Atomic StructureOxford AQA IGCSE Physics: Revision notes
Section 1
What is the structure of an atom?
An atom consists of a small, central, positively charged nucleus containing protons and neutrons, surrounded by electrons.
- The atomic radius is approximately 10⁻¹⁰ m
- The nucleus is much smaller than the whole atom, but contains almost all of the atom's mass
- Electrons occupy the space around the nucleus and have negligible mass in comparison
Section 2
What evidence shows how mass is distributed in an atom?
The scattering of alpha particles by thin metal foil provides evidence for how mass is distributed within an atom.
- Most alpha particles pass straight through the foil, showing that atoms are mostly empty space
- A small number are deflected at large angles, or bounce straight back, showing that there is a small, dense, positively charged nucleus that repels the (also positive) alpha particles
This evidence led to the modern nuclear model of the atom, replacing earlier models that assumed mass and charge were spread evenly throughout the atom.
Link each observation to what it proves: most particles pass through → atom is mostly empty space; some deflect/bounce back → a small, dense, positive nucleus exists.
Section 3
What are the relative masses and charges of subatomic particles?
| Particle | Relative mass | Relative charge |
|---|---|---|
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
| Electron | very small | −1 |
In a neutral atom, the number of electrons equals the number of protons, so the positive and negative charges balance out.
Atoms may lose or gain electrons to form ions — losing electrons makes a positive ion, gaining electrons makes a negative ion.
Section 4
How are atomic number, mass number and isotopes defined?
- Atomic number (proton number): the number of protons in the nucleus of an atom
- Mass number: the total number of protons and neutrons in the nucleus
- Isotopes: atoms of the same element (same atomic number) with different numbers of neutrons (and therefore different mass numbers)
Atoms are represented in the form: mass number above, atomic number below, before the element symbol X (e.g. for carbon-14: mass number 14, atomic number 6).
Carbon-12 and carbon-14 are isotopes of carbon: both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons.
Must Know
- Atoms have a small, positively charged nucleus (protons + neutrons) surrounded by electrons; atomic radius ≈ 10⁻¹⁰ m
- The nucleus is tiny compared to the atom but contains almost all its mass
- Alpha particle scattering by thin foil provides evidence for a small, dense, positive nucleus
- Proton: mass 1, charge +1; neutron: mass 1, charge 0; electron: mass very small, charge −1
- In a neutral atom, number of electrons = number of protons; ions form when electrons are lost or gained
- Atomic number = number of protons; mass number = protons + neutrons; isotopes have the same atomic number but different mass numbers
That's the notes covered.
Carry on to the next subtopic.