Write the atomic cores for potassium and chloride ions

This question is part of a bigger question.

The target is to determine the Lewis structer for the ionic compound KCl.

The atomic cores are the electrons but the outermost shell (i.e. valence electrons).

Potassium has atomic number (Z) 19.

So, its electron distribution is 1s2 2s2 2p6 3s2 3p6 4s

The valence electron is 4s, so the atomic core is 1s2 2s2 2p6 3s2 3p6.

Potassium ion, K+, has lost one electron, the outermost electron, so it has the same electrons as the atomic core of K.

Chlorine has atomic number 17.

The electron distribution of chlorine is 1s2 2s2 2p6 3s2 3p5.

The valence eletrons are 3s2 3p5, so the atomic core of chlorine is 1s2 2s2 2p6.

The ion Cl- has gained one electron so its electron configuration is 1s2 2s2 2p6 3s2 3p6.

To represent the Lewis structure you only use the valence electrons, so the representation is:

             **
K(+)  +*Cl(-) **
             **

Where the * marks represent the 7 valence electrons of chloride atom and the + mark represent the electron gained by chloride and released by potassium.

So, the ion K(+) has lost its only valence electron and the ion Cl(-) has increased its valence electrons to 8. In this way both have the stable configuration of a noble gas.