An atom consists of fast-moving electrons orbiting around a heavy nucleus. The nucleus contains neutrons and protons, which both have the same mass , but neutrons are electrically neutral, whereas protons have a positive charge. The electrons have very little mass and are negatively charged. Since an atom contains the same number of electrons as it does protons, it is electrically neutral. However, it is quite easy for an atom to lose an electron, and then it's called an ion. The important thing about an ion is that it has an electric charge (positive), which makes it behave very differently from atoms. A gas that contains ions is called a plasma.

Did you know?> What happens if an atom loses a proton or neutron? Protons and neutrons are very tightly held together in the nucleus, and so it is very difficult to break them apart. When it is done, the process is called fission and leads to an enormous release of energy. Nuclear power stations perform controlled fission to provide energy for people, while uncontrolled fission leads to the massive destruction caused by nuclear weapons.

Did you know? In everyday life it's not often that you find atoms by themselves. Instead you will tend to get molecules which consist of several (and sometimes thousands!) of atoms joined together. The Sun is so hot that any molecules get violently shaken about and the atoms break free from each other. However, sunspots are sufficiently cool to allow molecules to form sometimes and a few years ago some water molecules were actually found on the Sun!