The document discusses the periodic table and properties of elements in different groups. It defines valence electrons as electrons in the outermost shell that give elements their chemical properties. Elements in the same group have similar properties because they have the same number of valence electrons. Group 1 elements, known as alkali metals, have 1 valence electron and are extremely reactive, with reactivity increasing down the group. Group 2 elements, alkaline earth metals, have 2 valence electrons and are less reactive than Group 1. Halogens in Group 17 have 7 valence electrons and are very reactive nonmetals. Noble gases in Group 18 have a full outer shell so are stable and form almost no compounds.