Beta decay model 3
Reminder
The symbols represent isotopes of atoms. There are two numbers before each chemical symbol that tell us the atomic number and the atomic mass number.

Carbon-14 has atomic number 6; therefore it has 6 protons in the nucleus. Its atomic mass number is 14 so it has 14 particles in total in the nucleus. Therefore it must have 8 neutrons to add up to 14.

A radioactive atom can decay by emitting a beta particle which is a fast moving electron. A natural example of beta emission is the decay of carbon-14 into nitrogen-14. The equation for the decay is:
Nuclear decay equation of carbon 14 with rollover highlights
Notice that the equation balances for both the atomic number and the atomic mass number. The atomic mass number does not change because a beta particle has a much smaller mass than the atom. The atomic number goes up because a neutron has turned into an extra proton.
Flavour Charge
up 2/3
down -1/3
Table1
Beta and alpha
Beta decay is fundamentally different from alpha decay. An alpha particle is made of two protons and two neutrons. These particles were already in the nucleus. However, in beta decay, a fast moving electron is fired out of the nucleus. The nucleus did not originally contain any electrons so there must have been a change within the nucleus to produce an electron.
What is beta decay?
The quark composition of a neutron decaying into a proton with rollover highlights Quark composition of a proton produced by a neutron with rollover highlights
neutron to proton
Figure 1. Beta decay of a neutron.
On one level, we can think of the change as being a neutron turning into a proton plus an electron (Figure 1).

However, we now know that neutrons and protons, like all hadrons, are made up of quarks (Table 1). Beta decay happens when one of the down quarks in a neutron changes into an up quark, making it a proton. This change of flavour represents one fundamental particle changing into a different fundamental particle. This is different from any other change that we know. We can explain alpha decay and chemical reactions by the movement of particles rather than a fundamental change in a particle.

Force Has effect on
gravity anything with mass
electromagnetic anything with charge
strong nuclear quarks & therefore protons and neutrons
weak force all fundamental particles
Table 2. The weak force makes up the full set of four forces.
A new force
What force makes beta decay happen? It cannot be the strong nuclear force because this has no effect on electrons and the beta particle is an electron. Neither, as physicists know, can it be the electromagnetic force. In order to explain it, we need to identify a new force called the weak force.

The weak force is very short range and, as the name implies, it is not at all strong. Its effects are felt by all fundamental particles - quarks and leptons - and it is the fourth and final force in our model (table 2).

What are the neutrino and antineutrino?
M5
The story so far
In beta decay, a down quark changes into an up quark
Beta decay can only be explained by introducing the weak force
The weak force acts on quarks and leptons
There are four fundamental forces in the story so far
Question M3

a) What particle is given out in i) alpha decay ii) beta decay?

b) Why is alpha decay "fundamentally different" from beta decay?

c) Why can't the strong nuclear force explain beta decay?

The story so far
  • There are two families of fundamental particles: quarks and leptons.
  • Protons and neutrons are made from three quarks
  • Electrons are leptons
  • Nuclei can change by alpha and beta decay
  • There are three forces in the story so far: gravity, electromagnetism and the strong nuclear force