Question

Write down the formula of the compound formed by the combination of an element A of atomic number 12 with an element B of atomic number 17

Answer 1

A = 2,8,2
B = 2,8,7

The compound cannot be formed as valency of A is +2 and valency of 7 is -1

Answer 2

The element with atomic number 12 is Magnesium and the element with atomic number 17 is Chlorine

So the compound formed will be MgCl2... Two Chlorine atoms are joined because the valency of Mg is 2 and Cl needs only 1 electron to complete its octect....
Hope this helps you....