2.1.2 Stable and Unstable Nuclei

Question 1

What are the three forces acting on the nucleus?

Answer 1

• electromagnetic force
• gravitational force
• strong nuclear force

Question 2

What does the electromagnetic force do?

Answer 2

Causes the positively charged protons in the nucleus to repel each other.

Question 3

What does the gravitational force do?

Answer 3

Causes all the nucleons in the nucleus to attract to each other due to their mass.

Question 4

What does the strong nuclear force do?

Answer 4

Keeps nucleus stable by counteracting the electrostatic force of repulsion between protons in the nucleus (as they have the same charge).

Question 5

Draw the graph to show how the strong nuclear and electromagnetic forces vary with nucleon separation:

Answer 5

I I
I   I
I    I
------x---------x---------------------
I      I     \_\_------'''''''''''''''''''''
I       I   /
I        V

repulsive between 0 - 0.5fm
attractive between 0.5 - 3fm
negligible after 3fm

Question 6

List three statements about the strong nuclear force:

Answer 6

1. It has a very short range so can only hold nucleons together when they are separated by a few femtometres (size of nucleus) and quickly falls beyond this distance.
2. At very small distances, it must be repulsive to avoid the nucleus crushing itself to a point.
3. The force works equally between all nucleons and its size is the same for proton-proton, neutron-neutron or proton-neutron.

Question 7

State what you can learn from the strong nuclear force graph?

Answer 7

1. The SNF is repulsive until 0.5fm.
2. It’s attractive force increases and reaches a maximum after 0.5fm and falls rapidly after 3fm to zero.
3. The electromagnetic repulsive force extends until infinity.

Question 8

What is nuclear decay?

Answer 8

When an unstable nucleus emits particles to become more stable.

Question 9

What is alpha decay?

Answer 9

Only occurs in atoms containing +82 protons.
This is because the nuclei of these atoms are too big for the SNF to keep them stable.
They emit an alpha particle from the nucleus.
Alpha particles have very short range of only few cm in air- this can be seen through their tracks that they leave in cloud chambers or using a Geiger counter or spark counter.
These device measure amount of ionising radiation; if brought to an alpha source, the rate will dramatically drop as you drag it a few cm from the source.

Question 10

What is beta-minus decay?

Answer 10

The emission of an electron from the nucleus along with an antineutrino particle.
Beta decay happens in isotopes that are neutron rich.
When a nucleus rejects a beta particle, one of the neutrons in the nucleus changes to a proton.
The antineutrino particle released carries away some energy and momentum.
Beta particles have a range of a few m in air.

Question 11

State things about beta decay and the hypothesis of neutrinos:

Answer 11

Scientists thought the only particle emitted was an electron during beta decay but they discovered the energy of the particles was less after the decay which did not fit the principle of conservation of energy.
1930: Wolfgang Pauli suggest another particle with zero mass or almost must be emitted carrying away the missing energy with neutral charge.
Other discoveries led to his hypothesis being accepted and the particle was named neutrino.

Question 12

What is the equation for beta-minus decay?

Answer 12

element -> element + electron/beta particle + electron anti neutrino

(a,z) X -> (a,z+1) Y + (0,-1) β + ¯νe

Question 13

What is the equation for alpha decay?

Answer 13

element -> element + alpha particle

(a,z) X -> (a-4,z-2) Y + (4,2) α

Question 14

How can we find the range of alpha particles?

Answer 14

It can be seen by observing the tracks left by them in cloud chambers.

A geiger counter or spark counter can also be used bas it detects ionising radiation. The closer you are to the source the larger the radiation will be.