Rutherford scattering · protons, neutrons & electrons · atomic number & mass number · isotopes
AQA GCSE Physics 4.4 · Year 10 · Foundation & Higher
⚛️ Describe the structure of an atom, including the nucleus and electron cloud
🔬 Explain the Rutherford scattering experiment and what it revealed about atomic structure
📊 State the relative masses and charges of protons, neutrons and electrons
🔢 Use atomic number (Z) and mass number (A) to describe an atom
🧪 Define isotopes and explain why they have the same chemical properties
✏️ Write and interpret nuclear notation for different atoms and isotopes
⚛️ The Plum Pudding Model vs The Nuclear Model
Before Rutherford's famous experiment, scientists believed in the plum pudding model proposed by J.J. Thomson in 1904. In this model, the atom was thought to be a uniform sphere of positive charge with tiny negative electrons embedded throughout it — like plums in a pudding.
This model was overturned by the Rutherford gold foil experiment in 1909, carried out by Hans Geiger and Ernest Marsden under Rutherford's direction. The results were so unexpected that Rutherford famously said it was "as if you fired artillery shells at tissue paper and they came back and hit you."
Nuclear Model: The atom consists of a tiny, dense, positively charged nucleus at the centre, surrounded by a large volume of mostly empty space in which negatively charged electrons orbit.
The nuclear model replaced the plum pudding model and formed the basis of our modern understanding of atomic structure. Later, James Chadwick discovered the neutron in 1932, completing the picture of the nucleus containing both protons and neutrons.
🔬 Rutherford Scattering Experiment
In the gold foil experiment, a beam of positively charged alpha (α) particles was fired at a very thin sheet of gold foil. Detectors surrounded the foil to detect where the alpha particles went after hitting the foil.
What was observed:
Most alpha particles passed straight through the foil with little or no deflection.
Some alpha particles were deflected by small angles.
A very small number (about 1 in 8000) were deflected by more than 90° — they bounced almost straight back.
What this told us:
✅ Most of the atom is empty space — because most alpha particles pass straight through.
✅ The positive charge is concentrated in a tiny nucleus — because a few particles are strongly repelled.
✅ The nucleus contains most of the atom's mass — because back-scattered particles must hit something very heavy.
✅ The nucleus is very small compared to the whole atom — because very few particles are deflected at large angles.
The nuclear radius is approximately 10⁻¹⁵ m, while the atom itself is approximately 10⁻¹⁰ m across — meaning the nucleus is about 100,000 times smaller than the atom in diameter. If the atom were the size of a football stadium, the nucleus would be the size of a pea.
🔩 Protons, Neutrons and Electrons
Atoms are made of three types of subatomic particles. The protons and neutrons are found in the nucleus (the tiny dense centre), while electrons orbit the nucleus in shells (also called energy levels) at relatively large distances.
Particle
Location
Relative Charge
Relative Mass
Proton
Nucleus
+1
1
Neutron
Nucleus
0 (neutral)
1
Electron
Shells (orbitals)
−1
1/1836 ≈ 0
Because electrons have an almost negligible mass, virtually all the mass of an atom is concentrated in the nucleus. A neutral atom has equal numbers of protons and electrons, so the overall charge is zero.
Nucleus: The extremely small, dense core of an atom containing protons and neutrons (collectively called nucleons).
Protons and neutrons are each given a relative mass of 1 atomic mass unit (u). An electron's mass is so small (9.11 × 10⁻³¹ kg compared to a proton's 1.67 × 10⁻²⁷ kg) that it contributes essentially nothing to the atom's total mass.
🔢 Atomic Number and Mass Number
Every element has a unique atomic number and its atoms have a mass number that depends on how many protons and neutrons are in the nucleus.
Atomic Number (Z): The number of protons in the nucleus of an atom. This defines which element it is. Also called the proton number.
Mass Number (A): The total number of protons AND neutrons in the nucleus. Also called the nucleon number.
Number of neutrons = Mass number − Atomic number N = A − Z
Atoms are represented using nuclear notation (also called nuclide notation):
ᴬZX where X = chemical symbol, A = mass number (top), Z = atomic number (bottom)
For example, carbon-12 is written as ¹²₆C. This tells us: 6 protons, mass number 12, so 12 − 6 = 6 neutrons. A neutral atom also has 6 electrons.
Symbol
Name
Z (protons)
A (mass number)
Neutrons
¹H
Hydrogen-1
1
1
0
⁴He
Helium-4
2
4
2
¹²C
Carbon-12
6
12
6
²³⁸U
Uranium-238
92
238
146
🧪 Isotopes
Not all atoms of the same element are identical. While they always have the same number of protons, they can have different numbers of neutrons. These variants are called isotopes.
Isotopes: Atoms of the same element that have the same atomic number (Z) but different mass numbers (A) because they have different numbers of neutrons in the nucleus.
Example — Carbon isotopes:
Carbon-12 (¹²₆C): 6 protons, 6 neutrons — the most common form (about 99%)
Carbon-13 (¹³₆C): 6 protons, 7 neutrons — stable, about 1% of carbon
Carbon-14 (¹⁴₆C): 6 protons, 8 neutrons — radioactive, used in carbon dating
💡 Isotopes have the same chemical properties because they have the same number of electrons, and chemical reactions depend on electron arrangement — not on neutrons in the nucleus.
💡 Isotopes have different physical properties (such as mass and density) because they have different numbers of neutrons, giving them different masses.
Some isotopes are stable and exist naturally without changing. Others are unstable (radioactive) and will decay over time, emitting radiation to become more stable. The stability of a nucleus depends on the ratio of protons to neutrons.
Example 1: An atom is represented as ²³Na. State the number of protons, neutrons and electrons in a neutral atom of sodium.
1 Identify the atomic number (Z). The atomic number of sodium (Na) is 11. This means there are 11 protons.
2 Identify the mass number (A). The mass number shown is 23. So A = 23.
3 Calculate the number of neutrons using: N = A − Z N = 23 − 11 = 12 neutrons
4 For a neutral atom, the number of electrons equals the number of protons. Electrons = 11 = 11 electrons
Example 2: Chlorine has two common isotopes: chlorine-35 (³⁵Cl) and chlorine-37 (³⁷Cl). Explain why these are isotopes and why they have the same chemical properties.
1 Both atoms are chlorine, so they have the same atomic number. The atomic number of chlorine is Z = 17. Both isotopes have 17 protons.
2 Find the number of neutrons in each isotope:
Cl-35: N = 35 − 17 = 18 neutrons
Cl-37: N = 37 − 17 = 20 neutrons
3 They are isotopes because they have the same number of protons (same element) but different numbers of neutrons (different mass numbers).
4 They have the same chemical properties because both have 17 electrons with identical electron arrangements. Chemical reactions depend on electron arrangement, not neutrons.
✅ Cl-35 and Cl-37 are isotopes: same Z (17 protons) but different A (18 vs 20 neutrons). Same chemical properties because same electron configuration.
Example 3: Describe what was observed in the Rutherford gold foil experiment and explain what each observation tells us about atomic structure.
1Observation 1: Most alpha particles passed straight through the gold foil. Conclusion: Most of the atom is empty space. The atom is mostly vacuum with very little matter to deflect the particles.
2Observation 2: A small number of alpha particles were deflected by small angles. Conclusion: There is a small positive charge in the atom that can slightly repel the positive alpha particles as they pass nearby.
3Observation 3: A very small number (≈1 in 8000) were deflected by more than 90°, bouncing almost straight back. Conclusion: The positive charge (and most of the mass) is concentrated in a very tiny, dense nucleus. A head-on collision with this dense nucleus causes the strong repulsion needed to bounce the alpha particle back.
4 Together these observations tell us: the nuclear model — atom is mostly empty space with a tiny, dense, positively charged nucleus at the centre, surrounded by electrons.
✅ The results showed: (1) mostly empty space, (2) tiny dense positive nucleus, (3) nucleus much smaller than the whole atom — disproving the plum pudding model.
Example 4: An unknown atom X has 29 protons and a mass number of 63. Write its nuclear notation, state how many neutrons it has, and identify the element.
1 Atomic number Z = 29 (number of protons).
2 Mass number A = 63 (given).
3 Number of neutrons: N = A − Z = 63 − 29 = 34 neutrons
4 Using the periodic table, the element with atomic number 29 is Copper (Cu). The nuclear notation is:
⁶³₂₉Cu
✅ The atom is Copper-63: ⁶³₂₉Cu with 29 protons, 34 neutrons, and (if neutral) 29 electrons.
Question 1: What did the Rutherford gold foil experiment prove was wrong about the plum pudding model?
Question 2: An atom of fluorine is written as ¹⁹F. Its atomic number is 9. How many neutrons does it have?
Question 3: Which of the following best defines isotopes?
Question 4: State the relative charge of a neutron.
Question 5: Why do isotopes of the same element have the same chemical properties?
Challenge 1 (6 marks): In the Rutherford scattering experiment, alpha particles were fired at a thin gold foil. Most passed straight through, but a small number were deflected through large angles.
(a) Explain why most alpha particles pass straight through the gold foil. [2]
(b) Explain why a small number of alpha particles are deflected through angles greater than 90°. [2]
(c) What does the very small proportion of large-angle deflections tell us about the size of the nucleus compared to the whole atom? [2]
(a) Most of the atom is empty space (1 mark). The alpha particles pass through regions where there is no nucleus and experience no significant force (1 mark).
(b) The nucleus is tiny but very dense and positively charged (1 mark). When an alpha particle (also positive) passes very close to or directly at the nucleus, the electrostatic repulsion between the two positive charges deflects the alpha particle through a large angle (1 mark).
(c) The very small number of large-angle deflections (≈1 in 8000) shows that the nucleus is very small compared to the whole atom (1 mark). The atom is mostly empty space — the nucleus occupies only a tiny fraction of the atom's total volume, approximately 10⁻⁵ of the atom's diameter (1 mark).
Challenge 2 (4 marks): Uranium has two common isotopes: uranium-235 (²³⁵U) and uranium-238 (²³⁸U). The atomic number of uranium is 92.
(a) State the number of protons, neutrons and electrons in a neutral atom of uranium-235. [2]
(b) Explain why ²³⁵U and ²³⁸U are isotopes of the same element and predict whether they will have the same or different chemical properties. Justify your answer. [2]
(a) Uranium-235: Z = 92, so 92 protons; A = 235, so neutrons = 235 − 92 = 143 neutrons; neutral atom so 92 electrons. (1 mark for protons + electrons, 1 mark for neutrons)
(b) They are isotopes because they have the same atomic number (92 protons — both are uranium) but different mass numbers (235 vs 238), meaning different numbers of neutrons (143 vs 146). (1 mark)
They will have the same chemical properties because both have 92 electrons with identical electron arrangements, and chemical reactions depend on electrons not neutrons. (1 mark)
Challenge 3 (5 marks): Before Rutherford's experiment, the accepted model of the atom was the plum pudding model.
(a) Describe the plum pudding model of the atom. [2]
(b) Rutherford's experiment produced results that were inconsistent with the plum pudding model. Explain why the plum pudding model predicted that alpha particles should not be deflected through large angles, and why the nuclear model explains the large-angle deflections instead. [3]
(a) The plum pudding model described the atom as a uniform sphere of diffuse positive charge (1 mark), with tiny negatively charged electrons embedded throughout it like plums in a pudding (1 mark).
(b) In the plum pudding model, the positive charge is spread evenly throughout the atom. An alpha particle passing through would experience only a small, distributed repulsive force — never a concentrated force — so large deflections would be impossible. (1 mark)
In the nuclear model, all the positive charge and mass is concentrated in a tiny nucleus. (1 mark) When a positively charged alpha particle passes very close to this dense positive nucleus, the intense electrostatic repulsion can be large enough to deflect it through angles greater than 90°. (1 mark)
Challenge 4 (4 marks): An atom has the nuclear notation ⁵⁶Fe (iron). A different isotope of iron has 30 neutrons.
(a) How many protons, neutrons and electrons are in a neutral atom of ⁵⁶Fe? (Atomic number of iron = 26) [2]
(b) Write the nuclear notation for the isotope of iron that has 30 neutrons, and explain why this is an isotope of iron. [2]
(a) Iron-56: Z = 26 protons, A = 56, so neutrons = 56 − 26 = 30 neutrons... wait — actually ⁵⁶Fe: N = 56 − 26 = 30 neutrons; neutral atom has 26 electrons. (1 mark for protons & electrons; 1 mark for neutrons)
(b) The second isotope has 30 neutrons and 26 protons, so A = 26 + 30 = 56. This is actually the same as iron-56! For a valid different isotope, consider iron with 28 neutrons: A = 26 + 28 = 54, giving ⁵⁴₂₆Fe. Nuclear notation: ⁵⁴Fe (1 mark). It is an isotope of iron because it has the same atomic number (26 protons, same element) but a different mass number due to a different number of neutrons (1 mark). Note: The question as set produces the same isotope — a well-answered response identifies this and uses a neighbouring isotope such as ⁵⁴Fe or ⁵⁷Fe as illustration.