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Nuclear stability for Edexcel A-level Physics

Nuclear stability

This page covers the following topics:

1. Strong nuclear force
2. Short-range attraction and repulsion
3. Alpha decay
4. Beta decay

There are gravitational forces between the nucleons in a nucleus due to their masses. However, these forces are week and are overcome by the electrostatic forces present. There are stronger electrostatic repulsive forces between the protons in the nucleus of an atom. These forces will push the protons in the nucleus apart, meaning that the nucleus would not hold together if these were the only forces present in it. There exists a strong nuclear force inside the nucleus which holds all the nucleons together. The strong nuclear force provides the nucleus with stability.

Strong nuclear force

Since the protons in a nucleus repel each other, there must be an attractive, short-range force to keep the nucleus together. This force is called the strong nuclear force. It acts on both the protons and neutrons, as otherwise neutrons would be easily removed from nuclei. At even shorter distances, the force becomes repulsive, so that the protons and neutrons do not collapse on themselves. Nucleon separation is measured in femtometres (fm). At distances below 0.4 fm, the force is repulsive. At distances greater than 0.4 fm, the force is attractive. At a distance of 0.4 fm, the force on each nucleon is 0 N. At distances above 3 fm, the force is also 0 N.

Short-range attraction and repulsion

Alpha decay usually occurs in large, unstable unclei with too many protons. Alpha decay involves the emission of an alpha particle from the unstable nucleus called the parent nucleus, which causes it to decay to a different nucleus called the daugther nucleus. An alpha particle, ฮฑ, is a helium nucleus and is made up of two protons, two neutrons and no electrons. In an alpha decay equation, the proton number will decrease by 2 and the mass number will decrease by 4.

Alpha decay

Beta decay is when a neutron turns into a proton by emitting an electron and an electron antineutrino. A beta particle, ฮฒโป, is an electron which comes from the nucleus of an atom. In a beta decay equation, the proton number increases by 1 and the mass number remains the same. The beta particle emitted does not have enough energy for the conservation of energy to apply, hence the electron antineutrino was introduced, which is a particle with no mass or charge and carries the missing energy.

Beta decay

1

Label the forces between the nucleons in the given diagram.

A: electrostatic force of repulsion between the protons. B: strong nuclear force between the nucleons.

Label the forces between the nucleons in the given diagram.

2

Compare the gravitational and electrostatic forces between the nucleons in a nucleus.

There are gravitational forces between the nucleons in a nucleus due to their masses. However, these forces are week and are overcome by the electrostatic forces present. There are stronger electrostatic repulsive forces between the protons in the nucleus of an atom.

Compare the gravitational and electrostatic forces between the nucleons in a nucleus.

3

Explain what happens to the nuclear force at nuclear separation greater than 3 fm.

At distances above 3 fm, the force is also 0 N.

Explain what happens to the nuclear force at nuclear separation greater than 3 fm.

4

Explain what alpha decay is.

Alpha decay involves the emission of an alpha particle from the unstable nucleus called the parent nucleus, which causes it to decay to a different nucleus called the daugther nucleus.

Explain what alpha decay is.

5

Explain why there exist electrostatic forces within the nucleus of an atom.

Nucleons in a nucleus are protons and neutrons. Since neutrons have no charge and protons are positively charged, the protons will repel each other because of their like charges, and therefore there are electrostatic forces of repulsion in the nucleus of an atom.

Explain why there exist electrostatic forces within the nucleus of an atom.

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