3.2.1.2 Stable and Unstable Nuclei

Question 1

Name the two forces acting on the nucleons in the nucleus of the atom.

Answer 1

Electromagnetic Force

Gravitational Force

Question 2

How does the electromagnetic force act on the nucleus?

Answer 2

The electromagnetic forces cause the positively charged protons to repel each other within the nucleus.

Question 3

How does the gravitational force act in the nucleus?

Answer 3

The gravitational forces causes all the nucleons in the nucleus to attract to each other due to their mass.

Question 4

Which if larger, the electromagnetic repulsion or the gravitational attraction?

Answer 4

The electromagnetic repulsion is much larger.

Question 5

What would happen if these two forces were the only two acting in the nucleus?

Answer 5

The nucleons would fly apart.

Question 6

What is the nuclear strong force?

Answer 6

This force is the attractive force which holds stable nuclei together, so that they do not disintegrate.

Question 7

Is the nuclear strong force attractive or repulsive?

Answer 7

It is an attractive force.

Question 8

Why does the Nuclear strong force have to be stronger than the electromagnetic forces?

Answer 8

It has to be stronger because it must overcome the repulsion between the protons in the nucleus, and keep all the nucleons together.

Question 9

What is 1 femtometre equivalent to?

Answer 9

1 femtometre is equivalent to 10^-15 metres.

Question 10

What is the range of the nuclear strong force?

Answer 10

It has a very short range.
No more than about 3 - 4 femtometres.
This is about the same as the diameter of a nucleus.

Question 11

What does this fact about the range tell us about the force?

Answer 11

It tells us that the force can only hold nucleons together when they are separated by anything up to this range,

Question 12

What is the range of the electrostatic force acting on two charged particles?

Answer 12

It has an infinite range.

Question 13

How does the nuclear strong force act on nucleons?

Answer 13

It works equally between all nucleons. It has the same effect on two protons, as it does between two neutrons, or a proton and neutron.

This means that the size of the force is the same in all situations.

Question 14

What fact tells relates the range of the force to the nature of the force?

Answer 14

The nuclear strong force is an attractive force from 3-4 femtometres, down to about 0.5 fm.

Question 15

What is the nature of the nuclear strong force below 0.5 fm? Why?

Answer 15

It is a repulsive force that acts to prevent neutrons and protons being pushed into each other.

Question 16

Describe the graph that shows the relationship between the nuclear strong force and nucleon separation.

Answer 16

Attraction and Repulsion on the x-axis.
Nucleon Separation on the y - axis.
Repulsive force upwards on the x axis (>0)
Attractive force downwards on the x axis (<0)

Nuclear Strong force:
Curve which starts repulsive, then cuts the x-axis and becomes attractive to a maximum, then becomes less and less attractive, and flattens.
Cuts the x axis at 0.5fm and starts to plateau at 3 fm.

Electromagnetic Force:
Repulsive force which extends over a larger range.
Remains repulsive.
At low nucleon separations, it is high, but as the y variable increases, it decreases.

Question 17

Where on the graph is the equilibrium separation for the nuclear strong force?

Answer 17

Where the force curve crosses the x-axis at about 0.5 fm.

Question 18

Why do unstable nuclei emit particles?

Answer 18

To become more stable.

Question 19

What is it called when an unstable nucleus emits particles to become more stable?

Answer 19

It is called nuclear decay.

Question 20

When does alpha decay happen?

Answer 20

Alpha decay only happens in very big atoms with more than 82 protons.

Question 21

Why does alpha decay only happen in these kinds of atoms?

Answer 21

The nuclei of atoms like these are just too big for the nuclear strong force to keep them stable. To make them more stable, they must emit an alpha particle from their nucleus.

Question 22

What is an alpha particle?

Answer 22

An alpha particle is the nucleus of a helium atom.
4He2.
An alpha particle has 2 protons and 2 neutrons.

Question 23

What happens to the atom when an alpha particle is emitted?

Answer 23

The proton number decreases by two.

The nucleon number decreases by four.

Question 24

What is the range of alpha particles?

Answer 24

They have a very short range - only a few centimetres in air.

Question 25

What is beta-minus decay?

Answer 25

Beta-minus decay is the emission of a fast moving electron from a nucleus, and an antineutrino particle.

Question 26

When does beta decay happen?

Answer 26

Beta decay happens in isotopes that are neutron rich. This means that they have too many neutrons compared to protons in their nucleus.

Question 27

What are the properties of a beta-minus particle?

Answer 27

It has the same mass and charge of an electron.

Question 28

What happens to a neutron when a nucleus ejects a beta minus particle?

Answer 28

One of the neutrons becomes a proton. The beta particle is created when the change happens and is emitted instantaneously. In addition, an antiparticle with no charge called an antineutrino is emitted.

Question 29

What happens to the atomic and mass number when a beta particle is emitted?

Answer 29

The proton number increases by one, as a neutron changed into a proton. Therefore the element changes.

The nucleon number stays the same.

Question 30

What is the purpose of the antineutrino particle?

Answer 30

It carries away some energy and momentum.

Question 31

What is the range of a beta-particle?

Answer 31

Beta particles have a much greater range than alpha particles. They can travel up to several metres through air.

Question 32

What is gamma radiation?

Answer 32

Gamma radiation is electromagnetic radiation by an unstable nucleus.

Question 33

What are the properties of gamma radiation?

Answer 33

It can pass through thick metal plates.

It has no mass and no charge.

Question 34

When is gamma radiation emitted?

Answer 34

It is emitted by a nucleus with too much energy, following an alpha or beta decay.

Question 35

What happens to the atom when gamma radiation is emitted? Why?

Answer 35

The proton number and nucleon number does not change as gamma radiation is only energy and makes no difference to the isotope/ nucleus itself.