Helium and element ‘X’ have outer electronic configuration of \(ns^2\). Identify the element ‘X’.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Be
Helium has the configuration 1s2, so its outer shell is ns2 with n = 1. An element with the same outer configuration ns2 must have its last two electrons in an s orbital and nothing after them, which is the configuration of the group 2 (alkaline earth) elements.
Writing the configurations of the four options:
Na (Z = 11): [Ne] 3s1 — outer shell ns1.
B (Z = 5): [He] 2s2 2p1 — outer shell ns2np1.
Be (Z = 4): [He] 2s2 — outer shell ns2.
Ne (Z = 10): [He] 2s2 2p6 — outer shell ns2np6.
Only beryllium has the outer configuration ns2.