Dear student,
Valency is the combining capacity of an atom. It is equal to the  number of electrons the atom loses, or gains or shares when it combines  with one or more atoms.
Depending on whether the atom loses, gains or shares electrons, the valency may be positive or negative or neutral respectively.
Example: Valency of sodium is + 1, because it can lose one electron. It is represented as Na+. Valency of oxygen may be - 2 if it accepts 2 electrons i.e., O2-.
If two or more atoms combine by sharing electrons i.e., not losing or  gaining electrons, then the valency numbers are neither positive nor  negative. In the case of carbon dioxide, carbon and oxygen atoms, do  not, lose or gain electrons, and their valencies are 4 and 2  respectively.
Certain atoms exhibit different kinds of valencies. This is because  such elements can loose more than one electron from their outer most  shell depending on the available conditions. When such atoms exhibit  variable valency, the name of the atom with the lower valency ends as -ous, and the one with the higher valency, ends as -ic.
 

 

Lower Valency
 
Higher Valency

 


Cuprous
Cu+
Cupric
Cu++


Ferrous
Fe++
Ferric
Fe+++


Plumbous
Pb++
Plumbic
Pb++++


Mercurous
Hg+
Mercuric
Hg++


Aurous
Au+
Auric
Au+++



 
 
 
 
 
 
 















































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In this compound i.e. [AlCl(H20)5]^-2 ; covalency of Al is 6 not 2 because it is forming bonds with 6 legends i.e. 5 H20 molecules and 1 Cl ion