Since the element is in 17th group it will have 7 valence electrons and since it has 3 shells (and the valence shell has 7 electrons in it) it will have the other two shells filled completely. Hence, according to the octet rule the electronic configuration would be 2,8,7. This implies that the atomic number of this element would be 2+8+7 = 17.( Hence, the element is chlorine.) The subshell configuration (based on Aufbau's Principle) 1s1 2s2 2p6 3s2 3p5
Since it has 3 shells it will be in the 3rd Period.
Y's subshell configuration - 1s2 2s2 2p6 3s2 3p1
Valency of Y: 3 (It will form cation)
Valency of X: 1 (it will form anion)
Therefore the compound formed would be YX3
