form compounds, such as There was an error working with the wiki: Code[93]), which has "molecules" consisting of only a single atom.

History of atomic theory and discovery of atomic structure

Philosophical atomic ruminations date back to the ancient There was an error working with the wiki: Code[51] in the fifth and sixth centuries BCE. It was the There was an error working with the wiki: Code[94] (There was an error working with the wiki: Code[95] see below) who coined the term atomos, which meant "uncuttable". The first philosophical statements relating to an idea similar to atoms was developed by There was an error working with the wiki: Code[95] in Greece in the fifth century There was an error working with the wiki: Code[97] around 450 There was an error working with the wiki: Code[97]. The idea was lost for centuries until scientific interest was rekindled during the Renaissance Period.

In 1803, There was an error working with the wiki: Code[52], and why certain gases dissolved better in water than others. He proposed that each element consists of atoms of a single, unique type, and that these atoms could join up to form compound chemicals.

In 1897, There was an error working with the wiki: Code[99], through his work on There was an error working with the wiki: Code[100], discovered the electron and their subatomic nature, which destroyed the concept of atoms as being indivisible units. Later, Thomson also discovered the existence of isotopes through his work on ionized gases.

Thomson believed that the electrons were distributed evenly throughout the atom, balanced by the presence of a uniform sea of positive charge. However, in 1909, the There was an error working with the wiki: Code[101] of There was an error working with the wiki: Code[102] suggested that the positive charge of an atom and most of its mass was concentrated in a nucleus at the centre of the atom, with the electrons orbiting it like planets around a sun. In 1913, There was an error working with the wiki: Code[103] added Quantum mechanics into this model, which now stated that the electrons were confined to clearly defined orbits and could not freely spiral in or out.

In 1926, There was an error working with the wiki: Code[104], using There was an error working with the wiki: Code[105]'s 1924 proposal that electrons behave like waves in an atom, wrote out a wave equation for the electrons in an atom, which had solutions describing electron orbitals. A consequence of this notion, pointed out by There was an error working with the wiki: Code[106] a year later, is that it is mathematically impossible to obtain precise values for both the position and momentum of a particle at any point in time this became known as the There was an error working with the wiki: Code[107]. Instead, for any given value of position you 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 orbital zones around the nucleus where a given electron is most likely to exist.

Properties of the atom in present theory

Subatomic particles

Although the name "atom" was applied at a time when atoms were thought to be indivisible, it is now known that the atom can be broken down into a number of smaller components. The first of these to be discovered was the negatively charged Electron, which is easily ejected from atoms during There was an error working with the wiki: Code[108]. The electrons (or more specifically, electron clouds) orbit a small, dense body containing all of the positive charge in the atom, called the There was an error working with the wiki: Code[109]. This nucleus is itself made up of There was an error working with the wiki: Code[110]s: positively charged There was an error working with the wiki: Code[111]s and chargeless There was an error working with the wiki: Code[112]s.

Before 1961, the subatomic particles were thought to consist of only protons, neutrons and electrons. However, protons and neutrons themselves are now known to consist of still smaller particles called There was an error working with the wiki: Code[113]s. In addition, the electron is known to have a nearly massless neutral partner called a Neutrino. Together, the electron and neutrino are both There was an error working with the wiki: Code[114]s.

Ordinary atoms are composed only of quarks and leptons of the first There was an error working with the wiki: Code[53]. The proton is composed of two There was an error working with the wiki: Code[54].

The subatomic There was an error working with the wiki: Code[55] particles (called There was an error working with the wiki: Code[115]s) are also important to atoms. Electrons are bound to the nucleus by There was an error working with the wiki: Code[116]s carrying the There was an error working with the wiki: Code[117]. Protons and neutrons are bound together in the nucleus by There was an error working with the wiki: Code[118]s carrying the There was an error working with the wiki: Code[119].

Electron configuration

The nucleus of an atom is surrounded by a cloud of electrons, and it is primarily the interaction of these clouds that govern the chemical behavior of atoms (know as a There was an error working with the wiki: Code[120]). A popular concept is that the electrons orbit the atom in neat circles like planets around a sun, but this is an obsolete model that is nonetheless still taught to schoolchildren because it is simpler and sufficient for school-level chemistry. In the true modern model of the atom, the positions of the electrons around the atom's nucleus are described through probabilities&mdashthat is, an electron can theoretically be found at any arbitrary position around the nucleus, but is more likely to be found in certain regions than others. This pattern is referred to as its There was an error working with the wiki: Code[121] and the shape of its orbital depends on its energy level (or, more specifically, its quantum state).

Each atomic orbital can hold up to two electrons. The orbitals are organized into shells and subshells, based on their overall energy and angular momentum. Generally speaking, higher energy shells can hold more electrons and are located farther from the nucleus. A shell can hold up to 2n2 electrons (where n is the shell number). The electrons in the outermost shell, called the There was an error working with the wiki: Code[122]s, have the greatest influence on chemical behavior. Core electrons (those not in the outer shell) play a role, but it is usually in terms of a secondary effect due to screening of the positive charge in the atomic nucleus.

In the most stable There was an error working with the wiki: Code[56] to a higher energy level (that is, it absorbs energy from an external source and leaps to a higher shell), leaving a space in a lower shell. An excited atom's electrons will There was an error working with the wiki: Code[57] back to lower levels, emitting the energy it had gained as a photon. This behavior is the root of a substance's There was an error working with the wiki: Code[123].

Nucleon properties

The constituent There was an error working with the wiki: Code[124]s and There was an error working with the wiki: Code[125]s of the There was an error working with the wiki: Code[126] are collectively called There was an error working with the wiki: Code[127]s. The nucleons are held together in the nucleus by the There was an error working with the wiki: Code[128] which is carried by There was an error working with the wiki: Code[129]. Nuclei can undergo transformations that affect the number of protons and neutrons they contain, a process called There was an error working with the wiki: Code[130]. When nuclei transformations take place spontaneously, this process is called There was an error working with the wiki: Code[131]. Radioactive transformations proceed by a wide variety of modes, but the most common are There was an error working with the wiki: Code[132] (emission of a There was an error working with the wiki: Code[133] nucleus) and There was an error working with the wiki: Code[134] (emission of an electron). Decays involving electrons or There was an error working with the wiki: Code[135]s are due to the There was an error working with the wiki: Code[136].

In addition, like the electrons of the atom, the nucleons of nuclei may be pushed into There was an error working with the wiki: Code[137]s of higher energy. However, these transitions typically require thousands of times more energy than electron excitations. When an excited nucleus emits a photon to return to the There was an error working with the wiki: Code[138], the photon has very high energy and is called a There was an error working with the wiki: Code[139].

Nuclear transformations also take place in There was an error working with the wiki: Code[140]s. In There was an error working with the wiki: Code[141], two light nuclei come together and merge into a single heavier nucleus. In Nuclear fission, a single large nucleus is divided into two or more smaller nuclei.

Atom size and speed

Atoms are much smaller than the There was an error working with the wiki: Code[142]s of There was an error working with the wiki: Code[143] that human vision can detect, so atoms cannot be seen in any kind of optical There was an error working with the wiki: Code[144]. However, there are ways of detecting the positions of atoms on the surface of a solid or a There was an error working with the wiki: Code[145] so as to obtain images. These include: There was an error working with the wiki: Code[146]s (such as in There was an error working with the wiki: Code[147] (STM)), There was an error working with the wiki: Code[148] (AFM), There was an error working with the wiki: Code[149] (NMR) and There was an error working with the wiki: Code[150].

Nearly all the mass of an atom is in its nucleus, yet almost all the space in an atom is occupied by its There was an error working with the wiki: Code[151]. This cloud, which is composed of orbital space discussed, is the volume which the electrons occupy and exclude other electrons from. The electron cloud is far larger than the size of the individual electron(s) which comprise(s)it. Thus, particles such as neutrons and neutrinos, which do not interact with the electron cloud, do not "see" the same effective size for atoms, and often pass through them as though they were empty space.

Since the There was an error working with the wiki: Code[58], the distance between the centers of adjacent atoms can be easily determined by There was an error working with the wiki: Code[59]. If an atom were the size of the Earth, an electron would certainly be less than 10 cm in size, and might have no size at all (electrons are less than 10There was an error working with the wiki: Code[5]&nbspm in radius and may be point-particles). http://www.aip.org/pnu/2006/split/783-1.html

Atoms of different There was an error working with the wiki: Code[60] do vary in effective size, but the sizes (volumes) do not scale well with the mass of the atom. Heavier atoms do tend generally to be denser. The diameters of atoms are roughly the same to within a factor of less than three for the heavier atoms, and the most noticeable effect on size with atomic mass is a reverse one: atomic size actually shrinks with increasing mass in each periodic table row The reason for these effects is that heavy elements have large positive charge on their nuclei, which strongly attract the electrons to the center of the atom. This contracts the size of the <pesn type= (atomic weight about 192) are about the same size as There was an error working with the wiki: Code[153] atoms (atomic weight about 27), and this weight ratio of about seven for the individual atoms, is the major factor in the density ratio of more than eight between the two metals.

The Temperature of a collection of atoms is a measure of the average energy of motion of those atoms above the minimum zero-point energy demanded by quantum mechanics at 0 There was an error working with the wiki: Code[154]s (There was an error working with the wiki: Code[155]) atoms would have no extra energy above the minimum. As the temperature of the system is increased, the kinetic energy of the particles in the system is increased, and their speed of motion increases. At There was an error working with the wiki: Code[156], atoms making up gases in the air move at an average speed of 500&nbspm/s (about 1100&nbspmph or 1800&nbspkm/h).

Elements, isotopes and ions

Atoms are classified into chemical elements by their There was an error working with the wiki: Code[157] Z, which corresponds to the number of protons in the atom. For example, all atoms containing six protons (Z&nbsp=&nbsp6) are classified as There was an error working with the wiki: Code[158]. Atoms with the same atomic number share a wide variety of physical properties and exhibit almost identical There was an error working with the wiki: Code[159] (for the closest instance to an exception to this principle, see There was an error working with the wiki: Code[160] and There was an error working with the wiki: Code[161]). The elements may be sorted according to the There was an error working with the wiki: Code[162] in order of increasing atomic number. When this is done, certain repeating cycles of regularities in chemical and physical properties are evident.

The There was an error working with the wiki: Code[163] A, or nucleon number of an element is the total number of protons and neutrons in an atom of that element, so-called because each proton and neutron has a mass of about 1&nbspThere was an error working with the wiki: Code[164] (amu). A particular collection of a certain number of protons Z, and neutrons A-Z, is called a There was an error working with the wiki: Code[165].

Each element can have numerous different There was an error working with the wiki: Code[166] with the same Z (number of protons or number of electrons) but varying numbers of neutrons. Such a family of nuclides are called the There was an error working with the wiki: Code[167] of the element (isotope = "same place", because these nuclides share the same chemical symbol and place on the periodic table). When writing the name of a particular nuclide, the element name (which specifies the Z) is preceded by the mass number if written as a superscript, or else followed by the mass number if not a superscript. For example, the nuclide There was an error working with the wiki: Code[168], which may also be written 14C, is one of the isotopes of carbon, and it contains 6 protons and 8 neutrons in each atom, for a total mass number of 14. For a complete table of known nuclides, including radioactive and stable nuclides, see There was an error working with the wiki: Code[169].

The atomic mass listed for each element in the periodic table is an average of the isotope masses found in nature, There was an error working with the wiki: Code[61] by their There was an error working with the wiki: Code[62].

The simplest atom is the There was an error working with the wiki: Code[63] varies greatly with the element (There was an error working with the wiki: Code[170] has 10 stable isotopes see There was an error working with the wiki: Code[171]). There was an error working with the wiki: Code[172] (Z&nbsp=&nbsp82) is the last element which has stable isotopes. The elements with atomic number 83 (There was an error working with the wiki: Code[173]) and greater have no stable isotopes and are all radioactive.

Virtually all elements heavier than hydrogen and There was an error working with the wiki: Code[64] There was an error working with the wiki: Code[174], elements have been created through atomic number 118 (given the temporary name There was an error working with the wiki: Code[175]). These ultra-heavy elements are generally highly unstable and decay quickly.

Atoms that have lost or gained electrons to become electrically non-neutral, are called atomic Ions. Ions are divided into There was an error working with the wiki: Code[176] with positive (+) electric charge or There was an error working with the wiki: Code[177] with negative (-) charge.

Valence and bonding

The number of electrons in an atom's outermost shell (the There was an error working with the wiki: Code[65] together in the columns of the There was an error working with the wiki: Code[178] of the elements. There was an error working with the wiki: Code[179]s contain one electron on their outer shell There was an error working with the wiki: Code[180]s, two electrons There was an error working with the wiki: Code[181]s, seven electrons and various others.

Every atom is most stable with a full valence shell. This means that atoms with full valence shells (the There was an error working with the wiki: Code[66]. Alkali metals are therefore very reactive, with There was an error working with the wiki: Code[182], There was an error working with the wiki: Code[183], and There was an error working with the wiki: Code[184] being the most reactive of all metals. Also, atoms that need only few electrons (such as the halogens) to fill their valence shells are reactive. There was an error working with the wiki: Code[185] is the most reactive of all elements.

Atoms may fill their valence shells by Chemical bonding. This can be achieved one of two ways: an atom can either share electrons with other atoms (a There was an error working with the wiki: Code[186]), or it can remove electrons from (or donate electrons to) other atoms (an There was an error working with the wiki: Code[187]). The formation of a bond causes a strong attraction between two atoms, creating There was an error working with the wiki: Code[188]s or Ionic compounds. Many other types of bonds exist, including:

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Atomic spectrum

Since each element in the There was an error working with the wiki: Code[67] of its There was an error working with the wiki: Code[194]s and the number of electrons within them. Normally, an atom is found in its lowest-energy There was an error working with the wiki: Code[195] states with higher energy are called There was an error working with the wiki: Code[196]s. An electron may move from a lower-energy orbital to a higher-energy orbital by absorbing a There was an error working with the wiki: Code[197] with energy equal to the difference between the energies of the two levels. An electron in a higher-energy orbital may drop to a lower-energy orbital by emitting a photon. Since each element has a unique set of energy levels, each creates its own There was an error working with the wiki: Code[198] pattern unique to itself: its own spectral signature.

If a set of atoms is heated (such as in an There was an error working with the wiki: Code[199]), their electrons will move into excited states. When these atoms fall back toward the ground state, they will produce an There was an error working with the wiki: Code[200]. If a set of atoms is illuminated by a There was an error working with the wiki: Code[201], it will only absorb specific There was an error working with the wiki: Code[202]s (energies) of photon that correspond to the differences in its energy levels. The resulting pattern of gaps is called the There was an error working with the wiki: Code[203].

In spectroscopic analysis, scientists can use a There was an error working with the wiki: Code[68], producing high-energy There was an error working with the wiki: Code[204]s.

Exotic atoms

An There was an error working with the wiki: Code[69] if allowed to contact ordinary matter.

The most familiar examples of exotic atoms are the antiatom There was an error working with the wiki: Code[205] (composed of an There was an error working with the wiki: Code[206] and positron) which has been produced in tiny quantities, and There was an error working with the wiki: Code[207], an analogue to the hydrogen atom in which a positron is substituted for the usual proton nucleus. There was an error working with the wiki: Code[208] is unstable it is a common phase in the attraction between an electron and positron before the annihilation reaction in which the matter particles are destroyed and two There was an error working with the wiki: Code[209]s are emitted.

Atoms and the Big Bang Theory

Immediately after the start of the There was an error working with the wiki: Code[210], space expanded incredibly quickly for a very short time. This process, which lasted for the minutest fraction of a second, is called inflation. After that, expansion began to slow down and different kinds of There was an error working with the wiki: Code[211] including There was an error working with the wiki: Code[212] and There was an error working with the wiki: Code[213] made their appearance. Just one millionth of a second after the birth of the There was an error working with the wiki: Code[214], the There was an error working with the wiki: Code[212] had clumped together to from new particles called There was an error working with the wiki: Code[216] and There was an error working with the wiki: Code[217]. After a hundred seconds or so, some of the There was an error working with the wiki: Code[216] and nearly all of the There was an error working with the wiki: Code[217] gathered into bunches, consisting of two There was an error working with the wiki: Code[216] and two There was an error working with the wiki: Code[217]. Eventually, each bunch, or There was an error working with the wiki: Code[222], captured two There was an error working with the wiki: Code[213] to form a There was an error working with the wiki: Code[224] atom, and each remaining There was an error working with the wiki: Code[225] captured a single Electron to form a Hydrogen atom. The first building blocks of matter had been born.

In models of the There was an error working with the wiki: Code[70] of Hydrogen and There was an error working with the wiki: Code[226] formed abundantly, but almost no elements heavier than There was an error working with the wiki: Code[227]. Hydrogen makes up approximately 92% of the atoms in the universe (by number, not mass) helium makes up less than 7% and all other elements make up less than 1% (see There was an error working with the wiki: Code[228]). However, although nuclei (fully-Ionized atoms) were created, neutral atoms themselves could not form in the intense heat.

Big Bang chronology of the atom continues to approximately 379,000 years after the Big Bang when the cosmic temperature had dropped to just 3,000&nbspThere was an error working with the wiki: Code[71]. It was then cool enough to allow the nuclei to capture Electrons. This process is called There was an error working with the wiki: Code[229], during which the first neutral atoms took form. Once atoms become neutral, they only absorb There was an error working with the wiki: Code[230]s of a discrete There was an error working with the wiki: Code[231]. This allows most of the photons in the universe to travel unimpeded for billions of years. These photons are still detectable today in the There was an error working with the wiki: Code[232].

After Big Bang nucleosynthesis, no heavier elements could be created until the There was an error working with the wiki: Code[72]. These stars There was an error working with the wiki: Code[73] heavier elements through There was an error working with the wiki: Code[233] during their lives and through There was an error working with the wiki: Code[234] as they died. The seeding of the There was an error working with the wiki: Code[235] by heavy elements eventually allowed the formation of There was an error working with the wiki: Code[236]s like the There was an error working with the wiki: Code[237].

Atom size comparisons

Various analogies have been used to demonstrate the minuteness of the atom:

A human hair is about 1 million carbon atoms wide.

An There was an error working with the wiki: Code[238] There was an error working with the wiki: Code[239] is the width of 800 carbon atoms and contains about 100 million atoms total. An There was an error working with the wiki: Code[240] bacterium contains perhaps 100 billion atoms, and a typical human cell roughly 100 trillion atoms.

A speck of dust might contain 3x10There was an error working with the wiki: Code[6] (3 trillion) atoms.

The number of atoms in 12 grams of charcoal (about 6 x 1023) is more than 1,400,000 times the age of the universe in seconds.

See also

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Chemical bond

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References and external links

Patents

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Other

Kenneth S. Krane, Introductory Nuclear Physics (1987)

Atoms for Students www.studenthelpforum.com

Atomic sizes dl.clackamas.cc.or.us

How Atoms Work www.howstuffworks.com

Science aid - atomic structure A guide to the atom for teens.

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

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Comments