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Pearson Edexcel PhysicsAtomic structure

Atoms, isotopes and radiation

Describe atomic structure and compare nuclear radiation.

Start here

The key idea

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Atoms Isotopes And Radiation
nucleuselectrons

Use the labels to explain the scientific relationship shown.

Revision notes

The bit that matters

Short notes first. Learn the idea, then use the worked example and questions to check it properly.

1

Structure of the atom

An atom has a tiny central nucleus containing protons and neutrons, surrounded by electrons in energy levels (shells).The nucleus has a radius about 10000 times smaller than the atom, whose radius is around 1×10−101 \times 10^{-10} m.Protons have a positive charge, electrons a negative charge and neutrons no charge.Atoms have no overall charge because the number of protons equals the number of electrons.

2

Atomic number, mass number and isotopes

The atomic number is the number of protons, which defines the element.The mass number is the total number of protons and neutrons.Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so they have the same atomic number but different mass numbers.An ion is an atom that has lost or gained electrons and so carries a charge.

3

The development of the atomic model

The earliest idea was that atoms were tiny indivisible spheres.The plum pudding model suggested a ball of positive charge with electrons embedded in it.The alpha scattering experiment, where most alpha particles passed straight through gold foil but a few were deflected sharply, showed the atom is mostly empty space with a small, dense, positive nucleus.Bohr then proposed that electrons orbit in fixed energy levels.

4

Types of nuclear radiation

Unstable nuclei emit radiation to become more stable.An alpha particle is two protons and two neutrons (a helium nucleus); it is strongly ionising but only travels a few cm in air and is stopped by paper.A beta particle is a fast electron from the nucleus; it is moderately ionising and is stopped by a few mm of aluminium.A gamma ray is high-energy electromagnetic radiation; it is weakly ionising and is only reduced by thick lead or concrete.Neutron emission can also occur.

Key terms

Definitions to learn

Atomic number

The number of protons in the nucleus.

Mass number

The total number of protons and neutrons.

Isotope

Atoms of an element with the same protons but different neutrons.

Ion

An atom that has lost or gained electrons, giving it a charge.

Ionising radiation

Radiation that can knock electrons off atoms it passes through.

Worked example

An atom has 17 protons and 18 neutrons. State its atomic number and mass number.

1

Atomic number is the proton number.

2

Mass number is protons plus neutrons.

Final answer

Atomic number 17, mass number 35

Exam habit

For isotope questions, state proton number = atomic number and neutron number = mass number minus atomic number.When comparing radiation types, mention penetrating power, ionising ability and what stops each one.

Watch out

Do not count electrons when finding mass number.

Examiner tips

How to score full marks

  • 1Number of neutrons = mass number - atomic number; learn this rearrangement.
  • 2Match each radiation to its stopping material: alpha by paper, beta by aluminium, gamma by lead.
  • 3Quote atom size as about 1×10−101 \times 10^{-10} m and remember the nucleus is around 10000 times smaller.
Practice questions

Try these yourself

Start with the core skill, then open the answer only after you have attempted the full question.

1Compare the penetrating power of alpha, beta and gamma radiation.
Mark scheme
  1. 1.Order them from least to most penetrating.
Alpha is least penetrating, beta is moderately penetrating and gamma is most penetrating.
2Explain why alpha radiation is strongly ionising.
Mark scheme
  1. 1.Consider its charge and interactions.
An alpha particle has a relatively large charge and interacts strongly with atoms, removing electrons readily.
3An isotope has mass number 226 and atomic number 88. Find its number of neutrons.
Mark scheme
  1. 1.Subtract atomic number from mass number.
138
4State the charges of a proton, a neutron and an electron.[2 marks]
Mark scheme
  1. 1.Recall the charge of each particle.
Proton positive (+1) (1); neutron neutral/no charge and electron negative (-1) (1)
5An atom has an atomic number of 17 and a mass number of 37. State the number of protons, neutrons and electrons.[3 marks]
Mark scheme
  1. 1.Protons = atomic number.
  2. 2.Neutrons = mass number - atomic number.
  3. 3.Electrons = protons in a neutral atom.
17 protons (1); neutrons = 37 - 17 = 20 (1); 17 electrons (1)
6Define the term isotope.[1 mark]
Mark scheme
  1. 1.Compare protons and neutrons.
Atoms of the same element with the same number of protons but a different number of neutrons (1)
7State which type of radiation is the most ionising and which is the most penetrating.[2 marks]
Mark scheme
  1. 1.Recall the order for ionising power and penetration.
Alpha is the most ionising (1); gamma is the most penetrating (1)
8Explain how the results of the alpha scattering experiment led scientists to reject the plum pudding model and propose a nuclear model of the atom.[4 marks]
Mark scheme
  1. 1.Describe what most alpha particles did.
  2. 2.Describe the small fraction that deflected.
  3. 3.Link these to empty space and concentrated charge.
  4. 4.State the conclusion about the nucleus.
Most alpha particles passed straight through the gold foil, showing the atom is mostly empty space (1); a few were deflected through large angles and some bounced back (1); this could not happen if positive charge was spread out as in the plum pudding model (1); so the positive charge and most of the mass must be concentrated in a tiny central nucleus (1)
9Carbon-12 and carbon-14 are both isotopes of carbon. Carbon-12 has atomic number 6. State the number of protons and neutrons in each isotope and explain why they are the same element.[3 marks]
Mark scheme
  1. 1.Carbon-12: protons = 6, neutrons = 12 - 6.
  2. 2.Carbon-14: protons = 6, neutrons = 14 - 6.
  3. 3.Same element because same number of protons / same atomic number.
Carbon-12: 6 protons, 6 neutrons (1); carbon-14: 6 protons, 8 neutrons (1); they are the same element because the number of protons (atomic number) is the same and it is the atomic number that defines the element (1)
10A neutron is fired at a uranium-235 nucleus. The uranium nucleus splits (fissions) and releases three neutrons. Explain how this can lead to a chain reaction and describe the condition needed for the chain reaction to be sustained.[3 marks]
Mark scheme
  1. 1.Each new neutron can hit another nucleus.
  2. 2.Each fission releases more neutrons.
  3. 3.Chain reaction if each fission triggers at least one more.
  4. 4.Critical mass: enough fissile material.
Each fission releases three neutrons, which can go on to trigger further fission events in other uranium nuclei (1); those reactions release more neutrons, and so on, creating a chain reaction (1); for the chain reaction to be sustained, on average at least one of the neutrons from each fission must cause another fission — this requires a sufficient quantity (critical mass) of fissile material so neutrons are not all lost at the surface before finding another nucleus (1)
11Compare alpha, beta and gamma radiation in terms of their nature, ionising ability, penetrating power and what stops each one. Give one practical use that exploits each radiation's unique properties.[3 marks]
Mark scheme
  1. 1.Nature of each radiation.
  2. 2.Ionising ability order.
  3. 3.Penetration and stopping material for each.
  4. 4.One practical use linked to properties for each.
Alpha radiation consists of 2 protons and 2 neutrons (a helium nucleus); it is strongly ionising, travels a few centimetres in air and is stopped by paper; used in smoke detectors because it ionises air to carry a small current, and smoke particles absorb the alpha, reducing the current and triggering the alarm (1); beta radiation is a fast-moving electron from the nucleus; it is moderately ionising, penetrates a few mm of aluminium; used for monitoring the thickness of aluminium foil because some beta passes through and the amount reaching the detector varies with thickness (1); gamma radiation is high-energy electromagnetic radiation; it is weakly ionising but very penetrating, requiring thick lead or concrete to significantly reduce it; used in radiotherapy to destroy deep-lying cancer cells from outside the body (1) — award 1 per radiation type with use, max 3
12Explain why the development of the nuclear model of the atom was a significant advance in scientific understanding. Refer to the Geiger-Marsden experiment results, what the plum pudding model predicted, and how the nuclear model better fits the evidence.[4 marks]
Mark scheme
  1. 1.Plum pudding model prediction for alpha scattering.
  2. 2.Actual results of Geiger-Marsden.
  3. 3.Why plum pudding fails to explain large-angle deflection.
  4. 4.What nuclear model explains about large-angle deflections.
  5. 5.Significance: scientific consensus changed based on experimental evidence.
The plum pudding model predicted that alpha particles would pass through the gold foil with only slight deflections, because the positive charge was spread thinly throughout the atom, which would exert only a small repulsive force on the alpha particles (1); however, Geiger and Marsden found that most alpha particles passed straight through but a small fraction were deflected through large angles, and some even bounced back towards the source (1); this large-angle deflection cannot be explained if the charge is spread out — a diffuse positive charge would produce too small a repulsion to deflect the fast, massive alpha particles significantly (1); the nuclear model explains the results by proposing that almost all the positive charge and mass is concentrated in a very tiny, dense nucleus; most of the atom is empty space (explaining the straight-through majority), and only the rare alpha particle that passes very close to the nucleus experiences a large repulsive force and is deflected sharply (1); this change in model illustrates how experimental evidence that contradicts a current theory should lead scientists to revise or replace it — a key principle of scientific progress (1) — award max 4
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