Atoms history 1
The story of particle physics goes back 2000 years to the Greeks; and Isaac Newton thought that matter was made up of particles in the 17th century. However, it was John Dalton who formally stated in 1802 that everything is made from tiny atoms.
Aerial view of CERN Over the last two centuries scientists have attempted to discover what is in these atoms and how they came into being. Their motivation is still the same as Dalton’s – to find simple patterns that explain the complexity of nature. Their experiments have always been ingenious but have now outgrown the laboratory using enormous accelerator tunnels that are the size of a small town.
The photo shows an aerial view of the path of the accelerator tunnel at CERN which crosses the French/Swiss border. The tunnel is 27 km long. As well as discovering new particles, the scientists at CERN developed the technology that has now become the Internet.
The first particle model
In the eighteenth century, chemists began finding elements – substances that could not be made any simpler using chemical reactions. John Dalton suggested that these elements were made up of atoms and that the atoms of an element are all the same. The properties of the atoms determine the way that an element behaves and reacts. At the time, this meant that there were about fifty different types of particle – one for each known element . This simple model could then explain the millions of different materials around us
The pattern period
In 1869 Dmitri Mendeleev, a Russian chemist, grouped the elements into periods according to their chemical properties. However, his periodic table had gaps in it. Mendeleev was not put off; instead he used his periodic patterns to predict the existence and properties of the missing elements. Within a few years, germanium and gallium, for example, were discovered with the predicted properties. Similarly, a hundred years on, physicists were predicting the existence and properties of new fundamental particles, which were later discovered.
The families of hadrons and how they led to the proposal of quarks
Beyond the visible
When Dalton proposed the existence of atoms, he began an on-going quest into the invisible. He could never see an atom. And we can never see an atom either - however powerful our microscopes. This is because they are smaller than the wavelength of visible light. They have no effect on light, which goes straight past them. So how can we find out what is inside an atom?
Explaining the patterns
At the beginning of the twentieth century, chemists started to probe the structure of Dalton’s atoms. They could work out the number of electrons in an atom and found that this corresponded to its atomic number. However, while they were beginning to discover what is inside an atom, they did not know how it was arranged.
Cathode rays
The first clue came in 1897 when J J Thomson showed that cathode rays were a stream of electrons. This suggests that electrons are near the edge of an atom because they are easily removed.
Cathode ray tube with roll over highlightsA modern cathode ray tube
The cathode ray tube
A cathode ray tube has a small heater element sealed inside a vacuum. A large voltage is connected between the element and a small electrode a few centimetres away. There is a fluorescentscreen on the other side of this electrode. If the element is connected to the negative (cathode) then we can see a bright dot on the screen. The element is giving out an invisible beam which is attracted to the positive electrode, passes through it and hits the screen. The beam was known as a cathode ray.

In the 1890s, scientists knew that the beam must be negatively charged because it was attracted by the positive electrode. Thomson went a stage further and showed that it was made up of particles and that all of the particles were the same. These particles were named electrons. Later, it was shown that electrons have a mass much smaller than that of an atom and that all atoms contain electrons. We now think that the electron is a fundamental particle – the first one was discovered just over a hundred years ago.

The story so far
The Greeks and Newton thought that matter was made of particles
Dalton proposed tiny indivisible particles called atoms
Thomson showed that atoms contained electrons that were removed easily
Question H1

a) Why do scientists try to find smaller and smaller particles?

b) Which was the first sub-atomic particle to be discovered and by whom?

c) How did the scientists at CERN help you to see this e-source?

The story so far
  • The Universe is contains matter and radiation
  • Physicists are trying to work out what these consist of and how they behave with each other
  • They are constantly trying to make the model of matter as simple as possible