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Introduction page 1
Periodic table
Picture 1. In 2002, there were 115 known element. However, those above 109 are highly unstable and have been made in only tiny quantities.
From atoms to The Standard Model
For over 2 000 years, people have wondered about the fundamental building blocks of matter. As far back as 440 BC, the Greek Leucippus and his pupil Democritus coined the term atomos to describe the smallest particle of matter. It translates to mean something that is indivisible.

In the eighteenth century, the chemist John Dalton, revived the term when he suggested that each element was made up of unique atoms and the atoms of an element are all the same. At the time, there were about 35 known elements. This simple model could explain the millions of different materials around us. Differences between the atoms give the elements their different chemical properties.

Fundamental (or not)
For a while then, it seemed that atoms were fundamental. As their name suggests, they could not be split into anything simpler. This meant that all of matter was made of about seventy fundamental particles (because there were new elements being discovered all the time).

However, towards the end of the 19th century, it became clear that atoms are not fundamental ~ they themselves are made of smaller particles.

One of these atomic particles is the electron, which we now think is fundamental. It is a member of the lepton family, one of the two families of fundamental particles. The other family is the family of quarks. The theory which describes these families and the forces between them is called The Standard Model. We will look at these families later in this resource.

The Standard Model
The Standard Model is a theory that explains what matter consists of and what holds everything together. In The Standard Model, there are 12 fundamental particles (6 leptons and 6 quarks) each with an associated anti-particle. The Universe is made up of combinations of these particles and all normal matter is made from just three of these particles.

Although physicists are now looking beyond The Standard Model, it still forms the basis of our understanding of the structure of matter. In this electronic resource (e-source) we will look at the main discoveries and ideas behind The Standard Model, describing the main theories and the reasoning behind them.

Contents
The e-source is split into 6 chapters:
  1. The nuclear atom
  2. Isotopes and decay
  3. Half life
  4. Antimatter
  5. Probing matter
  6. The Standard Model
Using this e-source
There are a number of interactive features in this e-source:

A glossary of terms: any word with a glossary entry is highlighted like that. Clicking on the word will open a new window with a definition of that word.

Quick questions: at the end of each page is a quick question to test your understanding of that page. Type in your own answer then click on the button to see how well you did.

Roll over diagrams: many of the diagrams have highlights or sequences. You can see these by rolling your cursor over part of the picture or part of the text. The text has a roll over highlight like that.

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