Energy and particles · GCSE Physics
The atomic model
Teacher-written GCSE Physics revision on the atomic model: plum pudding to nuclear atom, protons, neutrons and electrons, isotopes, and how alpha-particle scattering changed the picture.
Atomic number is protons. Mass number is protons plus neutrons. Electrons equal protons in a neutral atom. The nucleus is tiny, dense and positive.
The important bits
What you need to know
- 1
Atoms have a tiny positive nucleus of protons and neutrons, with electrons in energy levels (shells) around it. Most of the atom is empty space.
- 2
Proton: charge +1, relative mass 1. Neutron: charge 0, relative mass 1. Electron: charge −1, relative mass very small (about 1/1835).
- 3
Atomic number Z = number of protons and defines the element. Mass number A = protons + neutrons. Neutrons = A − Z.
- 4
Isotopes are atoms of the same element (same Z) with different numbers of neutrons, so different A. Some isotopes are unstable and radioactive.
- 5
The plum pudding model was a ball of positive charge with electrons dotted through it. It could not explain large-angle scattering of alpha particles.
- 6
Rutherford’s alpha-particle scattering: most alphas went straight through (empty space); some deflected (positive nucleus); a few bounced back (nucleus is dense and massive).
- 7
Bohr placed electrons in fixed energy levels. Later work split the nucleus into protons and neutrons. Learn each model with the evidence that replaced the last one.
- 8
Ions form when electrons are added or removed. The nucleus does not change in ordinary ionisation; changing Z would change the element.
Quotations worth analysing
Short evidence. Real method.
“Most of the atom is empty space.”
Most alpha particles passed through gold foil undeflected. That single observation destroyed the idea of a filled pudding of charge.
“Isotopes are atoms of the same element with different numbers of neutrons.”
Same proton number, different mass number. Chemical behaviour matches because electron arrangement matches; nuclear stability may not.
“The nucleus is small, dense and positively charged.”
A few alphas rebounded. Only a concentrated positive mass could reverse a fast, heavy alpha particle. Pair that observation with this conclusion in the exam.
Go deeper
The gold foil experiment is a three-observation story
Learn the observations with the conclusions attached, not as a history essay. Most alpha particles passed through: atoms are mostly empty space. Some were deflected: the nucleus is positive and repels the positive alpha particle. A tiny fraction rebounded: the nucleus is extremely dense and holds nearly all the mass. That destroyed the plum pudding model, which predicted only weak deflections. Later work placed electrons in shells to explain why atoms only emit certain frequencies of light, then added neutrons to explain mass numbers. In the exam, match each observation to one conclusion. A paragraph that only says “Rutherford discovered the nucleus” is not enough for the full explain marks.
Go deeper
Count particles before you write a nuclear symbol
A typical exam atom is given as ²³Na or as “sodium, atomic number 11, mass number 23”. Protons = 11. Electrons in the atom = 11. Neutrons = 23 − 11 = 12. If it becomes Na⁺, it has lost one electron, so 10 electrons, but protons stay 11 — that is why the ion is positive. Students lose marks by changing the proton number when they make an ion. Radioactivity is different: alpha and beta decay do change the nucleus. Isotopes of carbon, ¹²C and ¹⁴C, both have 6 protons. ¹⁴C has two extra neutrons and is radioactive, which is why it is used in dating. Same chemistry, different nuclear stability.
See the idea in action
An aluminium nucleus is written ²⁷₁₃Al. Protons = 13, so Z = 13. Neutrons = 27 − 13 = 14. A neutral atom has 13 electrons. The ion Al³⁺ still has 13 protons and 14 neutrons, but only 10 electrons. Charge = +13 + (−10) = +3. The element has not changed because Z has not changed. If this nucleus emitted an alpha particle (⁴₂He), the new nucleus would have A = 23 and Z = 11, which is sodium — that is a nuclear change, not chemistry.
Exam technique
Turn knowledge into marks
Pair each scattering observation with its conclusion. For isotopes, stress same atomic number, different mass number. Never say ions change their number of protons. Show protons, neutrons and electrons as three separate numbers.
Common mistakes
Do not give these marks away
- 01
Swapping atomic number and mass number, or changing the proton count when an atom becomes an ion.
- 02
Describing the plum pudding model as having a nucleus, or stating a gold-foil conclusion without the observation that supports it.
- 03
Giving electrons a relative mass of 1, or putting neutrons in shells.
Why did a few alpha particles bounce back from gold foil?
AThe atom is a pudding of positive charge
BElectrons in shells repel alpha particles strongly
CThe nucleus is small, dense and positively charged, so it can reverse a fast alpha particle
DGold atoms are larger than alpha particles so they behave like solid balls
Show the answer
The nucleus is small, dense and positively charged, so it can reverse a fast alpha particle. Only a concentrated positive mass explains large-angle scattering. Most alphas still miss the nucleus, which is why most go straight through.
Quick questions
If this is the bit you searched
What did the plum pudding model get wrong?
It spread the positive charge through the whole atom, so it predicted only small deflections of alpha particles. The nuclear model was needed to explain rebound.
What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons, so they have different mass numbers.
How do you find the number of neutrons?
Mass number minus atomic number: A − Z. For ²³⁸U, Z = 92, so neutrons = 238 − 92 = 146.
Does ionisation change the element?
No. Ionisation adds or removes electrons. The element is defined by the proton number, which stays the same unless the nucleus decays.