Knowledge Organiser: Atomic Structure
Key Terms
- Atomic number (Z): Number of protons
- Mass number (A): Protons + neutrons
- Isotope: Same element, different neutron number
- Nucleon: Proton or neutron in nucleus
- Ion: Atom with gained or lost electrons
Key Facts
- Proton: charge +1, mass 1
- Neutron: charge 0, mass 1
- Electron: charge −1, mass ~0
- Neutral atom: protons = electrons
- Nucleus: 1/10,000 of atom diameter
- Atom is 99.9999999999999% empty space
Key Equation
- Neutrons = A − Z
- Example: ²³₁₁Na → 23 − 11 = 12 neutrons
Exam Tips
- Z = bottom number = protons
- A = top number = protons + neutrons
- Isotopes: same Z, different A
- Same chemical properties, different mass
Common Mistakes
- Confusing mass number and atomic number: The mass number (A) is the total of protons + neutrons; the atomic number (Z) is just the number of protons — A is always ≥ Z
- Calculating neutrons incorrectly: Number of neutrons = mass number − atomic number (A − Z) — not the other way round
- Saying isotopes have different chemical properties: Isotopes have the same number of protons (and electrons), so they have identical chemical properties — only their mass and nuclear stability differ
- Confusing electrons and protons in neutral atoms: In a neutral atom, the number of electrons equals the number of protons — ions have gained or lost electrons but the nucleus is unchanged
- Placing electrons in the nucleus: Electrons orbit the nucleus in shells — only protons and neutrons are in the nucleus
Memory Aid
- Particles and charges: "ProTons are PosiTive" — both have the letter T. Neutrons are Neutral (no charge). Electrons are negative (they're on the outside, "ex" means out, and they're negative).
- Atomic notation: "Big number on top = Mass (more letters = top), Small number below = atomic (Z = proton count)." Think: Z is at the bottom of the alphabet, and the atomic number is at the bottom of the symbol.
- Neutrons formula: N = A − Z. Think "Neutrons Are (A) Zapped (minus Z)."