Every element has two key numbers:
- Atomic number (Z) — The number of protons in the nucleus. This is what makes each element unique! All carbon atoms have 6 protons. All oxygen atoms have 8 protons.
- Mass number (A) — The total number of protons + neutrons. This tells you the mass of the atom.
The crucial formula:
Neutrons = Mass Number − Atomic Number
Example — Sodium (Na):
- Atomic number = 11 → 11 protons
- Mass number = 23 → 23 nucleons total
- Neutrons = 23 − 11 = 12 neutrons
- Since it's neutral: 11 electrons (same as protons)
This key facts covers Atomic Number and Mass Number within Atomic Structure for GCSE Chemistry. Revise Atomic Structure in Atomic Structure for GCSE Chemistry with 28 exam-style questions and 22 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 6 of 14 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Atomic Structure
What does the atomic number of an element tell you?
Explain what is meant by the relative atomic mass of an element and how it is calculated from isotopic data. [3 marks]
Quick recall flashcards
What is the mass number?
The total number of protons + neutrons in an atom
What are nucleons?
Particles in the nucleus — protons and neutrons together