atoms through history heading

Higher and foundation tiers

Early ideas

The Greek philosopher Empedocles suggested that all substances were made of a combination of four basic elements: fire, earth, air and water. He suggested that by changing the ratio of these four basic elements it was possible to change the properties of substances.

The ancient Greeks believed that all matter was composed of four basic elements, earth, air, fire and water.

However another Greek philosopher; Democritus developed a new theory of the composition of matter. However Democritus like all the Greek philosophers never carried any practical work or provided any evidence for his ideas.

Democritus' atomic theory was based on reasoning, logic and thinking. Democritus argued that if you cut a substance in half then each of the two halves had the same properties as the original whole substance. He then suggested that if you kept cutting the substance into smaller and smaller pieces you would eventually arrive at an infinitesimally small piece that could not be cut any further. He called these small infinitesimally small pieces of matter atomos.

Democritus' ideas for the existence of atomos or atoms are based on the idea that it is impossible to keep dividing matter infinitely, eventually you will end up at a atom which he believed could not be cut or divided any further. He also suggested that the atoms present in different substances were also different from each other and this gave different materials their different properties and textures. Unfortunately Democritus' ideas were not widely accepted or believed at the time.

Dalton's atomic theory

However Democritus' ideas were finally revisited by a number of scientists in the 17th and 18th centuries, including John Dalton.

Dalton was born in 1766 in the Lake District; England. He was interested in the sciences and 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:

Dalton was on the right track and for his time his theory were remarkable insightful and largely correct but ultimately he lacked the technology and resources to further his work. He also made an attempt at calculating the masses of some elements, though his calculations were not always correct.

The discovery of the electron

Portrait of J J thomsom working in his laboratory

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 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 Thomson is credited with providing proof of the existence of the electron.

Gases do not normally conduct electricity; however Thomson 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 called a cathode ray tube filled with a gas at very low pressure. There is a fluorescent screen at one end of the glass tube. 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.

Diagram of J.J. Thomson's cathode ray tube apparatus.

When working:

The plum pudding model of the atom

3d model of Thomson's Plum pudding model of the atom. What Thomson 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 metal 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 opposite charges will repel each other. 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.

Thomson 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.

Thomson developed a new model of the atom to explain his observations. Thomson's model of the atom is often called the plum pudding model. His idea of what an atom looks like is a sphere or cloud of positive charge in which are embedded the electrons, a bit like a plum pudding or chocolate chip cookie.

However as with any scientific model or idea it is only as good as the evidence available at the time. The next big breakthrough in working out the structure of an atom was taken by one of Thomson's student, Ernest Rutherford.

Key Points

Scientists models and theories change as new evidence becomes available

Practice questions

Check your understanding - Questions on Dalton's atomic theory and the Plum Pudding atom.

Check your understanding - Quick Quiz on atoms and the plum pudding model.