(i) Which of the following two elements have diagonal relationship? Li, Be, Al and Si.
(ii) Between `""_(29)Cu and ""_(19)K` which one has higher ionisation enthalpy and why?

Explanation

(i) Be and Al.
(ii) Electronic configuration of `""_(19)K:1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(1)`
Electronic configuration of `""_(29)Cu:`
`1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)3d^(10)4s^(1)`.
Nucleus of Cu-atom contains 10 more protons than that of K-atom. additional nuclear pull on outermost electron `(4s^(1))` In cu-atom is not counter-balanced by the shielding effect of ten 3d-electrons because d. orbitals have poor screening effect. thus the effective nuclear charge acting on the 4s-electron of copper is greater than that acting on the 4s-electron of potassium So Cu has greater ionisation enthalpy than that of K.