This makes the group somewhat exceptional. The number proceeds in a linearly increasing fashion for the most part, once on the left of the table, and once on the right see List of oxidation states of the elements , with some irregularities in the transition metals. Elements of the group have one s-electron in the outer electron shell. In history, several sets of group names have been used:  . Groups may also be identified using their topmost element, or have a specific name. The elements in a group have similar physical or chemical characteristics of the outermost electron shells of their atoms i. Disagreements mostly involve elements number 1 and 2 hydrogen and helium , as well as inner transition metals. By periodic table structure Groups 1 Alkali metals 2 Alkaline earth metals 3 4 5 6 7 8 9 10 11 12 13 14 15 Pnictogens 16 Chalcogens 17 Halogens 18 Noble gases.
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