Element groups are categories of elements in the periodic table that share similar chemical properties. These groups are often referred to as families, and they include elements that have the same number of electrons in their outer shell. For example, the alkali metals group includes elements like sodium and potassium, which are highly reactive and have one electron in their outer shell. Each group is numbered from 1 to 18, with specific names for some, such as the noble gases in group 18, which are known for their lack of reactivity due to having a full outer electron shell. Understanding element groups helps scientists predict how different elements will behave in chemical reactions.