Discovering the Structure of the Atom

Chronological

Greeks

1. Thales (624-546 BCE): Thought that the world was formed from water2. Anaximenes (585-528BCE): Believed that everything was made up of air3. Democritus (460-370BCE): Suggested that an atom is an ‘indivisible’ particle and concluded that everything

was made from atoms. They were of different shapes and sizes and joined to make more substances.4. Aristotle: Said that everything was made up of air, water, earth and fire. This idea was used for a long time.

French

5. Antoine Lavoisier (1743-1794): Set out his ‘Law of Conservation of Mass’ which stated that matter is neither can be created or destroyed in a chemical reaction.

6. Marie-Anne Pierrette Paulze (1758-1836): Helped her husband, Antoine Lavoisier, by translating other scientists’ work into French and made sketches of new pieces of apparatus that were invented for his investigations.

7. Joseph Proust (1754-1826): Devised the ‘Law of Definite Proportions’ which suggested that the elements in a compounder always present in definite proportion hi no matter how it is made.

English

8. John Dalton (1766-1844) : From his observations and from studying Lavoisier and Proust’s work, he put together his atomic theory which suggested that

a. All matter is composed of atoms.b. Atoms cannot be divided or destroyed.c. All atoms of an element have same mass and properties.d. Atoms of different elements have different mass and properties. e. Atoms combined in simple whole numbers when they form compounds.

9. William Prout (1785-1850): Studied the atomic weights of different elements and thought he could use them to explain the structure of atoms. Suggested that all elements were made up from different number of hydrogen atoms. This idea made later scientists think that atoms might have a structure within them.

10. Joseph J. Thomson: He investigated cathode rays and discovered that they were up of tiny particles called electrons. When he used different materials for the cathode, he found out that the electrons produced were the same. In 1904 he invented a model of the structure of the atom from his studies on electrons. He described the atom like a plum pudding with the negatively charged electrons with the positively charged ‘pudding’.

11. Ernest Rutherford (1871-1937): Worked with J. J. Thomson and they studied radioactive materials and radiation with Paul Viliard (1860-1934). They discovered the three types of radiation – Alpha particles, Beta

Particles and Gamma rays. Rutherford found that Alpha particles were much larger than electrons and positively charged so he used them to test Thomson’s idea about the plum pudding. He concluded that that an atom did not have a positively charged Pudding but instead a positively charged Nucleus which was surrounded by negatively charged electrons. Later he suggested that the unknown particle was a neutral doublet which consisted of a Proton and an electron.

12. James Chadwick (1891-1974): Fired alpha particles at beryllium atoms and knocked out particles that had a similar mass to protons, but no electrical charge. He had discovered the neutral doublet and he called it Neutron. Further research on this particle showed that it was not composed of a proton and an electron but was a particle with a similar structure to a proton and without electrical charge.