For elements in the first three periods of the periodic table (hydrogen - argon), the orbitals can simply be filled up by increasing energy energy level. 

A difficulty arises when we come to potassium. Potassium has an atomic number of 19.

The first 18 electrons can occupy the 1s, 2s, 2p, 3s and 2p orbitals: 1s² 2s² 2p⁶ 3s² 3p⁶. Where does the final electron go?

It would seem to make perfect sense to place the electron within the 3d orbitals. However, we find that the last electron enters the 4s orbital giving the overall electrons configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. This is very surprising since we assume the 4th energy level is higher in energy that the 3rd energy level. However, reality is more complex. 

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For potassium and calcium, the 4s orbital dips in energy below the 3d orbitals, and so electrons go into the 4s orbital before occupying the 3d orbitals. 

Consequently, potassium and calcium have the following electon configurations.

Potassium: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

Calcium: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

Writing out all the orbitals can be a bit laborious and so a frequent shorthand way is to write the electron configurations of potasisum and calcium as: [Ar] 4s¹ and [Ar] 4s² respectively, where [Ar] represents the electron configuration of argon.