In the periodic table:
Elements in group 1 and group 2 are metals. 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. Atoms of group 7 elements have seven electrons in their outer shell, and atoms of group 0 elements, except helium, have eight electrons in their outer shell.
The reactions of elements are related to the number of electrons in their outer shells:
See the study guide on groups in the periodic table for more information.