What is a group on the periodic table?

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A group on the periodic table is a vertical column of elements, and there are 18 groups numbered 1 to 18 in the modern periodic table. Elements in the same group, also called a family in many textbooks, have the same number of valence electrons in their outer shell (for the main groups), so they behave in similar chemical ways: group 1 contains the alkali metals, group 2 the alkaline earth metals, group 17 the halogens and group 18 the noble gases. Moving down a group adds an electron shell, so atomic radius grows and metals become more reactive, while nonmetals become less reactive. Groups 3 to 12 hold the transition metals, and the lanthanides and actinides sit below the table without group numbers in most layouts. Older tables use roman numerals with letters, such as IA to VIIIA, but the International Union of Pure and Applied Chemistry (IUPAC) recommends the 1 to 18 system. Group numbers help you predict ionic charges: group 1 elements form +1 ions, group 2 elements form +2 ions, group 17 elements form −1 ions and group 16 elements usually form −2 ions. Elements in a group also share trends in physical properties, such as the melting point of the alkali metals falling from lithium to cesium and the boiling point of the halogens rising from fluorine to iodine. Group names are widely used: group 1 is the alkali metals, group 2 the alkaline earth metals, group 17 the halogens and group 18 the noble gases, while groups 3 to 12 are often called the transition metals, and some books name group 13 the boron group, group 14 the carbon group, group 15 the pnictogens and group 16 the chalcogens. To find a group, count columns from the left or use the number printed at the top of the column; for example, calcium is in group 2, iron in group 8 and chlorine in group 17. Groups differ from periods, which are horizontal rows.