Isotopes and average atomic mass
An element is defined by its proton count, not its neutron count. Two isotopes of the same element therefore share an atomic number but have different masses.
Core idea
An element is defined by its proton count, not its neutron count. Two isotopes of the same element therefore share an atomic number but have different masses.
A concrete example
Natural chlorine contains mainly two isotopes. The periodic-table mass is an abundance-weighted average, not the mass of an imaginary chlorine atom.
Why it matters
This concept connects symbolic scientific language with what actually happens at atomic, molecular or experimental scales. Before applying a formula, always check the quantities, units and assumptions of the model.
Check it yourself
Restate the idea without technical vocabulary, then repeat the reasoning with a simple value. If your explanation and calculation tell different stories, revisit the assumptions.
Natural chlorine contains mainly two isotopes. The periodic-table mass is an abundance-weighted average, not the mass of an imaginary chlorine atom.