This topic summary covers Knowledge Organiser: Atomic Structure within Atomic Structure for GCSE Physics. Revise Atomic Structure in Atomic Structure for GCSE Physics with 16 exam-style questions and 25 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 12 of 12 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
- 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
Quick Check: Two atoms both have 8 protons. One has 8 neutrons, the other has 10. Are they isotopes? Explain your answer.
Yes, they are isotopes. They have the same number of protons (same element — oxygen), but different numbers of neutrons (mass numbers 16 and 18). These are oxygen-16 and oxygen-18.
Practice questions for Atomic Structure
What does the atomic number of an element tell you?
Describe the structure of an atom. Include the location and charge of the three main subatomic particles.