Question

the atomic number of 3 Element s a,bandc are 9,10 and 13 respectively which of them will form a caution

Answer 1

The element with atomic number 13 will form cation.

Answer 2

Electronic configuration of:
                                            a=2,7  {It will gain 1 more electron}
                                            b=2,8  {It has completely filled shells}
                                            c=2,8,3 {It will lose 3 electrons}
 
      Now, It is clear that only c can lose electrons.
          So, c will form a cation.