How the periodic table developed, and its chemical families
In 1869 Dmitri Mendeleev arranged the known elements by atomic mass and grouped elements with similar chemical and physical properties together, leaving gaps for elements not yet discovered and correctly predicting their properties, as later confirmed for gallium and germanium. In 1913 Henry Moseley showed that ordering elements by atomic number, the number of protons, fixes the few places where ordering by mass did not match the properties. Main-group elements in the same column generally share the same number of valence electrons (helium, with 2 instead of the other noble gases' 8, is the notable exception), which is why they form chemical families: the alkali metals (group 1, one valence electron, very reactive metals), the alkaline earth metals (group 2), the halogens (group 17, seven valence electrons, very reactive nonmetals), the noble gases (group 18, full outer shells, very unreactive), and the transition metals in the middle of the table, which share metallic properties and often form ions with more than one possible charge.
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