Inside the Atom

Most of our knowledge about the ‘building blocks’ of matter that we call atoms is less than 100 years old. But the idea that matter was made up of atoms was first suggested about 2500 years ago by the great philosopher and teacher Democritus. Since then, various theories and models of the atom have been accepted, rejected and modified. The flowchart below shows some of the important developments in our knowledge of the atom.

The model of the atom (click on the model to find out more) accepted today consists of a tiny, dense nucleus, made up of protons and neutrons, which is surrounded by electrons. It provides us with an explanation of many observable phenomena. However, it is likely that the model will continue to change or become more detailed as scientists continue with their research.

The energy emitted by radioactive substances is called nuclear radiation because it comes from the nucleus. Lord Rutherford showed that there were three different types of nuclear radiation: alpha particles, beta particles and gamma rays.

Alpha particles are helium nuclei that contain two protons and two neutrons. Alpha particles are positively charged. They cannot travel easily through materials and can be stopped by a sheet of paper or human skin. They pose little hazard to the external body but can cause serious damage if breathed in, eaten or injected. The symbol for alpha particles is α.

Beta particles are the same size and mass as electrons, can have a negative or positive electric charge and can travel at speeds as high as 99 per cent of the speed of light. Beta particles can penetrate human skin and damage living tissue, but they can not penetrate thin layers of plastic, wood or aluminium. The symbol for beta particles is β.

Gamma rays are not particles, but bursts of energy released after alpha or beta particles are emitted. Gamma rays travel at the speed of light and are highly penetrating. They can cause serious and permanent damage to living tissue and can be stopped only by a thick shield of lead or concrete. The symbol given to gamma rays is γ.

The isotope carbon-14 has a half-life of 5700 years. Radiometric dating with carbon is called radiocarbon dating. All living things contain the element carbon. A small amount of the carbon is radiocarbon. As long as organisms are alive, carbon (along with radiocarbon) is being replaced. Plants take in carbon dioxide, animals eat plants, and micro-organisms consume plant and animal matter or each other. All living things, therefore, contain a small amount of radiocarbon. When living things die, the decaying radiocarbon is no longer being replaced. Since all fossils were once living, their age can be determined by measuring the amount of radiocarbon remaining. After 5700 years, only half of the usual amount of radiocarbon will be left. A graph can be used to estimate the age of a fossil. After about 50 000 years, the amount of radiocarbon becomes too small to measure accurately.

Half-life Graph

Number of half-lives	Fraction remaining
1	1/2
2	1/4
3	1/8
4	1/16
n	1/2ⁿ

Half-life Table

Harry's Lab

Current Model of the Atom

Stability and Change Inside the Nucleus

Measuing Radioactivity

Three of a kind

The lives and half-lives of radioisotopes

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