Practise The structure of an atom. 13 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What an atom is made of, how tiny its nucleus is compared with the whole atom, and how electrons change energy level when they absorb or emit electromagnetic radiation. Expect short recall questions, size comparisons in standard form and 2 to 3 mark 'explain' questions on energy levels.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name the three particles in an atomProtons and neutrons are in the nucleus; electrons are arranged around the nucleus.
4
Give the relative charge of each particleProton +1, neutron 0, electron −1, so the nucleus is positively charged.
4
Recall the size of an atomThe radius of an atom is about 1 × 10−10 m.
5
Compare the sizes of atom and nucleusThe nucleus has a radius less than 1/10 000 of the atom's radius, yet contains almost all of its mass.
6
Explain how electrons change energy levelAbsorbing EM radiation moves an electron to a higher level, further out; emitting EM radiation moves it to a lower level, closer in.
6
Compare sizes using standard formDivide the larger size by the smaller one, e.g. 1 × 10−10 m ÷ 1 × 10−14 m = 10 000.
Notes
Inside an atom
An atom has a tiny, positively charged nucleus made of protons and neutrons.
Negatively charged electrons are arranged around the nucleus, at different distances from it.
Relative charges: proton +1, neutron 0 (no charge), electron −1.
Relative masses: proton 1, neutron 1, electron very small (about 1/2000). So almost all the mass of an atom is in its nucleus.
An atom has the same number of electrons as protons, so it has no overall charge.
How big is an atom?
The radius of an atom is about 1 × 10−10 m.
The radius of the nucleus is less than 1/10 000 of the radius of the atom, so it is about 1 × 10−14 m or smaller.
So an atom is mostly empty space. If an atom were 100 m across, its nucleus would be less than 1 cm across.
Even though it is so small, the nucleus contains almost all of the atom's mass.
Electrons and energy levels
Electrons can only be at certain distances from the nucleus. These are called energy levels (or shells).
If an electron absorbs electromagnetic radiation, it can move to a higher energy level, further from the nucleus.
If an electron emits electromagnetic radiation, it moves to a lower energy level, closer to the nucleus.
If an outer electron gains enough energy, it can leave the atom altogether. The atom then has more protons than electrons, so it becomes a positive ion. grade 7+
Cheatsheet
Nucleus = protons (+1) and neutrons (0); electrons (−1) are around the nucleus
Relative masses: proton 1, neutron 1, electron about 1/2000
Radius of an atom ≈ 1 × 10−10 m
Radius of nucleus < 1/10 000 of the radius of the atom
Almost all the mass of an atom is in the nucleus
Electron absorbs EM radiation → moves to a higher energy level (further from the nucleus)
Electron emits EM radiation → moves to a lower energy level (closer to the nucleus)
How to answer each type of question
Recall: the particles in an atom
1 to 3 marks3
Say where each particle is: protons and neutrons in the nucleus, electrons around it.
Give relative charges as numbers with a sign: +1, 0, −1.
Explain the charge of the nucleus from the particles in it.
Example. (a) Name the two types of particle found in the nucleus of an atom. (b) Give the relative charge of an electron. (c) Explain why the nucleus of an atom is positively charged.
Show the model answer
(a) Protons and neutrons (1) (b) −1 (1) (c) Protons are positive and neutrons have no charge, so the nucleus has an overall positive charge (1)
Calculate: compare the size of an atom and its nucleus
2 marks6
Write both sizes in the same unit (usually metres).
Divide the radius of the atom by the radius of the nucleus.
Check the answer is a large number: the atom is much bigger.
Example. The radius of an atom is 1.5 × 10−10 m. The radius of its nucleus is 3.0 × 10−15 m. Calculate how many times larger the radius of the atom is than the radius of the nucleus.
Show the model answer
1.5 × 10−10 ÷ 3.0 × 10−15 (1) = 50 000 (1)
Explain: electrons and energy levels
2 to 3 marks6
Say whether electromagnetic radiation is absorbed or emitted.
Say whether the electron moves to a higher or lower energy level.
Say whether it moves further from or closer to the nucleus.
Example. Light is shone on a gas. Some electrons in the gas atoms move further from the nucleus. A short time later, these electrons move back closer to the nucleus. (a) Explain why the electrons move further from the nucleus. (2 marks) (b) State what is given out when the electrons move closer to the nucleus. (1 mark)
Show the model answer
(a) The electrons absorb electromagnetic radiation / light (1) so they move to a higher energy level (1). (b) Electromagnetic radiation (1)
Shortcuts and memory tricks
Proton = Positive; Neutron = Neutral. The electron is the only negative one.
Energy levels are like stairs: going up (further out) needs energy absorbed, coming down (closer in) gives energy out.
Powers-of-ten check: atom about 10−10 m, nucleus about 10−14 m. The powers differ by 4, so the atom is about 104 = 10 000 times bigger.
Use your calculator's ×10x key for standard form, so you don't need brackets.
Where marks are lost
Putting electrons in the nucleus, or saying neutrons are negative.
Saying the nucleus is 10 000 times bigger than the atom. It is the other way round.
Mixing up absorb and emit: absorbing radiation moves an electron further from the nucleus, not closer.
Saying the mass of an atom is spread out evenly. Almost all of it is in the tiny nucleus.
Dividing the wrong way round in a size comparison and getting a tiny number.
Exam technique
In energy level answers, use the key terms: 'absorbs' or 'emits', 'electromagnetic radiation', 'higher' or 'lower energy level'.
Give sizes in standard form with the unit, e.g. 1 × 10−10 m.
When a question asks for a relative charge, give the number with its sign (+1, 0 or −1).
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
An electron absorbs electromagnetic radiation. Describe how the position of the electron can change.
It moves further from the nucleus, to a higher energy level.
State the relative charge of a proton.
+1
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy2 marks
(a) Which particle has a relative charge of 0 and a relative mass of approximately 1? Tick (✓) one box.[1]
Proton
Neutron
Electron
(b) State the relative charge of a proton.[1]
Show the answer and mark scheme
(a)Answer: Neutron
(b)Answer: +1
+1 (positive)
Question 2Medium6 marks
The radius of an atom is about 1 × 10−10 m. The radius of a nucleus is less than 1/10 000 of the radius of the atom.
(a) Calculate the maximum radius of the nucleus of an atom of radius 1 × 10−10 m.[2]
(b) A teacher makes a scale model of an atom. A bead of radius 2.0 mm represents the nucleus. Calculate the minimum radius of the atom in the model. Give your answer in metres.[2]
(c) Most of the mass of an atom is concentrated in the nucleus. Explain why an atom is described as mostly empty space.[2]
Show the answer and mark scheme
(a)Answer: 1 × 10−14 m
1 × 10−10 ÷ 10 000
1 × 10−14 (m)
(b)Answer: 20 m
2.0 mm × 10 000 = 20 000 mm
20 (m)
(c)Answer: The nucleus is at least 10 000 times smaller in radius than the atom and the electrons are far away from it, so almost all of the volume of the atom contains nothing.
the nucleus is tiny compared with the atom / at least 10 000 times smaller in radius
the electrons are far from the nucleus, so most of the volume of the atom contains no particles
Question 3Hard3 marks
A human hair has a diameter of about 80 micrometres (1 micrometre = 1 × 10−6 m). A typical atom has a diameter of about 2 × 10−10 m.
Estimate how many atoms would fit side by side across the diameter of the hair. Give your answer to 1 significant figure.[3]