[missUK11]

Can someone give me a good definition of what valance expansion is please ?

Many thanks

[gelminil]

You might be thinking of octet expansion.  This is where the species has greater than an octet around it (i.e. PF5).  Involves use of d-orbitals (low energy, empty ones).

Could also be increasing the number of bonds.  For instance, PF3 + F2 makes PF5 so the P undergoes valence expansion?