What is the atomic number of an element?

Atomic number = number of protons in the nucleus; uniquely identifies the element. Carbon has 6 protons (atomic number 6); oxygen has 8; iron has 26. The periodic table is organized by atomic number, increasing left-to-right and top-to-bottom. Atoms also contain: (1) Neutrons — in the nucleus, no charge, similar mass to protons; mass number = protons + neutrons; isotopes are atoms of the same element with different neutron counts; (2) Electrons — orbit the nucleus in shells; negative charge; mass much less than protons/neutrons; in a neutral atom, electrons = protons. Periodic table organization: Periods (rows) — elements have same number of electron shells. Groups (columns) — elements have similar chemical properties due to same number of valence (outer-shell) electrons. Group 1: alkali metals (highly reactive). Group 2: alkaline earth metals. Groups 3-12: transition metals. Group 17: halogens (highly reactive nonmetals). Group 18: noble gases (very unreactive). Metals on left side; nonmetals upper right; metalloids on the diagonal between.