Inside the atom model 2
Family Particle Fundamental
lepton electron yes
hadron proton no
neutron no
The nucleus of an atom normally contains two types of particle: the proton and the neutron. From what you have learned before, it may seem that protons and neutrons are fundamental particles. This would be a nice simple picture of matter: three particles that make up the Universe - protons, neutrons and electrons. However, protons and neutrons are only two particles in a large family called hadrons. Unlike the electron, hadrons are not fundamental - they are made up of even smaller particles called quarks.
Hunting the quark
Flavour Charge
up +2/3
down -1/3
Quarks are fundamental. They make up one family of fundamental particles. The other family is the leptons (the electron's family). Although the quarks are a family of fundamental particles, they never exist on their own. They are only ever found as combinations in protons, neutrons and the other hadrons.
Mixing flavours
The proposal of the hadran families
Protons and neutrons are made of two types of quark. These quarks are said to have different flavours: up and down. These up and down quarks are the only quarks that are found in normal matter and they are known as first generation quarks.
More generations of quarks
Nucleon Quarks Representation
proton up


A rollover change of a proton
neutron up


A rollover change of a neutron
Table 2
A proton is made from two up quarks and a down quark. A neutron is made from two down quarks and an up quark. Table 1 shows the properties of these quarks and how they combine to give the charges of protons and neutrons. You can read more about quarks in the extension page.
Picturing properties
We can never see these tiny particles because they are smaller than the wavelength of visible light but we can look into their properties. For example, we can work out their charge and measure their mass. Charge and mass are familiar properties because we can also measure the charge and mass of everyday objects.

However, sub-atomic particles have other properties that do not appear in everyday objects. One of these is the flavour of a quark. This is nothing like the flavour of something we eat (like ice cream) but it is a word that means something particular to physicists, who know when they use it exactly what they are talking about. (There are some much crazier names that you will come across.)

Holding the nucleus together
Changing image to show strong nuclear force
The forces between protons.
Force Has effect on
Gravity Anything with mass
Electromagnetic Anything with charge
Strong nuclear Quarks but not leptons
Table 3. The strong nuclear force is a new force needed to explain why protons and neutrons bind in the nucleus.
The nucleus of helium contains two protons. They are both positively charged and will repel each other. However, they stay bound in a helium nucleus. Therefore there must be another force that holds them together.

This is the strong nuclear force, which is the third force in our model (Table 3). It is an attractive force that only has an effect over a very short range (about 10-15 metres - the size of the nucleus). The strong nuclear force binds protons and neutrons together to make the nucleus.

The strong nuclear force is actually a force between quarks and is carried by particles called gluons. Protons and neutrons are made of quarks and they feel the strong nuclear force as well.

What causes forces?
Familiar feelings
Electrons do not feel the strong nuclear force. Fundamental particles that don't feel the strong nuclear force are all in the family of leptons. Particles in the quark family do feel the strong nuclear force. So do the hadrons like protons and neutrons, which are made up of quarks.
The story so far
Quarks are fundamental particles
Quarks have charge and flavour
Normal matter is made from only two flavours of quark - up and down
Protons and neutrons are made of up and down quarks
There are three fundamental forces in the story so far
Leptons, including electrons, do not feel the strong nuclear force
Question M2

a) Which of these are not properties of quarks?
mass, flavour, smell, charge, colour

b) How many quarks make up a proton?

c) How many up quarks are there in a nucleus of helium-4 (atomic number =2)?

d) Which force is felt by quarks but not by leptons?

The story so far
  • Atoms have a tiny nucleus that contains protons and neutrons
  • Electrons are outside the nucleus
  • Electrons are fundamental particles - part of the family of leptons
  • Protons and neutrons are members of the hadron family
  • There are two fundamental forces in the story so far: gravity and electromagnetism