The structure of the nucleus

Scientists once thought the most fundamental building block of matter was a particle called the atom. Now we know that the atom is made of many smaller pieces, known as subatomic particles.

Every atom contains a very small, dense, central core called the nucleus. Apart from hydrogen, the nucleus of every other atom is made up of particles called protons and neutrons. The nucleus is surrounded by mostly empty space, except for very tiny particles called electrons that orbit the nucleus.

One way to picture the hydrogen atom is to think about a large sports stadium. Imagine a grain of rice placed in the centre of the field. This represents the nucleus. The outer row of seats in the stadium is the limit of the electron’s influence. The rest of the atom is empty space. The electron seems to be everywhere at once like the seats surrounding the playing area.

Quarks

Scientists now believe that protons and neutrons are made of even smaller particles known as quarks.

Quarks are thought to come in a variety of forms. Protons and neutrons are thought to be each made up of three quarks arranged in a slightly different way. The quarks are bonded very tightly together by another type of particle called a gluon. The gluons effectively ‘glue’ the quarks together. Gluons are thought to be responsible for the strong nuclear force that binds the nucleus together. They are called force carrier particles.

In terms of size, picture a large city, like Christchurch, with people moving in the central city square:

• The outer boundary of the city is the limit of the atom.
• The central city square is the nucleus.
• The people in the city square are the protons and neutrons.
• Freckles on the faces of the people are the quarks.

Today, physicists don't know of anything smaller than quarks and electrons, but they don't know for sure whether these are the simplest building blocks of matter.

While we can't see the particles themselves, physicists have designed ingenious experiments that allow them to see the paths, or tracks, of moving particles. Just as skid marks on a road can tell you about a car's behaviour just before an accident, particle tracks tell scientists a lot about how the building blocks of matter behave.

In fact, particle tracking has allowed physicists to identify more than a hundred different kinds of particles and learn important information about them – such as their size and mass, how they interact with other particles and their role in the universe.

Probing more deeply

A series of experiments has been planned for the Large Hadron Collider, the world’s largest particle accelerator. The data obtained will, on careful analysis, allow scientists to gain a deeper understanding of the origin of the universe as well as the structure of matter.

The experiments being conducted at the Large Hadron Collider will allow physicists to probe even more deeply into atomic structure. The circulating particles in the collider can be raised to extremely high energies. When the particles are allowed to collide, a state of energy-matter that existed in the initial few microseconds following the Big Bang origin of the universe will be formed. The data collected from the collider will be used to confirm or dispute some current theories physicists have about atomic structure and how atoms were formed in the instant of time after the Big Bang.