Question

An atom contains a total of 25 electrons. When the atom is in the ground state, how many different principal energy levels will contain electrons?

Answer 1

Well, with `25` electrons, whatever atom it is, it need not be neutral... but since it's within the neighborhood of a transition metal, we can assume it's one of them... i.e. it may be `"Mn"`, or perhaps `"Co"^(2+)`.

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With `25` electrons, we can immediately write the electron configuration. Argon has `18` electrons and is the previous noble gas, and thus we use it as the noble gas core.

`1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^2 -> [Ar] 3d^5 4s^2`

And you can convince yourself that `18 + 5 + 2 = 25` electrons total.

This lands us in the first-row transition metals, and so we have involved `bb(n = 1, 2, 3, )` and `bb(n = 4)`. Those are the only principal energy levels involved... These contain the following orbitals:

`n = 1`: `1s`
`n = 2`: `2s, 2p`
`n = 3`: `3s, 3p, 3d`
`n = 4`: `4s`

We ignore the `4p, 4d, 4f` obviously because they are empty.