Octet Rule

The electron configurations for these four elements and their most stable ions are as follows:

Chlorine (atomic number 17): $1s^2 2s^2 2p^6 3s^2 3p^5$

$\ce{Cl^-}$ : $1s^2 2s^2 2p^6 3s^2 3p^6$

Fluorine (atomic number 9): $1s^2 2s^2 2p^5$

$\ce{F^-}$ : $1s^2 2s^2 2p^6$

Sodium (atomic number 11): $1s^2 2s^2 2p^6 3s^1$

$\ce{Na^+}$ : $1s^2 2s^2 2p^6$

Potassium (atomic number 19): $1s^2 2s^2 2p^6 3s^2 3p^6 4s^1$

$\ce{K^+}$ : $1s^2 2s^2 2p^6 3s^2 3p^6$

Chloride and potassium ions have the same electron configuration as argon (atomic number 18).

Fluoride and sodium ions have the same electron configuration as neon (atomic number 10).