Question

the atomic number of three elements A B and C are 9 and 13 respectively Which of them will form a cation

Answer 1

According to given Values. 
A electronic configuration - 2,7
B's electronic configuration is - 2 ,8, 3

As A element cannot loos electron ( they won't get octet configuration)
Whereas B loos 3 electrons to become stable. therefore element B Forms a cation.

Hope my answer is Clear.