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Physics · Atomic structure

The structure of an atom

Describe the nuclear model of the atom: the size of atoms and nuclei, protons, neutrons and electrons, and electron energy levels.

  • 6 key terms
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Teacher resources

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Student handouts

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Warm-up

Answer each one, then check.

  1. 1

    What is an atom?

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    The smallest part of an element that can exist

  2. 2

    What charge does an electron have?

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    Negative

  3. 3

    What charge does a proton have?

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    Positive

  4. 4

    What is standard form?

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    A number written as \(a \times 10^n\)

  5. 5

    What is electromagnetic radiation?

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    Waves such as light and X-rays

Learning Objectives

  1. 1Describe the basic structure of an atom.
  2. 2Recall the size of an atom and how it compares with the size of a nucleus.
  3. 3State the relative charge and mass of protons, neutrons and electrons.
  4. 4Explain how electrons change energy levels by absorbing or emitting electromagnetic radiation.

THE ATOM

Atoms have a radius of about \(1 \times 10^{-10}\) m. The nucleus has a radius of less than \(\tfrac{1}{10\,000}\) of the atom's radius but contains most of the mass.

The nucleus is positively charged (protons and neutrons) and is surrounded by negatively charged electrons at different distances (energy levels).

Particles in an Atom

Learn the relative charge and mass.

  • Proton

    Relative charge: +1. Relative mass: 1. Where: Nucleus

  • Neutron

    Relative charge: 0. Relative mass: 1. Where: Nucleus

  • Electron

    Relative charge: −1. Relative mass: Very small (about 1/2000). Where: Energy levels around the nucleus

Size of the Nucleus

The radius of an atom is about 1 × 10⁻¹⁰ m. The radius of its nucleus is less than 1/10 000 of this. Calculate the largest possible radius of the nucleus.

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  1. 1 Divide \(\dfrac{1 \times 10^{-10}}{10\,000}\)
  2. 2 Write 10 000 as a power of ten \(10\,000 = 1 \times 10^{4}\)
  3. 3 Answer \(1 \times 10^{-14}\) m

AnswerThe nucleus has a radius of less than \(1 \times 10^{-14}\) m.

Electron Energy Levels

Describe what happens when an electron absorbs electromagnetic radiation.

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  1. 1 Energy The electron gains energy from the radiation
  2. 2 Movement It moves to a higher energy level, further from the nucleus
  3. 3 Reverse If it emits radiation it falls to a lower energy level, closer to the nucleus

AnswerAbsorbing radiation moves an electron further from the nucleus; emitting radiation moves it closer.

Key Facts

Points examiners ask about.

  • Neutral atoms

    The number of electrons equals the number of protons, so there is no overall charge.

  • Mass

    Most of the mass of an atom is in the nucleus.

  • Space

    Most of an atom is empty space.

  • Positive ions

    An atom that loses one or more outer electrons becomes a positive ion.

Atom Sizes

An atom has a radius of 1 × 10⁻¹⁰ m and a nucleus of radius 1 × 10⁻¹⁵ m. How many times larger is the atom's radius than the nucleus's radius? Where is most of the mass?

1. Divide the two radii.

2. Use powers of ten.

A good answer shows: The atom is 100 000 times larger (10⁻¹⁰ ÷ 10⁻¹⁵ = 10⁵). Most of the mass is in the nucleus.

Can I...?

  1. 1Describe the parts of an atom.
  2. 2Give the size of an atom.
  3. 3Compare atom and nucleus sizes.
  4. 4State charges and masses.
  5. 5Explain why atoms are neutral.
  6. 6Describe electron energy levels.
  7. 7Say what happens when radiation is absorbed.
  8. 8Use standard form.

Summary & Exam Focus

  • Atoms are about 1 × 10⁻¹⁰ m in radius.
  • The nucleus is less than 1/10 000 of the atom's radius.
  • Protons +1, neutrons 0, electrons −1.
  • Electrons move to higher levels by absorbing radiation.

Exam focus

Describe the structure of an atom. (3 marks) (3 marks)

Nucleus with protons and neutrons; electrons around it.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Atom
The smallest part of an element that can exist.
Nucleus
The positively charged centre of an atom containing protons and neutrons.
Proton
A positive particle in the nucleus.
Neutron
A neutral particle in the nucleus.
Electron
A negative particle that orbits the nucleus.
Energy level
A distance from the nucleus at which an electron can be.

Questions and answers

10 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question Describe 3 marks Easier

Describe the structure of an atom.

Mark scheme — 3 marks available

  • Small central nucleus — 1 mark
  • Nucleus contains protons and neutrons — 1 mark
  • Electrons around the nucleus — 1 mark

Model answer

A small central nucleus containing protons and neutrons, with electrons arranged at different distances (energy levels) from the nucleus.

2. Exam question Complete 3 marks Easier

Complete the table to show the relative charge of a proton, a neutron and an electron.

Mark scheme — 3 marks available

  • +1 — 1 mark
  • 0 — 1 mark
  • −1 — 1 mark

Model answer

Proton +1; neutron 0; electron −1.

3. Exam question Calculate 3 marks Easier

The radius of an atom is 1 × 10⁻¹⁰ m. The radius of its nucleus is less than 1/10 000 of the radius of the atom. Calculate the largest possible radius of the nucleus. Give your answer in standard form.

Mark scheme — 3 marks available

  • Divides by 10 000 — 1 mark
  • Correct power — 1 mark
  • 1 × 10⁻¹⁴ m — 1 mark

Model answer

\(1 \times 10^{-10} \div 10\,000 = 1 \times 10^{-14}\) m

4. Exam question Explain 2 marks Easier

An atom has no overall electrical charge. Explain why.

Mark scheme — 2 marks available

  • Number of electrons equals number of protons — 1 mark
  • Charges cancel — 1 mark

Model answer

The number of electrons is equal to the number of protons, so the negative and positive charges cancel.

5. Exam question Explain 3 marks Easier

An electron in an atom absorbs electromagnetic radiation. Describe what happens to the electron. Then say what happens when the electron emits radiation.

Mark scheme — 3 marks available

  • Absorbs radiation: moves to higher energy level or further from nucleus — 1 mark
  • Emits radiation: moves to lower energy level — 1 mark
  • Closer to the nucleus — 1 mark

Model answer

It moves to a higher energy level, further from the nucleus. When it emits radiation it moves to a lower energy level, closer to the nucleus.

6. Multiple choice 1 mark Easier

The nucleus contains...

  1. A electrons and protons
  2. B protons and neutrons Correct
  3. C electrons only
  4. D neutrons and electrons

Why: Electrons are outside the nucleus.

7. Multiple choice 1 mark Core

The charge on a neutron is...

  1. A +1
  2. B −1
  3. C zero Correct
  4. D +2

Why: Neutrons are neutral.

8. Multiple choice 1 mark Core

Most of the mass of an atom is in the...

  1. A electrons
  2. B space around
  3. C outer shell
  4. D nucleus Correct

Why: Protons and neutrons are heavy.

9. Multiple choice 1 mark Core

An atom loses an electron. It becomes a...

  1. A positive ion Correct
  2. B negative ion
  3. C neutral atom
  4. D neutron

Why: It has more protons than electrons.

10. Multiple choice 1 mark Stretch

The radius of an atom is about...

  1. A 1 × 10⁻² m
  2. B 1 × 10⁻¹⁰ m Correct
  3. C 1 × 10⁻¹⁵ m
  4. D 1 m

Why: About 0.1 nanometres.