Why elements No. 20 and No. 21 are located in the same period in different groups and subgroups

Elements 20 and 21 (calcium and scandium) are in one (fourth) period of the periodic table, because they both have four energy levels along which electrons revolve around the nucleus.

Calcium is in group II because it has two electrons on the external energy level. The element is in subgroup A, because all previous levels are already filled with electrons.

Scandium is a Group III element because it has three electrons on the outer energy level. The element belongs to subgroup B, because it begins to fill with electrons d – the sublevel of the previous third energy level. This is a d-element.