How to find valency of an element in a compound

So compounds in general do not posses valencies, it is the ions or the combining atoms making up the compound that have different valencies and these valencies give rise to different chemical formulae for different components.

For ions, the valency is equal to the charge possessed by the molecule.

For example, let's work with sodium chloride (NaCl). It consists of a sodium ion (Na^+ ion) and a chloride ion (Cl^- ion). Both of the ions have a charge of 1 and thus, the valencies of both these elements is 1.

Try CaCl2 for another example. It consists of a Ca^+2 cation, which carries a +2 charge, and two Cl^- ions, each of which carry a charge of -1. Thus, the valency of Ca is 2 and the valency of Cl is 1.

For atoms, valency is the number of electrons gained, lost or shared by an atom to complete its octet (basically, to achieve its noble gas configuration).

If the number of electrons in the outermost shell is more than 4, then the valency of that atom will be (8-number of electrons in outermost shell). For example, if the number of valence electrons are 5, the valency of the atom is 3. [8 – 5 = 3]

If the number of electrons in the outermost shell is less than 4, then the valency is the same as the number of electrons in the outermost shell. For example, if the number of valence electrons are 2, the valency of the atom is 2.So compounds in general do not posses valencies, it is the ions or the combining atoms making up the compound that have different valencies and these valencies give rise to different chemical formulae for different components.

For ions, the valency is equal to the charge possessed by the molecule.

For example, let's work with sodium chloride (NaCl). It consists of a sodium ion (Na^+ ion) and a chloride ion (Cl^- ion). Both of the ions have a charge of 1 and thus, the valencies of both these elements is 1.

Try CaCl2 for another example. It consists of a Ca^+2 cation, which carries a +2 charge, and two Cl^- ions, each of which carry a charge of -1. Thus, the valency of Ca is 2 and the valency of Cl is 1.

For atoms, valency is the number of electrons gained, lost or shared by an atom to complete its octet (basically, to achieve its noble gas configuration).

If the number of electrons in the outermost shell is more than 4, then the valency of that atom will be (8-number of electrons in outermost shell). For example, if the number of valence electrons are 5, the valency of the atom is 3. [8 – 5 = 3]

If the number of electrons in the outermost shell is less than 4, then the valency is the same as the number of electrons in the outermost shell. For example, if the number of valence electrons are 2, the valency of the atom is 2.