Question

What is the ground state electron configuration of a `""_27 "Co"` atom in the gas phase?

Answer 1

Cobalt is atomic number `27`, which means it has `27` electrons as a neutral atom (protons and electrons cancel out when charge = `0`).

If you looked at the periodic table, it's a `3d` metal, in the fourth period, so the ground-state electron configuration is:

`color(blue)(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^7)`

That adds up to `2+2+6+2+6+2+7 = 27`. It's not `"Cu"` or `"Cr"`, so it's not a strange configuration. It's implied that it's in the gas phase so it's not really necessary to state that.

In this configuration, three `3d` orbitals are singly occupied.

CHALLENGE: Is cobalt paramagnetic or diamagnetic as an atom? How do you know? What is its closest-in-energy first-excited configuration (it cannot promote into the `4s`, and it cannot promote into the `4d` in one step)?