atomic weight
Summary
Atomic weight is the average mass of an element's atoms as found in nature, weighted by isotope abundance, in atomic mass units (u). It is the periodic table value and equals molar mass in g/mol. Testable point: it is a weighted average, so it lies closer to the more abundant isotope's mass.
Detail
Meaning: Atomic weight is the average mass of an atom of an element as found in nature, weighted by the abundance of each isotope. It is given in atomic mass units (u), where 1 u is 1/12 the mass of a carbon-12 atom. It is the periodic table value, so it is rarely a whole number, and it equals the molar mass in g/mol.
Equation: Atomic weight = Σ (fractional abundance × isotope mass). Abundances are unitless fractions that sum to 1; isotope mass is in u (1 u ≈ 1.66 × 10⁻²⁷ kg).
On the MCAT: Passages give isotope masses and abundances and ask for the atomic weight, or give the atomic weight and ask which isotope is more abundant. Chlorine, about 75% Cl-35 and 25% Cl-37, averages about 35.5 u. The average lies closer to the more abundant isotope. Atomic weight also converts grams to moles in stoichiometry.
Common confusion: Atomic weight is not mass number. Mass number is the whole-number count of protons plus neutrons in one isotope, so no single chlorine atom has a mass of 35.5 u. It is also not atomic number, which counts protons.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- General Chemistry (Petrucci)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.