What is nuclear fission?

a large nucleus splitting. A heavy nucleus splits into two lighter ones.

What starts the fission of a uranium-235 nucleus?

a neutron. Neutrons have no charge, so the nucleus does not repel them.

Where does nuclear fusion happen naturally?

in stars. Stars are hot and dense enough for hydrogen nuclei to fuse.

In a nuclear reactor, what do the control rods do?

absorb neutrons. Absorbing neutrons controls the rate of the chain reaction. (Slowing them is the moderator's job.)

Why is fusion hard to achieve on Earth?

Nuclei repel each other, so very high temperatures are needed. The nuclei need enormous speeds to overcome their repulsion.

What makes a chain reaction possible in fission?

the neutrons released. Each fission releases neutrons that can cause more fissions.

Nuclear fission and fusion · Edexcel IGCSE Science (Single Award)

In plain words

There are two ways to get energy out of nuclei. Fission splits a big nucleus (such as uranium) into two smaller ones: it's what nuclear power stations do. Fusion joins two small nuclei (such as hydrogen) into a bigger one: it's what powers the Sun.

Three things to know

Fission: a large nucleus (such as uranium-235) absorbs a neutron and splits into two smaller nuclei, releasing energy and two or three more neutrons, which can cause a chain reaction.
Fusion: two small nuclei (such as hydrogen) join to make a larger one, releasing energy. It powers the Sun and other stars.
Fusion needs very high temperature and pressure, because the nuclei are positive and repel each other.

Tips and tricks

Fission is splitting (think "fissure", a crack). Fusion is joining (think "fuse together").
In a reactor, the control rods absorb neutrons to slow the chain reaction down. The moderator slows the neutrons themselves down. Don't mix them up.

6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Nuclear fission and fusion: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What is nuclear fission?
- two nuclei joining
- a large nucleus splitting (the answer)
- an electron leaving an atom
- a nucleus emitting light
A heavy nucleus splits into two lighter ones.
What starts the fission of a uranium-235 nucleus?
- an electron
- a proton
- a neutron (the answer)
- an alpha particle
Neutrons have no charge, so the nucleus does not repel them.
Where does nuclear fusion happen naturally?
- in the Earth's core
- in power stations
- in stars (the answer)
- in the atmosphere
Stars are hot and dense enough for hydrogen nuclei to fuse.
In a nuclear reactor, what do the control rods do?Stretch
- slow the neutrons down
- absorb neutrons (the answer)
- produce neutrons
- cool the reactor
Absorbing neutrons controls the rate of the chain reaction. (Slowing them is the moderator's job.)
Why is fusion hard to achieve on Earth?
- Hydrogen is rare.
- Nuclei repel each other, so very high temperatures are needed. (the answer)
- It produces no energy.
- It needs uranium.
The nuclei need enormous speeds to overcome their repulsion.
What makes a chain reaction possible in fission?
- the energy released
- the smaller nuclei produced
- the neutrons released (the answer)
- the gamma rays released
Each fission releases neutrons that can cause more fissions.

6 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

4 questions, 9 marks. Write your answers on paper, then check them.