In every atom we have shells that contain electrons, and these shells are surrounding the nucleus
Each circle around the nucleus represents one shell, and a normal atom will usually have a maximum of 7 shells, but the most common ones will have 1,2,3,4 shells. The last shell it is known as the valence shell, it is the outermost shell of the atom, having valence electrons, core shells and electrons are any shell that is not the last one, for example, Sulphur has 3 shells, we can see that by looking at its electron configuration:
1s2 2s2 2p6 3s2 3p4
The third shell will be the valence one, the last one with 6 electrons (2+4), and the core electrons are present in the core shells, 1 and 2, having a total of 10 electrons (2+2+6).
3s2 will have valence electrons because it is part of the last shell of Sulphur
