Question

what is the difference between valency and oxidation number.

Answer 1

Valence is the older of the two concepts, and refers to the number of bonds that an element can form. That seems simple enough, but as chemists continued to study compounds, complications arose. For example, does the nitrogen in NH4Cl have valency 3 (the usual valence of nitrogen), 4 (the number of covalent bonds in NH4+), or even 5 (adding in the ionic bond)? It became clear that more precise concepts were needed.

Coordination number is the preferred term for the number of neighboring (usually taken to be bonded) atoms. In NH4Cl, nitrogen has coordination number 4, and in [Cr(NH3)6]Cl3, chromium has coordination number 6.

Oxidation state (aka oxidation number) is a signed number which shows the charge on the atom given a certain assignment of electrons to atoms. The rules for assigning the electrons to an atom are a little complicated, but the basic idea is to always assign the electrons of a bond to the more electronegative atom if the atoms are different, and to split the electrons of a bond 50:50 if the atoms are the same. In NH4Cl, nitrogen has oxidation number -3, and in [Cr(NH3)6]Cl3, chromium has oxidation number +3.

By the way, it is not true that valency is constant for a given element. For example, we distinguish divalent, trivalent, and hexavalent chromium according to the oxidation state it has in a given compound.

One more caveat: sometimes neither coordination number nor oxidation state corresponds to valence as traditionally defined. Example: in CH2O (formaldehyde), carbon has coordination number 3 and oxidation state 0, but is still tetravalent because it still forms 2 single bonds and 1 double bond.

Answer 2

$\huge\mathcal\purple{Valency}$

1. It is the combining capacity of element. No plus or minus sign is attached to it.
2. Valency of an element is usually fixed.
3. Valency is always whole number.
4. Valency of an element is never zero except in noble gas.

$\huge\mathcal\purple{Oxidation\: Number}$

1. Oxidation number is the charge present on the atom of the element. It may be plus or minus sign.
2. Oxidation number of an element may have different values.
3. Oxidation number of the element may be a whole number or fractional.
4. Oxidation number of an element may be zero.

For example :

In compounds of carbon, the Oxidation number varies from -4 to +4 but the valency of carbon is always 4