Valency

is the bonding potential or combining power of an atom or group, measured by the number of hydrogen ions (H⁺, or equivalent) that the atoms could combine with or replace. In an ionic compound, the charge on each ion represents the valency, e.g., in NaCl, both Na⁺ and Cl^- have a valency of one. In covalent compounds, the valency is represented by the number of bonds formed. In carbon dioxide, CO₂, carbon has a valency of 4 and oxygen 2.

The electronic theory of valency explains bonds through the assumption that specific arrangements of outer electrons in atoms (outer shells of eight electrons) give stability (as with the inert gases, which have such a structure) through the transfer or sharing of electrons. Thus with the combination of sodium with chlorine, sodium has one electron in the outer shell, which it loses to chlorine to form the stable structure of the inert gas neon. Similarly, the gain of one electron by chlorine gives it the stable structure of argon.

Taken from Dictionary of Science

_____________________________________________________________

In chemistry, valence, also known as valency or valency number, is a measure of the number of chemical bonds formed by the atoms of a given element. Over the last century, the concept of valence evolved into a range of approaches for describing the chemical bond, including Lewis structures (1916), valence bond theory (1927), molecular orbitals (1928), valence shell electron pair repulsion theory (1958) and all the advanced methods of quantum chemistry.

Taken from Wikipedia