The periodic system: periods and groups

In the periodic system elements stand in horizontal rows — periods — and vertical columns — groups. There are 7 periods: periods 1–3 are small (one row each, with 2, 8 and 8 elements), periods 4–7 are large (two rows each; 18, 18, 32 and 32 elements). Every period starts with an alkali metal and ends with a noble gas (the first period goes from H to He). There are 8 groups, marked with Roman numerals, and each is divided into a main and a side subgroup: for example, the main subgroup of Group I holds Li, Na, K, the side subgroup holds Cu, Ag, Au. In main subgroups the number of outer-shell electrons and the highest valency towards oxygen equal the group number (except O and F). Along a period from left to right metallic character weakens, down a main subgroup it strengthens; so the strongest nonmetal is fluorine, and the most active metals are at the lower left (francium, caesium).