Atom


 * For alternative meanings see atom (disambiguation).

An atom is a microscopic structure found in all ordinary matter around us. Atoms are composed of 3 types of subatomic particles:
 * electrons, which have a negative charge;
 * protons, which have a positive charge; and
 * neutrons, which have no charge.

Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions. An atom is the smallest particle differentiable as a certain chemical element; when an atom of an element is divided, it ceases to be that element. Only 91 elements have been identified as occurring naturally on Earth.

Each element is unique by the number of protons in each atom of that element. Every atom has a number of electrons equal to its number of protons; if there is an imbalance, the atom is called an ion. Atoms of the same element can have different numbers of neutrons, as long as the number of protons or electrons does not change. Atoms with different numbers of neutrons are called isotopes of a chemical element.

Other elements have been artificially created, but they are usually unstable and spontaneously change into stable natural chemical elements by the processes of radioactive decay.

Though only 91 naturally occurring elements exist, atoms of these elements are able to bond into molecules and other types of chemical compounds. Molecules are made up of multiple atoms. A molecule of water is a combination of 2 hydrogen and one oxygen atoms.

Because of their ubiquitous nature, atoms have been an important field of study for many centuries. Current research focus on quantum effects, such as in Bose-Einstein condensate.

Atomic theory
Main article: Atomic theory

The atomic theory is a theory of the nature of matter. It states that all matter is composed of atoms.

Structure
The most widely accepted model of an atom is the wave model. It is based on the Bohr model, but takes into account recent developments and discoveries in quantum mechanics.

It states that:
 * Atoms are composed of subatomic particles (protons, electrons, and neutrons).
 * However, much of an atom's volume is empty space.
 * At the center of the atom is a tiny, positively charged nucleus composed of protons and neutrons (known as nucleons).
 * The nucleus is more than 100,000 times smaller than the atom. If an atom was expanded to the size of Heathrow Airport, the nucleus would be about the size of a golf ball.




 * Most the atom's space is taken up by orbitals containing electrons in a certain electron configuration.
 * Each orbital can hold up to two electrons, and is governed by three quantum numbers: the principal, azimuthal, and magnetic.
 * Each electron in an orbital has a unique value for the fourth quantum number, spin.
 * Orbitals are not physical contructs, but are actually probability distributions of where two electrons having equal values for the first three quantum numbers might be. The "edge" of an orbital is generally considered to be where the probability of an electron's presence drops below 90%.
 * As electrons join an atom, they fall into the lowest energy shell; that is, the orbitals closest to the nucleus (the first shell). Only the electrons in the outermost orbital (the valence shell) are available for atomic bonding; see "Valence and bonding" for more information.

Elements and isotopes
Atoms are generally classified by their atomic number, which corresponds to the number of protons in the atom. The atomic number defines which element the atom is. For example, carbon atoms are those atoms containing 6 protons. All atoms with the same atomic number share a wide variety of physical properties and exhibit the same chemical behavior. The various kinds of atoms are listed in the periodic table in order of increasing atomic number.

The mass number, atomic mass number, or nucleon number of an element is the total number of protons and neutrons in an atom of that element, because each proton or neutron essentially has a mass of 1 amu. The number of neutrons in an atom has no effect on which element it is. Each element can have numerous different atoms with the same number of protons and electrons, but varying numbers of neutrons. Each has the same atomic number but a different mass number. These are called the isotopes of an element. When writing the name of an isotope, the element name is followed by the mass number. For example, carbon-14 contains 6 protons and 8 neutrons in each atom, for a total mass number of 14.

The simplest atom is the hydrogen atom, which has atomic number 1 and consists of one proton and one electron. The hydrogen isotope which also contains 1 neutron is called deuterium or hydrogen-2; the hydrogen isotope with 2 neutrons is called tritium or hydrogen-3.

The atomic mass listed for each element in the periodic table is an average of the isotope masses found in nature, weighted by their abundance.

Valence and bonding
The chemical behavior of atoms is largely due to interactions between electrons. Electrons of an atom must remain within certain, predictable electron configurations. Electrons fall into shells based on their relative distance from the nucleus (see Atomic structure for more details). The electrons in the outermost shell, called the valence electrons, 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.

Each shell, numbered from the one closest to the nucleus, can hold up to a limited number of electrons due to its differing number and type of orbitals: Electrons fill orbitals and shells from the inside out, beginning with shell one. Higher numbered shells only exist when made necessary by the number of electrons. Whichever extant shell is currently most outward is the valence shell, even if it only has one electron.
 * Shell 1: 2 electrons
 * Shell 2: 8 electrons
 * Shell 3: 8 or 18 electrons (depending on the element)

The number of electrons in an atom's valence shell governs it's bonding behavior. Therefore, elements with the same number of valence electrons are grouped together in the periodic table of the elements. Group (i.e. column) 1 elements contain one electron on their outer shell; Group 2, two electrons; Group 3, three electrons; etc. As a general rule, the fewer electrons in an atom's valence shell, the more reactive it is. Group 1 metals are therefore very reactive, with caesium, rubidium, and francium being the most reactive of all elements.

Every atom is much more stable (i.e. less energetic) with a full valence shell. This can be achieved one of two ways: an atom can either share electrons with neighboring atoms (a covalent bond), or it can remove electrons from other atoms (an ionic bond). Another form of ionic bonding involves an atom giving some of its electrons to an other atom; this also works because it can end up with a full valence by giving up its entire outer shell. By moving electrons, the two atoms become linked. This is known as chemical bonding and serves to build atoms into molecules. Five major types of bonds exist:
 * ionic bonds;
 * covalent bonds;
 * coordinate covalent bonds;
 * hydrogen bonds; and
 * metallic bonds.

Atoms in the universe and our world
Using inflation theory, the number of atoms in the universe can be estimated to be between 4.0e78 and 6.0e79. However, because of the possibly infinite nature of the universe, a more exact value may never be known.

Atoms in industry
Atoms serve many important roles in industry, including in nuclear power plants, industrial materials science, and many roles in the chemical industry.

Atoms in science
The study of atoms has been a major focus of scientific research for decades. Atomic theory has wide-ranging impact on many fields of science, including nuclear physics, spectroscopy, and all of chemistry, among many other topics. Today, most scientific research associated with atoms is in the field of quantum mechanics. Subatomic particle research is also a popular field.

The study of atoms was done by largely indirect means through the 19th century and early 20th century. In recent years, however, new techniques have made the identification and study of atoms easier and more accurate. The electron microscope, invented in 1931, has allowed pictures to be taken of actual, individual atoms. Methods also exist to identify atoms and compounds. Mass spectrometry methods allow the exact identification of the types and amounts of atoms in a substance. Gas chromatography-mass spectrometry is often used by forensics investigators to identify unknown substances. X-ray crystallography reveals the structure of certain atomic and molecular substances.

Historical theories
Democritus and Leucippus, Greek philosophers in the 5th century BC, presented the first theory of atoms. They held that each atom had a different shape, like a pebble, the governed the atom's properties. Dalton's work in the 19th century proved that matter was made up of atoms, but he knew nothing of their structure. This goes against the theory of infinite divisibility, which states that matter can always be divided into smaller parts.

Through this time, atoms were thought to be the smallest possible piece of matter. However, it was later shown that atoms are made up of subatomic particles. Thomson's experiments discovered the electron, the first of the subatomics to be discovered. This showed that atoms are actually divisible. Rutherford's work helped to show that the positively charged nucleus exists. All recent models of the atom have taken into account the existence of subatomic particles.

Since Democritus' time, many theories for the structure of the atom have been suggested, including:
 * Plum pudding model,
 * Cubical atom,
 * Bohr model, and
 * Wave model, which is widely accepted today. See Structure, above.

While Democritus's theory has been widely disproven, the recent string theory is based on a similar idea of shape and vibration governing a subatomic particle's properties.

Etymology of "atom"
The word "atom" is derived from the Greek atomos, "indivisible", from a-, not, and tomos, a cut. Until the 19th century and the development of the Bohr model, it was believed that atoms were tiny, indivisible particles. This idea dates back to Democritus and Leucippus, Greek philosophers in the 5th century BC.

External links and references

 * The American Heritage Dictionary entry on atom (contains popups)
 * The Universe, at Sacramento Peak

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