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 25 exam-style questions and 22 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 6 of 13 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 6 of 13
Practice
25 questions
Recall
22 flashcards
🔢 Atomic Number and Mass Number
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)