Atoms - the inside story. First steps into the atom.

1. The nuclear atom

page 2

	First steps into the atom

		As peviously described, the Greeks defined atomos to mean something that cannot be split. In the 18th century, the term atom began to be used in a scientific (rather than philosophical) sense to describe the unique particle that is the building block of a given element. Again, it was assumed that it could not be split. However, by the end of the 19th century, a series of observations contradicted the belief that the atom was the fundamental particle of matter. These observations came from: electron beams (thermionic emission of electrons) line spectra.

Cathode ray tube with roll over highlights

Picture 1.1 In 1897, J J Thomson showed that cathode rays were a stream of electrons. See page 18 for more detail.

Electron beams
In 1897, J J Thomson investigated thermionic emissions. These emissions occur when a heated metal element releases an invisible beam. This beam can be fired at a a fluorescent screen by placing a small electrode a few centimetres away from the heated element and connecting a voltage between the element and the electrode. The electrode has to be positive (anode) and the element has to be negative (cathode). The invisible beam produces a bright dot on the screen. The beam is known as a cathode ray.

Because the beam is attracted to the anode, Thomson deduced that cathode rays were a stream of negative particles. He called these electrons. This suggests that electrons have been released from the metal atoms. Later, it was shown that all atoms contain electrons i.e. the electron is one of the building blocks of a non-fundamental atom.

We now believe that the electron itself is a fundamental particle.

Picture 1.2 A discharge tube. Atoms are broken up by fast moving electrons.

Picture 1.3 Part of the line spectrum of stronium, showing discrete strips of colour.

Line spectra
When light passes through a vapour of an element (or a gas, such as helium), some of the light is absorbed. The absorbtion happens at specific wavelengths, i.e. the absorbtion spectrum is a continuous spectrum with gaps in it. These gaps are always the same for a given element.

We can also get a sample of an element to give out light by heating it or passing an electric current through it in a discharge tube. The light they give out does not form a continuous spectrum. Instead, there are discrete lines in the spectrum. Each line corresponds to the release of a quantum of energy from the atom.

This suggests that atoms must have excited states. They can absorb energy to push them up to an excited state (in which they have more internal energy). They then release energy (often as light) when their internal energy drops.

A system whose internal energy can change must be made up of two or more particles with a force between them. This leads us to the conclusion that atoms must be made up of other, simpler particles. They are not fundamental.

	Take a look inside

		It would be useful to look inside an atom to understand its structure. But this is impossible because an atom is smaller than the wavelength of visible light. So other ways are needed to determine what they are made of. It was Ernest Rutherford who devised a method of probing the atom using alpha particles. We'll look at this on the next page.