How many electrons can the fourth energy level hold according to Niels Bohr?

The fourth energy level has 18 electrons.

The fourth energy level of the periodic table includes the 4s 3d and 4p orbitals.

The 4s orbital holds 2 electrons
The 3d orbital holds 10 electrons
and
The 4p orbital holds 6 electrons.

There is a 4d orbital with 10 electrons which coincides with the 5th energy level of the periodic table.

There is also a 4f orbital with 14 electrons, but this coincides with the 6th energy level of the periodic table.

I hope this was helpful.
SMARTERTEACHER

(This was originally a comment.)

@Brian M. wrote that it was `18` electrons, but here are my reasons for it being `32` electrons instead.

Method 1:
The expression that we can use to calculate the maximum number of electrons given energy level is `2n^2`, with `n` being the energy level.
The fourth energy level, therefore, should have `2*(4)^2 = 2*16 = 32` electrons.

Method 2:
The fourth energy level doesn't necessarily equate to the fourth period in the periodic table.

The maximum number of electrons to be gained in the fourth period is `18`, because we have:

• `2` electrons from the `4s` orbital series;
• `10` electrons from `3d` orbital series; and
• `6` electrons from `4p` orbital series.

`2 + 10 + 6 = 18` electrons.

However, this doesn't include other fourth energy level orbitals, like `4d` and `4f`. It also includes the `3d` orbitals, which is in the third energy level.

The fourth energy level, however, is composed of the `4s, 4d, 4p,` and `4f` orbital series—the `4` in front means that it's in the fourth energy level.

That would give us:

• `2` electrons from the `4s` orbital series;
• `6` electrons from `4p` orbital series;
• `10` electrons from `4d` orbital series; and
• `14` electrons from `4f` orbital series.

`2 + 6 + 10 + 14 = 32` electrons.