Question

The number of electrons in the outermost shell of a phosphorus is 5 then find what is its valanc–e and why?

Answer 1

valency of phosphorus can be both 3 and 5, it depends upon the molecule

. P has a lone pair in s orbital and 3 unpaired electron in p orbital of valence shell so generally it form 3 bonds with other atoms , hence valency 3 like PH3

. If more electronegative atom is attached with P like florine and chlorine then partial positive charge develops on P and thus energy gap between s orbital and vacant d orbital decrease. So one electron from lone pair in s orbital excite to d orbital and now there are 5 unpaired electrons. So P is able to form 5 bonds, thus valency become 5 in this case. Eg: PF5 , PCl5

Example:

Take hydride phosphene (PH3)

Valency of H = 1

Criss cross the subscript numbers

P          H

3           1

From this we can observe that valency of phosphorus is 3