# Atomic Structure and Isotopes — GCSE Physics
Everything around you is made of atoms — incredibly tiny particles that are the building blocks of all matter. Understanding atomic structure is fundamental to both physics and chemistry, and forms the basis for understanding radioactivity, nuclear energy, and the periodic table.
1. Structure of the Atom
An atom consists of:
Nucleus (centre):
- Contains protons (positive charge, +1) and neutrons (no charge, 0)
- Very small (radius ~ m) but contains almost all the mass
- Held together by the strong nuclear force
Electrons (orbiting the nucleus):
- Negative charge (−1)
- Arranged in shells (energy levels) around the nucleus
- Atom radius ~ m (about 10,000× larger than the nucleus)
- Very tiny mass compared to protons/neutrons
Subatomic Particles
| Particle | Relative Mass | Relative Charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | ~1/2000 | −1 | Shells around nucleus |
2. Atomic Number and Mass Number
Atomic number () = number of protons in the nucleus
- Defines the element (e.g., all carbon atoms have 6 protons)
- In a neutral atom: number of protons = number of electrons
Mass number () = number of protons + neutrons in the nucleus
Notation
Example: means carbon with 6 protons and neutrons.
3. Isotopes
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons (different mass numbers).
Examples: Carbon Isotopes
| Isotope | Protons | Neutrons | Mass Number |
|---|---|---|---|
| Carbon-12 () | 6 | 6 | 12 |
| Carbon-13 () | 6 | 7 | 13 |
| Carbon-14 () | 6 | 8 | 14 |
All three are carbon (6 protons), but they have different numbers of neutrons.
Key Facts About Isotopes
- Same chemical properties (same electron configuration)
- Different physical properties (different mass → different density, melting point)
- Some isotopes are stable, others are radioactive (unstable)
4. Ions
When an atom gains or loses electrons, it becomes an ion:
- Losing electrons → positive ion (cation): more protons than electrons
- Gaining electrons → negative ion (anion): more electrons than protons
The nucleus doesn't change when forming ions — only the electron count changes.
5. Development of the Atomic Model
| Model | Year | Key Idea |
|---|---|---|
| Dalton | 1803 | Atoms are tiny solid spheres that cannot be divided |
| Thomson | 1897 | "Plum pudding" model — electrons embedded in a positive sphere |
| Rutherford | 1911 | Nuclear model — small, dense, positive nucleus with electrons orbiting |
| Bohr | 1913 | Electrons in specific energy levels (shells) |
| Modern | 1920s+ | Electron cloud model — electrons in probability clouds/orbitals |
Rutherford's Alpha Scattering Experiment
- Fired alpha particles at thin gold foil
- Most passed straight through → atom is mostly empty space
- Some deflected slightly → nucleus is positive (repels positive alphas)
- Very few bounced back → nucleus is very small and dense
Worked Example: Example 1
Question: An atom of sodium has the symbol Na. State the number of protons, neutrons, and electrons.
Protons = 11, Neutrons = , Electrons = 11 (neutral atom)
Worked Example: Example 2
Question: Chlorine has two isotopes: Cl and Cl. Explain why these are isotopes.
Answer: Both have 17 protons (so both are chlorine), but Cl has 18 neutrons while Cl has 20 neutrons. They have the same atomic number but different mass numbers.
7. Practice Questions
- Describe the structure of an atom in terms of subatomic particles. (3 marks)
- Define the term "isotope." (2 marks)
- An atom has 26 protons and a mass number of 56. Name the element and calculate the number of neutrons. (2 marks)
- Explain why the alpha scattering experiment led scientists to replace the plum pudding model. (4 marks)
- A sodium atom (Na) forms a Na⁺ ion. How many protons and electrons does this ion have? (2 marks)
Answers
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Summary
- Atoms: nucleus (protons + neutrons) + electrons in shells
- Atomic number = protons; Mass number = protons + neutrons
- Isotopes: same protons, different neutrons
- Ions: atoms that have gained or lost electrons
- Atomic model evolved: Dalton → Thomson → Rutherford → Bohr → Modern