Mendeleev · Science

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.

Understand it before converting

Mass is not just a classroom quantity: it appears constantly in cooking, laboratories, transport, industry and science. Units are chosen for practical reasons. Some are convenient in everyday life, while others dominate scientific or technical work. This page deliberately focuses on familiar units — milligrams, grams, kilograms, tonnes, ounces and pounds — instead of presenting a long catalogue of rarely used formats. Every pair links to a dedicated conversion page with the calculation rule, worked examples and useful reference values. The aim is to get the answer quickly while still understanding what is being converted and whether the result makes physical sense.

Mass