Question

Sol. Key concept: Outermost-shell electrons are those in the highest principal quantum shell.

(a) Na has 3s¹, so its outermost shell has one electron; Sr has 5s², so it has two. The numbers are unequal.

(b) Se has 4s²4p⁴ and Te has 5s²5p⁴ in their outermost shells: six electrons each.

(c) Mn is [Ar]3d⁵4s² and Fe is [Ar]3d⁶4s². Their highest principal quantum number is n = 4, containing two electrons in both atoms. The differing 3d counts belong to an inner shell.

(d) As has 4s²4p³ and Bi has 6s²6p³ in their outermost shells: five electrons each. Therefore (b), (c) and (d) are correct. Do not equate “outermost-shell electrons” with every electron that may participate in transition-metal bonding.