chem

The number of dots around the symbol is the number of valence electrons in the atom.

Do one for Flouride?

For HCN, the group number of C (group 4A) is lower than that of N (group 5A), making C the central atom. The bonding arrangements that will give each atom an octet are:

HCN contains an H atom (with one valence electron) a C atom (with four valence electrons) and an N atom (with five valence electrons) for a total 1 4 5=10 valence electrons. So only the structure with 10 electrons is valid.