Valence electron

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In chemistry, valence electrons are the electrons located within the outermost energy level of an atom. These electrons participate in chemical reactions. Elements with a full outer shell are very unreactive. Elements with an almost full or almost empty outer shell, such as the alkali metals and halogens, tend to be very reactive.

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Determining the number of valence electrons

Valence Electrons
Image:Atom.png
Helium atom model This helium (He) model displays two valence electrons located in its outermost energy level. Helium is a member of the noble gases and contains two protons, neutrons, and electrons.

To determine the quantity of valence electrons an element has, one must look at the family (vertical column) in which the element is categorized. With the exception of groups 3–12 (transition metals), the number within the unit's place identifies how many valence electrons are contained within the elements listed under that particular column.

Group	Quantity of valence electrons
Group one (1) (alkali metals)	One
Group two (2) (alkaline earth metals)	Two
The (transition metals) (3-12)	One to two
Group three (13) (boron group)	Three
Group four (14) (carbon group)	Four
Group five (15) (nitrogen group)	Five
Group six (16) (oxygen group)	Six
Group seven (17) (halogens)	Seven
Group eight / zero (18) (noble gases)	*Eight

The asterisk (*) is used to indicate that all of the noble gases contain eight valence electrons with the exception of helium (He), which only contains two.

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Valence electrons in chemical reactions

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 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.

Image:HAtomOrbitals.png

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:

Shell number	Electron capacity	Sublevels	Orbitals
One	2	S	1
Two	8	S, P	4
Three	18	S, P, D	9
Four	32	S, P, D, F	16

To determine the electron capacity of a shell, the formula 2n² is used, where n is the shell number or principal 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.

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).

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. 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.

Every atom is much more stable, or less reactive, 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: