Atom

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

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Atom sizes

The size of an atom is not easily defined since the electron orbitals just gradually go to zero as the distance from the nucleus increases. For atoms that can form solid crystals, the distance between adjacent nuclei can give an estimate of the atom size. For atoms that do not form solid crystals other techniques are used, including theoretical calculations. As an example, the size of a hydrogen atom is estimated to be approximately 1.2×10-10m. Compare this to the size of the proton which is the only particle in the nucleus of the hydrogen atom which is approximately 8.7×10-16m. Thus the ratio of the sizes of the hydrogen atom to its nucleus is about 100,000:1. Atoms of different elements do vary in size, but the sizes are roughly the same to within a factor of 2 or so. The reason for this is that elements with a large positive charge on the nucleus attract the electrons to the center of the atom more strongly.

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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.

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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.

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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.

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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.

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Valence and bonding

:see main article valence electrons and chemical bond

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The chemical behavior of atoms is largely due to interactions between electrons. Electrons of an atom remain within certain, predictable electron configurations. Electrons fall into shells based on their relative energy level which is usually visualized as their mean distance from the nucleus. 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.

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Each shell, numbered from the one closest to the nucleus (lowest in energy), can hold up to a specific number of electrons due to its differing sublevel and orbital capacity:

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• Shell 1: 2 electron capacity - s sublevel - 1 orbital
• Shell 2: 8 electron capacity - s and p sublevels - 4 orbitals
• Shell 3: 18 electron capacity - s, p, and d sublevels - 9 orbitals
• Shell 4: 32 electron capacity - s, p, d, and f sublevels - 16 orbitals

To determine the electron capacity of a shell, the formula 2n² is used, where n is the shell number or principle quantum number. Electrons fill orbitals and shells from the inside out, beginning with shell one. Whichever occupied shell is currently most outward is the valence shell, even if it only has one electron.

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The reason why shells fill up in order is that the energy levels of electrons in the innermost shells are significantly lower than the energy levels of electrons in outer shells. So if the inner shells were not completely full, the electron in an outer shell would quickly "fall" into the inner shell (with the emission of a photon that would carry away the difference in the energy.

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The number of electrons in an atom's outermost valence shell governs its 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 metals.

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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 another 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 or ionic compounds. Five major types of bonds exist:

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• ionic bonds;
• covalent bonds;
• coordinate covalent bonds;
• hydrogen bonds; and
• metallic bonds.

Theory: Theory has a number of distinct meanings in different fields of knowledge, depending on the context and their methodologies....

Matter: Matter is commonly referred to as the substance of which physical objects are composed. In physics, it is everything that is constituted of elementary fermions. Philosophically, matter constitutes the formless substratum of all things, which exists only potentially and from which reality is produced...

Proton: :For alternative meanings see proton (disambiguation)....

Atom related Images and Photos (experimental)

Atom Bomb	Atom Watch	I [Atom] Science	Atom Man Vs. Superman 1948
Atom Man Vs. Superman 1948	Nerf Atom Blasters Reactor	Atom Ant and Huckleberry Hound Nodnik 2-Pack	Real Steel Atom and Max Kubrick 2-Pack
Pink Floyd Atom Heart Mother Fabric Poster Wall Hanging	The Big Bang Theory Atom Zip Front Hoodie	Women's: DC-The Atom	Design of the Atom Created in a Lab at Columbia University