What is the correct ground-state electron configuration of a cobalt atom?
I saw the correct answer is [Ar] 3d7 4s2. I find this confusing. Cobalt is in period 4. So why is it 4S, but 3d? I feel I must have forgotten something. Like for Cl, if we really named all the shells, it would be 1s2 2s2 2p6 3s2 3p5. All the numbers match. I though Cobalt would be the same, so that it would be [Ar] 4d7 4s2. What makes it a 3 and not a 4, if we are in period 4?
The Correct Answer and Explanation is :
You’re right to be confused, but I can help clarify the situation. Let’s break it down!
The electron configuration of elements follows a specific order based on the increasing energy levels of the atomic orbitals. The confusion comes from the fact that the 3d orbital is actually lower in energy than the 4s orbital, despite the fact that cobalt (Co) is in the 4th period of the periodic table.
Why is it [Ar] 3d7 4s2 for Cobalt?
- Period and Orbitals:
Cobalt is indeed in period 4, and you’d expect the electron configuration to start from the 4th period’s orbitals. However, it’s not just the principal quantum number (n) that matters when determining electron placement; we also need to consider the energy levels of the orbitals. The 4s orbital is higher in energy than the 3d orbital in its unoccupied state, which is why electrons fill the 4s orbital first when the atom is in its ground state. - Electron Filling:
When cobalt’s electrons are added, the 4s orbital is filled first because it is initially lower in energy. Once the 4s orbital is filled, the 3d orbitals start to fill. So, cobalt has a configuration of [Ar] 3d7 4s2, where 7 electrons are in the 3d orbital, and 2 electrons are in the 4s orbital.
Why is 3d, not 4d?
- Energy Levels and Quantum Numbers:
The key point is that the 3d orbitals are actually lower in energy than the 4s and 4p orbitals, and as a result, the 3d orbitals get filled before the 4d orbitals. The 4d orbitals only come into play in elements that are further along in the periodic table, specifically those in period 5 and beyond (starting with Yttrium, Y).
Example Comparison with Chlorine (Cl):
For chlorine, the electron configuration is indeed 1s² 2s² 2p⁶ 3s² 3p⁵ because it’s in period 3, and the numbers correspond to the energy levels for each orbital type, as you correctly mentioned. So, the period number directly correlates to the principal quantum number (n) for the outermost orbitals, which is why Cl’s configuration corresponds directly to its period.
Summary:
Cobalt’s electron configuration is [Ar] 3d7 4s2 because the 3d orbitals are lower in energy than the 4s, even though it is in period 4. The energy ordering of orbitals plays a more significant role than the period number itself when determining electron configuration.