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How did our model of the atom develop, and what is inside an atom?

The structure of the atom; subatomic particles, their relative charges and masses; atomic number, mass number and isotopes; the development of the model of the atom from plum pudding to nuclear.

A focused answer to AQA GCSE Chemistry 4.1.1, covering the sub-atomic particles, atomic and mass number, isotopes and relative atomic mass, and how the model of the atom developed from Dalton through the plum pudding model to the nuclear model.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. Sub-atomic particles
  3. Atomic number, mass number and isotopes
  4. How the model developed
  5. Try this

What this dot point is asking

AQA wants you to describe the sub-atomic particles and their relative charges and masses, define atomic number, mass number and isotopes, calculate relative atomic mass, and explain how the model of the atom changed as new experimental evidence appeared. This dot point combines factual recall with a calculation (relative atomic mass) and an evaluation of how evidence changes scientific models.

Sub-atomic particles

Atoms have a radius of about 1×10−101 \times 10^{-10} m. The nucleus is tiny, about 1×10−141 \times 10^{-14} m across, but contains almost all the mass of the atom because the protons and neutrons are far heavier than electrons.

Atomic number, mass number and isotopes

The atomic number is the number of protons; it defines the element (every carbon atom has 6 protons). The mass number is the total number of protons plus neutrons. The number of neutrons is the mass number minus the atomic number.

Relative atomic mass (ArA_r) is the average mass of the isotopes, weighted by their abundance:

Ar=∑(isotope mass×abundance)100A_r = \frac{\sum (\text{isotope mass} \times \text{abundance})}{100}

How the model developed

  • Before electrons: atoms were thought to be tiny solid spheres that could not be divided (Dalton, early 1800s).
  • Plum pudding model: after J.J. Thomson discovered the electron, the atom was modelled as a ball of positive charge with negative electrons embedded in it like fruit in a pudding.
  • Nuclear model: the alpha particle scattering experiment showed most alpha particles passed straight through gold foil but a few were strongly deflected, so the mass and positive charge must be concentrated in a tiny central nucleus (Rutherford).
  • Bohr showed electrons orbit at fixed distances (shells), explaining why atoms are stable, and later the neutron was discovered in the nucleus (Chadwick).

Try this

Q1. An atom has 17 protons and 18 neutrons. Give its atomic number and mass number. [2 marks]

  • Cue. Atomic number 1717; mass number 17+18=3517 + 18 = 35.

Q2. State what the alpha scattering experiment showed about the atom. [2 marks]

  • Cue. Mass and positive charge are concentrated in a small central nucleus; most of the atom is empty space.

Q3. An element has two isotopes: mass 10 (20 percent) and mass 11 (80 percent). Calculate the relative atomic mass. [2 marks]

  • Cue. ((10×20)+(11×80))/100=(200+880)/100=10.8((10 \times 20) + (11 \times 80)) / 100 = (200 + 880) / 100 = 10.8.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksDescribe how the alpha particle scattering experiment led scientists to replace the plum pudding model with the nuclear model of the atom. Refer to the observations and the conclusions drawn from them.
Show worked answer →

A 4-mark Paper 1 question on the history of the atom (an AO1 recall and AO3 evaluation task).

Observations (2 marks): most alpha particles passed straight through the thin gold foil; a small number were deflected through large angles, and a very few bounced almost straight back. Conclusions (2 marks): because most passed through, the atom is mostly empty space; because a few were strongly deflected or bounced back, the positive charge and most of the mass must be concentrated in a tiny, dense central nucleus. This replaced the plum pudding model, in which positive charge was spread out, since spread-out charge could not deflect the alpha particles so sharply.

Markers reward pairing each observation with the correct conclusion.

AQA 20214 marksChlorine has two isotopes, chlorine-35 (abundance 75 percent) and chlorine-37 (abundance 25 percent). Define the term isotope, and calculate the relative atomic mass of chlorine. Give your answer to one decimal place.
Show worked answer →

A 4-mark question combining a definition with a relative-atomic-mass calculation.

Isotope (1 mark): atoms of the same element with the same number of protons but different numbers of neutrons (same atomic number, different mass number).

Calculation: relative atomic mass =(35×75)+(37×25)100=2625+925100=3550100=35.5= \dfrac{(35 \times 75) + (37 \times 25)}{100} = \dfrac{2625 + 925}{100} = \dfrac{3550}{100} = 35.5 (3 marks for the weighted-mean method and answer).

Markers reward the weighted-mean approach using the abundances; a plain average of 36 scores zero for the calculation.

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