How to find core electrons of an element?

Hey I'm not sure on how to calculate the number of core electrons of each element. I need to know this in order to calculate effective nuclear charge. I know how to find the total number of electrons just not the core ones. I know that core electrons=total electrons-valence. But how do I find valence electrons? Thanks.

Obviously atomic number = no of protons = total number of electrons in an atom.
Sodium 2,8,1 (1s1 2s2 2p6 3s1) has 13 electrons and is a group 1 element so has 1 valence electron (1 electron in outer shell). So 12 core and 1 valence
Carbon 2,4 (1s1 2s2 2p2) has 6 electrons, is in group 4 and a valency of 4. So it has 2 core and 4 valence electrons
Chlorine 2,8,7 (1s1 2s2 2p6 3s2 3p5) has 17 electrons, in in group 7 but has a valency of 1. However it is classed as having 10 core electrons and 7 valence electrons.
More complex ones like cobalt has an arrangement of 2,8,15,2 or 1s2 2s2 2p6 3s2 3p6 4s2 3d7. When cobalt reacts the 4s electrons and the 3d electrons are involved. So there are 9 valence electrons and 18 core electrons
Scandium has an arrangement of 2,8,9,2 or 1s2 2s2 2p6 3s2 3p6 4s2 3d1. When scandium reacts the 4s electrons and the 3d electrons are involved. So there are 3 valence electrons and 18 core electrons

Valence electrons are involved in bonding, core electrons are not.

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