The columns on the periodic table of elements are called groups. The elements in a group share the same configuration of valence electrons, which gives the elements similar chemical properties. The number of valence electrons in a group is sometimes represented with a Roman numeral above the column.
Group IA of the periodic table consists of the alkali metals. These elements have one valence electron which is easily lost, making these elements highly reactive. Group VIIA consists of the halogens, which are highly reactive non-metals. The halogens have seven valence electrons and are especially prone to forming compounds with the alkali metals. Group VIII contains the noble gases which are all relatively inert. These gases have a full outer electron shell, which causes them to be stable and non-reactive.