Question

How do I know how many occupied shells are in an atom of any element?

Answer 1

This question is a little vague, but the very basic one is the number of the period (look at the element(s) in the periodic table from left to right) tells you how many shells there are and the number of the group (vertically/up and down) is the number of electrons in the outer most shell (or valence electron) with a few exceptions (transition metals or the metals in the middle of the table). Also the atomic number tells you the number of all the electrons the element have.

For example: Mg (magnesium) is in Period 3, Group 2 and has an atomic number of 12. You know that Mg has 3 shells and there are 2 electrons in the outer most shell. The first shell can only hole 2 electrons the next shell can only hold 8 (for all basic elements) and the 3rd shell (which in this case your last shell) can hold another 8 electrons but since we know that it only has a total of 12 electrons we can put the last 2 in there. The electron configuration will be 2-8-2

Answer 2

hold 2 electrons* not hole sorry lol
and to clarify "since we know it only has a total of 12 electrons" I meant Magnesium have 12 electrons only (the atomic number) and not the shells