A trivalent anion, X³⁻ , is isoelectronic with a noble gas. If this noble gas contains exactly 18 p-electrons, the total number of s-electrons present in the ground state of the neutral atom X is

Step-by-step solution

First, identify the noble gas. The p-subshells are filled in the order 2p , 3p , 4p , etc. Each fully filled p-subshell holds 6 electrons. For the noble gas to have exactly 18 p-electrons, it must have fully filled 2p , 3p , and 4p subshells ( 6 + 6 + 6 = 18 ). The electronic configuration of this noble gas is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ , which corresponds to Krypton ( Z = 36 ). The trivalent anion X³⁻ is isoelectronic with Krypton, meaning X³⁻ has 36 electrons. The neutral atom X must therefore have 36 - 3 =