Chemistry 11 · Chemistry · Lesson 7.2
Atomic number, mass and isotopes
- Level
- Grade 11
- Reading time
- 13 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Protons, neutrons, electrons, isotopes and average atomic mass.
What are isotopes?
Short answer
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, so they share an atomic number but differ in mass number. Because chemical behaviour is set by the electrons, isotopes of an element react almost identically. The atomic mass on the periodic table is the weighted average of an element’s naturally occurring isotopes.
Concept preview
What you will be able to do
- 01Relate atomic number and atomic mass to protons, neutrons and electrons
- 02Define and identify isotopes
- 03Calculate an average atomic mass from isotope abundances
Elements are told apart by the number of protons in the nucleus. The atomic number is defined as the number of protons in an atom's nucleus, which is also the charge on the nucleus. Every hydrogen atom has 1 proton, every helium atom 2, every lithium atom 3 — so the atomic number is the identity of the element. Any atom with 11 protons must be sodium, no matter what else is true of it.
School Plan
Atomic number, mass and isotopes
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Related topics
Frequently asked questions
- Do isotopes of an element react differently?
- No, they react almost identically, because chemistry depends on electrons and isotopes have the same electron arrangement.
- Why is atomic mass usually not a whole number?
- It is the weighted average of all naturally occurring isotopes, based on their masses and relative abundances.