What is the atomic core of potassium ion?

The nucleus is an electron, but in the outermost shell (= valence electron).

The atomic number (Z) of potassium is 19.

Therefore its electron distribution is 1s2 2s2 2p6 3s2 3p6 4s

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

The potassium ion, K+, has lost one of its outermost electrons, so it has the same electrons as the nucleus of K.