Reading the Periodic Table for Mass

The number under an element symbol is its standard atomic weight — a weighted average of naturally occurring isotopes. Chlorine’s ~35.45 reflects roughly three parts chlorine-35 to one part chlorine-37.

For classroom molar mass work, use the values your instructor specifies. For research-grade work, cite IUPAC standard atomic weights and note when a sample’s isotopic composition differs from terrestrial averages.

Diatomic elements (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂) require doubling the atomic mass for the molar mass of the elemental gas or liquid as it exists under standard conditions.