Assertion (A) Electronic configuration of boron (Z=5) is [He]2s22p1. So, in first ionisation ( 1IE1 or ∆iH1 ) removal will take place from unpaired p1 -electron. Whereas that of Be will be from paired 2s2 -electrons which requires more energy. Electronic configuration of Be(Z=4):[He]2s2 So, the Assertion is a correct statement. Reason (R)s -orbital is being symmetrical in shape (spherical), it shields nuclear force (nuclear charge) strongly. So, 2p1 -electron of B is experiences lesser nuclear attractive force for ionisation. As a result, IE1 or ∆iH1:B<Be So, the Reason is correct explanation for Assertion.