1) the capacity of an element or radical to combine with another to form molecules, as measured by the number of hydrogen or chlorine atoms which one radical or one atom of the element will combine with or replace (e.g.: oxygen has a valence of two, i.e., one atom of oxygen combines with two hydrogen atoms to form the water molecule, HO)
any of the units of valence which a particular element may have
2) the valence or valency of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules. The concept of valence was developed in the second half of the 19th century and was successful in explaining the molecular structure of inorganic and organic compounds. The quest for the underlying causes of valence led to the modern theories of chemical bonding, including the cubical atom (1902), Lewis structures (1916), valence bond theory (1927), molecular orbitals (1928), valence shell electron pair repulsion theory (1958), and all of the advanced methods of quantum chemistry.
What is Valency?
The valency of an element is the number of hydrogen atoms that can combine with or replace (either directly or indirectly) one atom of the element. In other words, the valency of an element is the number of electrons an atom of the element uses to combine with atoms of other elements - it is the combining power of an atom of the element. In an atom, the valence electrons are the electrons that can be used in combining with other atoms - these are the electrons in the orbitals of the outermost shell (also called valence shell).
Notice that it is not in all cases that the valency of an atom equals the total number of its valence electrons. For example, oxygen has six valence electrons, but its valency is 2. Some elements may have more than one combining power (or valency), while others have just one.
For example, H →1; Mg→2; Al→3; C→4; N→3, and 5; P→3 and 5; O→2; S→ 2, 4 and 6; Cl→ 1; and Ne→0.
The valencies of radicals are same as the number of charge they carry. For example, NH4+ →1; OH- → 1 ; and SO42- → 2.