Isotopes

Definition

Variants of an element that share the same proton count but differ in neutron count, giving them identical chemistry but different mass.

Because most elements occur as a stable mixture of isotopes in predictable proportions, their averaged mass is effectively fixed for practical work. Carbon-12, with six protons and six neutrons, is the reference standard for the atomic mass unit. Atomic weight is the abundance-weighted average across an element’s isotopes.

Related concepts

Neutron · Atomic mass and atomic weight · Atomic mass unit (u)

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Part of The Blueprint of Water · 🧪 chemistry