The 19th century scientists achieved a great deal and it was tempting for them to believe that the ultimate truth of the nature of matter was just around the corner. They had great confidence in the wonderful structure which the Periodic Table had given to chemistry. The truth was, however, that the unsplittable atom, which Dalton believed was the fundamental particle of matter, had hardly begun to reveal its secrets. For the scientists of that time there were, of course, some questions still to be answered. For instance, what decided an atom's chemical properties?
Some believed that shape was important and imagined that molecules contained atoms of different shapes. Oxygen was considered to be like a doughnut which could hold two ballshaped atoms of hydrogen at its center.
Many models such as this were suggested, but none could explain all the facts.
Then, just before the end of the century, chemistry received a severe jolt. Professor Henri Becquerel (1852-1908), working in Paris, had as his assistant a young Polish girl named Marie Sklodowska. One day, in the laboratory, she used some old pieces of uranium to weigh down photographic plates. When the plates were developed, they revealed a curious pattern of lines. There was little doubt that the lumps of uranium were responsible. Becquerel experimented with his uranium rays and found them to be in some way rather similar to the X-rays discovered by Wilhelm Rontgen (1845-1923) only a year before.
Marie Sklodowska, fascinated by these 'radioactive' rays, decided to write a thesis on them, and so began one of the most remarkable investigations in the history of science. The young assistant, who is better known today as Marie Curie (1867-1934), devoted her whole life to the study of natural radioactivity and, together with her husband Pierre, discovered two new elements which she called polonium and radium.
These new and very strange elements emitted three types of rays, named after letters of the Greek alphabet: alpha rays, which do not appear to be very energetic; beta rays, which move quite fast and were soon to be identified as electrons; and gamma rays, which appeared to be very similar to X-rays. What was really amazing was that her radioactive elements were turning themselves into new elements as a result of the radioactive emissions - doing of their own accord what the alchemists of old had tried to do for centuries.
The important discovery from the chemist's point of view was the fact that the alpha rays closely resembled atoms of the gas helium. How could an atom of one element suddenly be shot like a bullet out of the center of another when atoms were supposed to be unsplittable?
This single piece of evidence destroyed for ever the belief in an indestructible atom, and made scientists realize that their billiard ball model, though useful in explaining many phenomena, had not enough detail to enable them to understand the principles behind natural radioactivity.
Modern scientists will never claim that all is now known to them, or that their models are in any way perfect. In the light of experience, they realize that scientific progress in any one branch of science is almost impossible without corresponding progress in another.
One science feeds another with new ideas and all are inter-related.
Chemistry, in the latter half of the 19th century, had to wait until the science of electricity was firmly established before it could progress further.