Early ideas

The idea of atoms is not a new one! The Greeks philosopher Democritus suggested over 2500 years ago that matter was made up of solid tiny balls called atoms. Democritus thought of atoms as unbreakable spheres. These ideas about atoms changed little for thousands of years and it is really only in the last 120 years or so that any real insight was made into the nature and structure of atoms. One of the first pioneers on this journey was John Dalton.
Dalton was born in 1766 in the Lake District, England. He was interested in the sciences, in particular chemistry, physics and meteorology. Dalton is best known for his atomic theory and his gas laws, which you will probably meet in your physics course. According to Dalton's atomic theory:

• All substances are made up of atoms and the atoms of any one element are all identical.
• Atoms of different elements are different and have different properties.
• Atoms of different elements have different masses.
• Compounds are formed when atoms of more than one element join. The atoms in any one compound are always present in a fixed ratio.

The discovery of the electron

Sir John Joseph Thomson was professor of physics at Cambridge University in 1884. Thomson is credited with the discovery of the electron. The idea that atoms were not made of indestructible solid balls had been around for around 30 years or so and the idea of electrons had been around since 1874 when G.J Stoney suggested the name to explain some observations in his work. But Thompson is credited with providing proof of its existence.
Now gases do not normally conduct electricity, however Thompson was investigating how their conductivity changed at very low pressures when very high voltages were applied across them. He set up an experiment similar to the one shown below. It basically consists of a glass tube filled with gas at very low pressure, with a fluorescent screen at one end. Half way up the tube were 2 charged plates, one positively charged and one negatively charged. At the other end was a piece of metal which was connected to the negative terminal of a power supply, this was the cathode. A metal disc with a slit in it was connected to the positive terminal of the power supply. A diagram of his apparatus is shown below.

The plum pudding model of the atom

What Thompson observed while running the experiment was a green glow from the fluorescent screen where a "beam" or "ray" (shown as blue dots in the diagram) coming from the cathode had struck it. He noticed that the "ray" was deflected or bent away from the negatively charged plate. This told him that the "ray" had a negative charge. Since these rays were coming from the cathode he called them cathode rays!. Calculations showed that these cathode rays were over 1840 lighter than a hydrogen atom and that no matter which gas was placed in the tube or if a different metal was used as the cathode they were always produced.

Thompson realised that these "corpuscles", as he called them had a negative charge and they were present in all atoms. So atoms were not solid indestructible balls after all. The name corpuscles was later changed to the electron.

Thompson developed a new model of the atom to explain his observations. Thompson's model of the atom is often called the plum pudding model. His idea of what an atom looks like is a sphere of positive charge in which are embedded the electrons. A bit like a plum pudding or chocolate chip cookie. Thompson's calculations had shown that the electron was 1/1840^th the mass of a hydrogen atom.

Key Points

• Scientific models change and develop with new ideas, thoeries and discoveries. Early ideas considerd atoms as solid spheres that could not be split.
• J.J Thompson showed through a series of simple but ground breaking experiments that atoms were not solid spheres but were made up of smaller particles. The idea that atoms were solid spheres was abandoned and the plum pudding model of atomic structure took its place.
• The plum pudding model considered atoms as spheres of positive charge with negatively charged electrons spread through the positively charged sphere.



Practice questions

Check your understanding - Plum Pudding atoms

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