The periodic table: organization and families

Dmitri Mendeleev built the first widely accepted periodic table in 1869 by arranging known elements in order of increasing atomic mass and noticing that their properties repeated in a pattern -- so reliable a pattern that he left gaps for elements not yet discovered and successfully predicted their properties before anyone had found them. The modern table instead arranges elements by increasing atomic number (a refinement made possible by Henry Moseley's work in 1913), but the core idea Mendeleev found -- that properties repeat periodically -- is exactly where the table gets its name.

The table's rows are called periods, and moving across a period, electrons are filling the same outer energy level while proton count steadily increases. The table's columns are called groups (or families), and elements in the same group share the same number of valence electrons, which is why they share similar chemical behavior -- group membership predicts behavior far more than being in the same row does.

A jagged staircase line running from boron down to astatine divides the table into metals (to the left, most of the table -- shiny, conduct heat and electricity, malleable, tend to lose electrons), nonmetals (to the right -- dull, poor conductors, brittle if solid, tend to gain electrons), and metalloids (straddling the staircase -- properties in between, useful as semiconductors). Several groups have their own family names worth knowing by heart: group 1 (except hydrogen) is the alkali metals, extremely reactive and never found uncombined in nature; group 2 is the alkaline earth metals, reactive but less so than group 1; group 17 is the halogens, highly reactive nonmetals; and group 18 is the noble gases, essentially unreactive.