Chemistry - PERIODIC TABLE Question with Solution | TestHub

(A) The electron gain enthalpy ($\Delta H_{eg}$) for an isolated gaseous anion is always positive

For a gaseous anion (X⁻), it requires energy to remove an electron , so $\Delta H$ for the formation of an anion is positive. Correct.

(B) Pauling's scale of electronegativity is a relative scale

Pauling's scale is not absolute; it compares bond energies relative to H.

(C) The number of electrons having $l=2$ in La ($Z=57$) is equal to 21

La ($Z=57$) electronic configuration: $[Xe]5d^{1}6s^2$

d-orbitals: $l=2$

Count: $3d^{10}+4d^{10}+5d^1\to 21$ electrons with $l=2$. Correct.

(D) $I{E}_2(Mg)>I{E}_2(Al)$

Mg has the configuration $1s^{2}2s^{2}2p^{6}3s^2\to I{E}_2$ removes the second $3s$ electron $\to 1450kJ/mol$.

Al: $1s^{2}2s^{2}2p^{6}3s^{2}3p^1\to I{E}_2$ removes a $3s$ electron $\to 1817kJ/mol$.

So $I{E}_2(Al)>I{E}_2(Mg)$. Incorrect.