Atomic structure and the periodic table

Inside an atom

Protons (positive) and neutrons (neutral) sit in the nucleus; electrons (negative) move around it in shells. An atom has equal numbers of protons and electrons, so it is neutral overall. The atomic number is the number of protons; the mass number is protons plus neutrons.

Electron shells

The first shell holds up to $2$ electrons, the next two hold up to $8$ each at this level. Sodium ($11$ electrons) is $2,8,1$. The number of electrons in the outer shell decides how an element reacts — that single idea is why the periodic table has the shape it does.

Groups and periods

A group is a column: all its elements have the same number of outer electrons and so behave alike. A period is a row: the number of shells stays the same across it. Group 1 metals are very reactive; Group 7 are reactive non-metals; Group 8 (noble gases) have full outer shells and hardly react at all.