Atomic StructureEdexcel GCSE Physics: Revision notes
Section 1
What is the structure of an atom?
An atom consists of a tiny, positively charged nucleus (containing protons and neutrons) surrounded by negatively charged electrons. The nuclear radius is much smaller than the radius of the atom as a whole, yet almost all of the atom's mass is concentrated in the nucleus.
Typical atoms and small molecules have a diameter of the order of 10⁻¹⁰ m.
In a neutral atom, the number of protons equals the number of electrons, so the positive and negative charges balance and the atom has no overall charge.
Section 2
How are isotopes described using atomic and mass number?
Nuclei are described using:
- Atomic (proton) number — the number of protons in the nucleus, which gives an element its characteristic positive charge
- Mass (nucleon) number — the total number of protons and neutrons in the nucleus
This is written in the format ¹³₆C, where 13 is the mass number and 6 is the atomic number.
Isotopes are atoms of the same element (same atomic number) that differ in mass number because they have different numbers of neutrons.
Carbon-12 and Carbon-14 both have 6 protons (atomic number 6) but 6 and 8 neutrons respectively — they are isotopes of carbon.
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 (~1/1836) | −1 |
| Positron | very small (~1/1836) | +1 |
A positron has the same mass as an electron but the opposite (positive) charge — it is the antimatter equivalent of an electron.
Section 4
How do electrons behave, and how do atoms form ions?
Electrons orbit the nucleus at different set distances (energy levels/shells). Electrons can change orbit (energy level) when the atom absorbs or emits electromagnetic radiation — moving to a higher orbit on absorbing energy, and dropping to a lower orbit while emitting energy as radiation.
Atoms can form positive ions by losing outer (least strongly held) electrons — this leaves more protons than electrons, giving the atom an overall positive charge.
Section 5
How has the atomic model changed over time?
- Plum pudding model: atoms were thought to be a ball of positive charge with negative electrons embedded throughout, like plums in a pudding.
- Rutherford's alpha particle scattering experiment: firing alpha particles at thin gold foil showed most passed straight through, but a small number deflected strongly or bounced back — this proved most of the atom is empty space, with a tiny, dense, positively charged nucleus at the centre.
- Bohr model: proposed that electrons orbit the nucleus at specific, fixed distances (energy levels), which better explained experimental observations than a model where electrons could be at any distance.
Examiners want the deflection evidence linked directly to conclusions: most alpha particles pass straight through = mostly empty space; some deflect/bounce back = small, dense, positive nucleus.
Must Know
- Atom = small positive nucleus (protons + neutrons) surrounded by orbiting electrons; nucleus much smaller than atom but holds almost all the mass
- Atomic number = protons; mass number = protons + neutrons; isotopes have same protons, different neutrons
- Proton +1, neutron 0, electron −1, positron +1 (electron/positron mass much smaller than proton/neutron)
- Number of protons = number of electrons in a neutral atom
- Electrons change orbit by absorbing or emitting electromagnetic radiation; losing outer electrons forms positive ions
- Model evolved: plum pudding → Rutherford scattering (nuclear model) → Bohr model (fixed electron orbits)
That's the notes covered.
Carry on to the next subtopic.