This topic summary covers Knowledge Organiser: Atomic Structure 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 14 of 14 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: Atomic Structure
Key Terms
- Atom: Smallest particle of an element
- Atomic number (Z): Number of protons
- Mass number (A): Protons + neutrons
- Isotope: Same protons, different neutrons
- Nucleon: Proton or neutron in the nucleus
Must-Know Facts
- Protons: +1 charge, mass 1, in nucleus
- Neutrons: 0 charge, mass 1, in nucleus
- Electrons: −1 charge, mass ~0, in shells
- Neutrons = mass number − atomic number
- Neutral atom: protons = electrons
- Thomson → plum pudding; Rutherford → nuclear; Bohr → shells
- Isotopes: same chemical properties, different masses
Key Equations
- Neutrons = mass number − atomic number
- Ar = Σ(isotope mass × abundance) ÷ 100
- Atomic number = number of protons (= electrons in neutral atom)
Common Mistakes
- Swapping atomic number and mass number: Atomic number is always the smaller number (protons only)
- Saying electrons have mass: Electron mass is negligible — only protons and neutrons count towards mass number
- Isotopes have different chemical properties: Chemical properties depend on electrons (= protons), not neutrons — isotopes react identically
- Relative atomic mass is always a whole number: It is a weighted average, so it can be a decimal (e.g. Cl = 35.5)
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