The Structure of an Atom
Master The Structure of an Atom for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Atoms have a tiny central nucleus of protons and neutrons surrounded by electrons, with a radius of about 1 × 10⁻¹⁰ m.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Atoms have a radius of about 1 × 10⁻¹⁰ metres. The nucleus is far smaller, with a radius less than 1/10000 of that of the atom, yet it contains almost all of the atom's mass.
The nucleus contains protons and neutrons. Protons have a relative charge of +1 and neutrons are uncharged, so the nucleus is positively charged. Electrons, with a relative charge of −1 and negligible mass, orbit the nucleus at different distances.
Electrons can move between energy levels. Absorbing electromagnetic radiation moves an electron to a higher level, further from the nucleus. Emitting radiation moves it to a lower level. If an atom loses an outer electron altogether it becomes a positive ion.
Revision notes
Sizes and structure
An atom has a radius of about 1 × 10⁻¹⁰ m. The nucleus has a radius less than 1/10000 of this.
Despite its tiny size the nucleus contains almost all of the atom's mass, because protons and neutrons are far more massive than electrons.
The particles
Protons: relative charge +1, relative mass 1, in the nucleus.
Neutrons: relative charge 0, relative mass 1, in the nucleus. Electrons: relative charge −1, negligible mass, in energy levels around the nucleus. Atoms are neutral because protons and electrons are equal in number.
Energy levels and ions
Electrons occupy energy levels at different distances from the nucleus.
Absorbing electromagnetic radiation moves an electron to a higher energy level; emitting radiation moves it to a lower one. Losing an outer electron makes the atom a positive ion.
Key points
- An atom has a radius of about 1 × 10⁻¹⁰ m.
- The nucleus is less than 1/10000 of the atom's radius.
- Almost all the mass is in the nucleus.
- Protons have charge +1 and neutrons are uncharged.
- Electrons have charge −1 and negligible mass.
- Losing an electron makes a positive ion.
Worked examples
Example 1
State the relative charges of a proton, a neutron and an electron. [3 marks]
Model answer
Proton: +1 (1 mark). Neutron: 0 (1 mark). Electron: −1 (1 mark).
Example 2
Explain why almost all the mass of an atom is concentrated in the nucleus. [2 marks]
Model answer
The nucleus contains the protons and neutrons, which each have a relative mass of 1 (1 mark), whereas the mass of an electron is negligible in comparison (1 mark).
Example 3
Explain what happens to an electron when an atom absorbs electromagnetic radiation. [2 marks]
Model answer
The electron moves to a higher energy level (1 mark), further away from the nucleus (1 mark).
Common mistakes
Giving electrons a relative mass of 1.
Their mass is negligible compared with protons and neutrons.
Saying the nucleus is negatively charged.
It contains protons and neutrons, so it is positive.
Confusing absorption and emission.
Absorbing radiation moves an electron up; emitting it moves the electron down.
Saying an atom becomes negative when it loses an electron.
Losing a negative electron leaves a positive ion.
Exam tips
- Learn the three particles as a table of charge, mass and location.
- Give the atomic radius in standard form.
- Link absorption to moving up an energy level.
- Explain ion formation through electron loss or gain.
Key terms
- Nucleus
- The central part of an atom containing protons and neutrons.
- Energy level
- A shell in which electrons orbit the nucleus.
- Ion
- A charged atom that has lost or gained electrons.
- Negligible
- So small it can be ignored.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.