The History of the Atom

1. The History of the Atom

2. Democritus(c.460-c.370 BCE) • Democritus was a philosopher in ancient Greece. • He thought that all matter was made of tiny particles that could not be divided. • The Greek word atomos means ‘indivisible’.

3. John Dalton(1766-1844) • Dalton was an English scientist. • He developed modern atomic theory. • His model of the atom is sometimes called the ‘billiard-ball model’.

4. Dalton’s Billiard-Ball Model • All matter is made of tiny particles called atoms. • Atoms of the same element are identical. • Atoms can combine to form compounds. • Chemical reactions change the grouping of atoms, but not the atoms themselves.

5. Sir J. J. Thomson(1856-1940) • Thomson was a British physicist. • He did experiments on cathode rays and discovered the electron. • In 1906, he was awarded a Nobel prize for his discovery. • His model of the atom is called the ‘plum-pudding model’.

6. Thomson’s Plum-Pudding Model • Dalton realised that negatively charged electrons could come from an atom. • He proposed the ‘plum-pudding’ model of the atom, suggesting that atoms consist of negatively-charged electrons in a ‘sea’ of positive charge.

7. Ernest Rutherford(1871-1937) • Rutherford was a chemist from Nelson, New Zealand. • Based on the results of his gold-foil experiment, he proposed that most of the mass of an atom is concentrated in a central nucleus. • He won the Nobel Prize for Chemistry in 1908 for his work.

8. Rutherford’s Gold-Foil Experiment • In his famous gold-foil experiment, Rutherford fired alpha particles at a thin sheet of gold foil. • He found that some particles were deflected through large angles, and some even bounced back.

9. Rutherford’s Nuclear Model Rutherford concluded that: • most of the mass in an atom must be in a very small, positively-charged nucleus in the centre of the atom • electrons spin around this central nucleus • there was a basic unit of positive charge in the nucleus, called the proton.

10. Louis de Broglie (1892-1987) proposed that electrons can be regarded as waves, resulting in an ‘electron cloud’ around the nucleus. Sir James Chadwick (1891-1974) discovered the neutron, a nuclear particle with similar mass to a proton but no electrical charge. Niels Bohr (1885-1962) realised that the electrons could only occupy fixed orbits around the nucleus.

11. References Atom Image: http://www.turbosquid.com/FullPreview/Index.cfm/ID/197928 Democritus: http://smccd.net/accounts/goth/MainPages/Chron/Democritus.jpeg Dalton: http://en.wikipedia.org/wiki/John_Dalton Thomson: http://en.wikipedia.org/wiki/J_J_Thomson Plum-Pudding Atom: https://reich-chemistry.wikispaces.com/file/view/348px-Plum_pudding_atom_svg.png Ernest Rutherford: http://en.wikipedia.org/wiki/Earnest_Rutherfordhttp://nobelprize.org/nobel_prizes/chemistry/laureates/1908/rutherford-bio.html Gold-Foil Experiment: http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter02/Text_Images/FG02_05.JPG Nuclear Atom: http://upload.wikimedia.org/wikipedia/commons/thumb/e/e1/Stylised_Lithium_Atom.svg/180px-Stylised_Lithium_Atom.svg.png Electron Cloud: http://www.csmate.colostate.edu/cltw/cohortpages/viney_old1/atom.jpg