RefTables

The atomic number of an element is the number of protons in the nucleus of an atom of that element. It uniquely identifies the element - hydrogen always has 1 proton, carbon always has 6, and gold always has 79. No two elements share an atomic number.

Atomic mass (or atomic weight) is the average mass of all atoms of an element, weighted by natural isotope abundance, expressed in atomic mass units (u or Da). Carbon (C) is defined as exactly 12 u, which is the reference standard.

Metals (like iron, copper, gold) are shiny, conductive, and malleable. Nonmetals (like oxygen, nitrogen, sulfur) are poor conductors and tend to be brittle as solids. Metalloids (like silicon, germanium, arsenic) have properties between metals and nonmetals and are key to semiconductor technology.

Noble gases (helium, neon, argon, krypton, xenon, radon) are group 18 elements with full outer electron shells. This makes them extremely stable and unreactive. They are used in lighting (neon signs), insulation, and as inert atmospheres in scientific applications.