While writing the subshell electronic configuration of elements with higher atomic number, the symbol of the noble gas preceding that element may be written in square brackets followed by the electronic configuration of the remaining subshells.

For eg, potassium(K) has the electronic configuration
1S² 2S² 2P⁶ 3S² 3P⁶ 4S¹

The preceding noble gas Argon has the electronic configuration
1S² 2S² 2P⁶ 3S² 3P⁶

So we can write the configuration of potassium using the symbol argon as [Ar]4S¹ For sodium it will be [Ne] 3S¹