The Periodic Table

The number of protons in the nucleus — which defines the element and is unique to each one.

A horizontal row. Elements in the same period have the same number of electron shells.

A vertical column. Elements in the same group share the same number of valence electrons and have similar chemical properties.

Group 1 elements (Li, Na, K, Rb, Cs, Fr) — highly reactive metals that react vigorously with water to form hydrogen gas and a metal hydroxide.

Group 18 elements (He, Ne, Ar, Kr, Xe, Rn) — they have full valence electron shells, giving them no tendency to gain or lose electrons.

Group 17 elements (F, Cl, Br, I, At) — highly reactive nonmetals with 7 valence electrons that readily form salts when reacting with metals.

Electronegativity is an atom's tendency to attract electrons. It increases left to right across a period and decreases top to bottom in a group.

Elements in groups 3–12, found in the middle of the periodic table. They have variable oxidation states and often form colorful compounds.

Atomic radius is the size of an atom. It decreases left to right across a period (more protons pull electrons closer) and increases top to bottom in a group (more electron shells).

Dmitri Mendeleev (1869) arranged elements by atomic mass and predicted the existence of undiscovered elements. The modern table, arranged by atomic number, was refined by Henry Moseley.