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)
Memory Aid
- PEN = Protons, Electrons, Neutrons — the three subatomic particles.
- For charges: "ProPlus" = Protons are Positive. Electrons are the opposite (negative). Neutrons are Neutral.
- For isotopes: "Same Pro, Different Neu" — Same number of Protons, Different number of Neutrons.
- Atomic model order: "Dalton Then Rutherford Bohr Chadwick": Dalton (solid sphere) → Thomson (plum pudding) → Rutherford (nuclear) → Bohr (shells) → Chadwick (neutron).