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Physics Paper 1 · P4 · Lesson 4.2

Atomic number, mass number and isotopes

Read two numbers, then count protons, neutrons and electrons.

Key ideas1 of 3

What do the two numbers mean?

Atomic number is the number of protons. Mass number is protons + neutrons. All atoms of the same element have the same number of protons.

Carbon isotope notation has mass number 12 above atomic number 6. Mass number counts protons plus neutrons; atomic number counts protons.

In this notation the mass number is above the atomic number.

Next: Electrons, ions and energy levels →
Topic resources and sources

Daily 5 · Core exam practice · Higher Tier questions · A4 revision sheet

Lessons 4.1–4.10 cover the core. Lesson 4.11 is Higher Tier only. Daily 5 practises core facts. Answer positions are shuffled on each new attempt; lessons begin with very simple questions.

AQA Combined Science Trilogy specification · 6.4.1.2. Original notes, questions and diagrams. Experimental numbers are illustrative.

Photos: Geiger counter by Boffy b, enhanced by Marco Plassio; caesium source by Wusel007, enhanced by Partynia, both CC BY-SA 3.0. Rutherford at McGill, 1905, unknown photographer / Wellcome Images, public domain. Photos displayed without cropping or other alterations.