HSW: The development of a tomic theory

1. HSW: The development of atomic theory

2. Democritus I call them ATOMS The very first Atomic theory was put forward by around 400 BC! Democritus believed all matter was made up of tiny particles that could not be split up into anything smaller, and that between atoms was nothing but space. Greek Dictionary atomos: Cannot be cut.

3. The first atomic theory John Dalton proposed the first atomic theory in 1803, for the first time based on actual experiments instead of ideas! The main points • Atoms (think snooker balls!) are indivisible and indestructible • Atoms of the same element have the same mass and chemical properties • When atoms react they join together to form COMPOUNDS

4. The Plum Pudding Atom JJ Thomson discovered the electron in 1897. It was the first evidence that the atom itself must have some kind of structure. Thomson’s plum pudding model was widely accepted. However it did not explain all the things that were known about atoms and elements at the time. It was superceeded by Rutherford’s model of the atom 12 years later. He first thought that the hydrogen atom must contain around 2000 electrons to account for its mass! As atoms are neutral he suggested that the electrons were embedded in a positive cloud, with the electrons being like plums baked into a positively charged pudding!

5. Rutherford’s Experiment • In 1909 Geiger and Marsden were working for Rutherford at Manchester University. • They firedα particles at a thin gold foil. • The particles were scattered, but a tinyamount were bounced backwards. • The plum pudding model could not account for this….

6. Rutherford’s Atom • Rutherford was amazed at the results from the experiments he said “it was like firing a cannon at tissue paper and it bouncing off!” • The only way he could make this work was if there was a tiny dense nucleus with a positive charge, surrounded by orbiting electrons

7. Rutherford Strikes again! • Rutherford carried out more experiments with alpha particles. • When he bombarded nitrogen with α particles, he detected hydrogen nuclei had been formed (and an isotope of oxygen) • The only place the hydrogen nucleus could have come from was the nitrogen gas. • He deduced that the hydrogen nucleus must be an ELEMENTARY PARTICLE the proton! α H O

8. The Neutron • Rutherford knew that the electron did not really contribute to the mass of the atom. • The positively charged nucleus contained protons, but the mass of the proton was not enough to make up the known masses of the atoms. • He decided that there must be another particle in the nucleus, uncharged, about the same mass as the proton to account for the masses of the elements, but other scientists did not agree with him until…..

9. James Chadwick …… (another of Rutherford’s team!) fired α particles at Beryllium atoms. • No radiation was detected BUT something emitted by the beryllium atoms caused paraffin wax to emit protons. • Rutherford said something undetectable emitted from the beryllium atoms must be causing the protons to be knocked out of the paraffin. Like an invisible man in a crowd, seen as he bumps into other people. • THE NEUTRON was discovered! (in 1932)

10. James Chadwick …… (another of Rutherford’s team!) fired α particles at Beryllium atoms. • No radiation was detected BUT something emitted by the beryllium atoms caused paraffin wax to emit protons. • Rutherford said something undetectable emitted from the beryllium atoms must be causing the protons to be knocked out of the paraffin. Like an invisible man in a crowd, seen as he bumps into other people. • THE NEUTRON was discovered! (in 1932)