Given, Atomic number of element B=Z ( ∵ noble gas ∴ belong to zero group) Atomic number of element A=Z−1 (ie, halogens) Atomic number of element C=Z+1 (ie, group IA) Atomic number of element D=Z+2 (ie, group IIA) ∵ Element B is a noble gas ∴ Element A must be a halogen ie, have highest electron affinity. and element C must be an alkali metal and exist in +1 oxidation state. and element D must be an alkaline earth metal with +2 oxidation state.