Valency Electrons

the electrons present in the outermost shell of an atom of an element and therefore the ones involved in forming bonds with other atoms when they are shared, lost or gained in the formation of a compound or ION. Some elements always have the same number of valence electrons, e.g., hydrogen has one, ordinary oxygen has two, and calcium has two.

Taken from Dictionary of Science

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In chemistry, valence electrons are the outermost electrons of an atom, which are important in determining how the atom reacts chemically with other atoms. Atoms with a complete shell of valence electrons (corresponding to an electron configuration s²p⁶) tend to be chemically inert. Atoms with one or two valence electrons more than a closed shell are highly reactive because the extra electrons are easily removed to form positive ions. Atoms with one or two valence electrons less than a closed shell are also highly reactive because of a tendency either to gain the missing electrons and form negative ions, or to share electrons and form covalent bonds.

Valence electrons have the ability, like electrons in inner shells, to absorb or release energy in the form of photons. This gain or loss of energy can trigger an electron to move (jump) to another shell or even break free from the atom and its valence shell. When an electron absorbs energy in the form of one or more photons, then it moves to a more outer shell depending on the amount of energy gained. (See also : electrons in an excited state). When an electron loses energy (photons), then it moves to a more inner shell.

Taken from Wikipedia