Elements in the same group of the periodic table have similar properties because their electronic configurations have the same number of electrons in the outermost shell. Elemental behavior is almost completely reliant on this outermost shell configuration, with inner shells playing a less important role in determining properties.
Elements with three electrons or fewer in their outermost shells tend to be metals, while those with five or more tend to be nonmetals. Those with exactly four electrons in the outermost shell behave as semimetals. Metals are electropositive, tending to lose their outermost electrons and become positive ions when they undergo chemical reactions. Nonmetals are electronegative, tending to gain additional electrons and become negative ions.
The fewer the electrons an element needs to lose or gain to empty or fill its outermost shell and reach a noble configuration, the more reactive the element is. This makes elements at the very left and right of the periodic table the most reactive, excluding the noble gases. The elements at the very left of the periodic table belong to group IA and are called alkali metals, while those at the very right of the periodic table belong to group VII and are called halogens.