1.2.2—Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same electron arrangement and therefore the same chemical properties, while their different masses can give different physical properties.

Convert percentage abundances to fractions, multiply each isotope mass by its fractional abundance, then add the contributions. The result is a weighted mean, not an unweighted average.

For a two-isotope sample containing 75% mass 35 and 25% mass 37, the weighted mean is (35 × 0.75) + (37 × 0.25) = 35.5. An answer between the isotope masses is a useful check; simply averaging 35 and 37 would ignore abundance.

Read isotope identity from the pair of nuclear symbols: ³⁵₁₇Cl and ³⁷₁₇Cl are both chlorine because Z = 17 in each, but they contain 18 and 20 neutrons respectively because A differs. Mass number belongs to one nuclide; relative atomic mass is the abundance-weighted mean for a sample.