What is the correct ground state electron configuration of the cobalt(II) ion, Co2+, written using noble gas core notation?

Introduction / Context:
This question tests your understanding of electron configurations of transition metal ions. Transition metals often lose electrons from the 4s orbital before the 3d orbital when forming cations. Recognising this pattern is crucial for writing correct electron configurations, predicting magnetic behaviour and understanding coordination chemistry. Here, you are asked to write the configuration of the cobalt(II) ion, Co2+, in noble gas core notation.

Given Data / Assumptions:
- The element is cobalt (Co) with atomic number 27.
- We must write the configuration for the Co2+ ion, which has lost two electrons compared with the neutral atom.
- We express the answer in noble gas core notation using [Ar] for the core, since argon has atomic number 18.
- We use the rule that 4s electrons are removed before 3d electrons when forming transition metal cations.

Concept / Approach:
First, we determine the electron configuration of the neutral cobalt atom. Then we remove two electrons to obtain Co2+. For neutral Co (Z = 27), the configuration is [Ar] 3d7 4s2, because electrons fill 4s before 3d but 3d actually lies slightly lower in energy once filled. When cobalt forms the 2+ ion, it loses two electrons from the 4s orbital first, not from 3d. Therefore, Co2+ has configuration [Ar] 3d7 with no electrons remaining in the 4s orbital.

Step-by-Step Solution:
Step 1: Write the ground state electron configuration of neutral cobalt. Starting from [Ar] (18 electrons), we add 9 more electrons: 4s2 and 3d7. So Co: [Ar] 3d7 4s2. Step 2: Identify that we need the configuration of Co2+, which has 2 fewer electrons than neutral cobalt. Step 3: Apply the rule for transition metal cations: electrons are removed first from the 4s subshell, then from 3d. Step 4: Remove the two 4s electrons from [Ar] 3d7 4s2. This leaves [Ar] 3d7 as the configuration for Co2+. Step 5: Confirm that we have accounted for all 25 electrons remaining (27 - 2 = 25) in the configuration [Ar] 3d7.

Verification / Alternative check:
To verify, count electrons explicitly. [Ar] represents 18 electrons. The 3d7 subshell adds 7 electrons, giving a total of 25 electrons for Co2+, which matches 27 - 2. If we had chosen [Ar] 3d6, that would represent only 24 electrons and thus be suitable for Co3+, not Co2+. Options that still include 4s electrons, such as [Ar] 3d5 4s2 or [Ar] 3d7 4s1, would correspond to different electron counts and would contradict the rule that 4s electrons are removed first when forming cations.

Why Other Options Are Wrong:
[Ar] 3d6: This configuration corresponds to cobalt(III), Co3+, where three electrons have been removed, not two.
[Ar] 3d5 4s2: This still has 7 valence electrons beyond argon (5 in 3d and 2 in 4s), giving 25 electrons, but it is not consistent with actual energy ordering and removal rules; Co2+ does not retain 4s2 in its ground state.

Common Pitfalls:
Students often forget that when transition metals form cations, 4s electrons are removed before 3d electrons, because 4s becomes higher in energy in the presence of a filled 3d subshell. Another common mistake is miscounting the total number of electrons after ion formation. To avoid these errors, always write the neutral atom configuration carefully, then explicitly remove electrons from the 4s subshell first, and finally recount to confirm the correct number of electrons.

Final Answer:
The correct ground state electron configuration of the cobalt(II) ion, Co2+, is [Ar] 3d7.