The diagram above shows each shell as a circle around the nucleus, ... The number of electron shells of all the elements in a period is the same as the period number. Figure caption, ...

Atoms of group 1 elements have one electron in their outer shell, and atoms of group 2 elements have two electrons in their outer shell. Elements in groups 6, 7 and 0 are non-metals.

The outermost shell contains 1 electron, which makes sodium highly reactive and likely to form chemical bonds. Understanding the atomic structure helps explain why elements have different ...