Higher and foundation tiers
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.
However another Greek philosopher; Democritus developed a new theory to explain the composition of matter bases on atoms. 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 or atoms.
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.
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:
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 for the existence of the electron.
In the 1800s many scientists were investigating the effects of applying high voltages to gases at very low pressures inside glass tubes called discharge tubes. 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. Here a large voltage is applied to a glass tube with a negatively charged metal plate at one end and a positively charged metal plate at the other end.
When the high voltage was turned on Thomson observed that a bright green glowed covered the inside of the glass tube, similar in many ways to the glow produced in many modern advertising neon signs. The few gas particles that were present in the glass tube get broken down by the high voltage present and will conduct an electrical current, now the stream of particles, which Thomson called cathode rays were emitted from the negatively charged metal plate and were attracted to the positively charged metal plate at the opposite end of the glass tube. These cathode rays that Thomson observed were in fact electrons that were being pulled out of the metal atoms in the negatively charged metal plate by the high voltage across the glass tube. Some of the very fast moving electrons emitted from the negatively charged metal plate strike the sides of the glass tube causing it to glow, the tube can also be lined with a fluorescent screen which glows showing the path of the cathode rays or electrons inside the glass discharge tube.
The cathode rays can also be shown to have a negative charge by introducing two charged metal plates, one positively charged and one negatively charged sitting just outside the glass tube, as shown in the diagram below. Here the cathode rays (or electrons as we now know them to be) are attracted to the positively charged plate showing that the cathode rays contain negatively charged particles. The cathode rays are also deflected by a magnetic field in a way consistent with them have a negative charge. Following his work Thomson concluded that the cathode rays were indeed negatively charged particles that had been remove from the metal atoms present in the negatively charged metal plate, so atoms are not small indestructible particles after all! They can be broken up into smaller parts.
Thomson found that no matter which gas was placed in the glass tube or if a different
metal was
used as the cathode the same cathode rays
were always produced.
Thomson realised that these cathode rays as he called them had a
negative charge and they were present in all atoms. So atoms were not solid indestructible
balls after all.
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 negatively 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.
