1. History of the Atom

2. Democritus of Abdera Democritus: 460-370 BCE Greek mathematician and philosopher Co-author (with Leucippus) of the idea that all matter is made of small, indivisible particles called atoma Proposed that the behavior of substances were due to the type of atom

3. Alchemists Great many 'experiments' to try to convert one element to another Many discoveries, but very secretive

4. Robert Boyle Robert Boyle: 1627-1691 Performed experiments with controls and published detailed results 1661: Published claiming that Aristotle's ideas were incorrect, and that some 'elements' (like mercury) did not contain the 4 Aristotlean elements (earth, air, fire, water)

5. Antoine Lavoisier Lavoisier: 1743-1794 Experimented and established the Law of Conservation of Mass Investigated combustion and cellular respiration in terms of energy and mass

6. Charles de Coulomb Coulomb 1736-1806 Developed Coulomb's Law, the definition of electrostatic force

7. Joseph Proust Proust: 1754-1826 Controversy with another scientist (Berthollet) led him to discovery of the Law of Definite Proportions (sometimes called Proust's Law) A chemical compound always contains the same proportion of elements by mass There are exceptions, which are called non- stoichiometric (or Berthollide) compounds

8. John Dalton Dalton: 1766-1844 Law of Multiple Proportions (1803): elements combine in mass ratios of small whole numbers. Elements may combine with the same element in different proportions to make a different compound. Atomic theory contains 4 hypotheses

9. Atomic Theory 1. Elements are made of small particles called atoms. (Dalton viewed atoms as small spheres of differing characteristics.) 2.All atoms of an element are identical. All atoms of one element are different from atoms of other elements. 3.Compounds are made of atoms of more than one element. The ratio of the elements is a simple fraction. 4.A chemical reaction involves separation, combination or rearrangement of atoms.

10. James Clerk Maxwell 1831-1879 Showed that electricity and magnetism were related, and were related to atoms Predicted that accelerating charges would make waves (electromagnetic radiation)

11. Cathode Ray Tube Experiments Glass tube with wire at each end; as much air pumped out as possible Charge passed across tube makes fluorescent glow William Crookes  Tube coated with fluorescent material can be made to glow in one focused dot  Rays travel in straight lines Ray carries negative charge

12. Joseph John Thomson 1856-1940 Used a study of the cathode ray tube to determine the presence of electrons 1897 Suggested the plum pudding model of the atom and the existance of isotopes Won the Nobel Prize in Physics in 1906

13. J. J. Thomson’s 4 Experiments Used magnetic field to show cathode rays had negative charge Used electric charge to show cathode rays were particles with negative charge Used varying electric currents to determine charge to mass ratio  Either particles very small or  Carried huge charge  Evidence suggested particles very small and came from atom

14. Robert A. Millikan 1858-1953 Used the 'falling drop method' to determine the charge of the electron (- 1.6022 x 10 -19 C) and mass of electron as 9.10 x 10 -28 g Investigated photoelectric effect and spectroscopy of elements Won the Nobel Prize in Physics in 1923

15. Wilhelm Röntgen 1825-1943 Rays produced by cathode ray tube interaction with other matter could pass through objects, affect photographic plates and cause some materials to fluoresce (X-rays) Nobel Prize in Physics in 1901

16. Antoine Henri Becquerel 1852-1908 Investigated possible link between X-rays and natural phosphorescence Discovered that the rays emitted by uranium were different from X-rays Nobel Prize in Physics 1903 (shared with Pierre and Marie Curie)

17. Marie Curie 1867-1934 Worked with huband Pierre to study radioactivity Isolated elements polonium and radium Shared Nobel Prize in Physics 1903, won Nobel Prize in Chemistry 1911 for work with radioactive elements.

18. Ernest Rutherford 1831-1937 Investigated release and measurement of different forms of radiation from elements (α, β, γ)‏ Gold foil experiment with Geiger and Marsden led to new model of an atom with dense central nucleus and vast empty areas Nobel Prize in Chemistry 1938

19. Model of the atom in the early 1900s Nucleus: has positive charge, very small, but very dense Proton: same charge as electron, but opposite. Mass=1.67262 x 10 -24 g Vast empty space around nucleus contains electrons

20. James Chadwick 1891-1974 Discovered the neutron and worked on fission of atoms Discovery of neutron (mass=1.67493 x 10 - 24 g) explained the mass problem of many atoms Nobel Prize in Physics 1935

21. Mass and Atomic Number Each element's atoms have a specific number of protons in the nucleus (Z=atomic number)‏ For a neutral atom, the number of protons is equal to the number of protons The mass number (A) is the number of protons plus the number of neutrons (the number of particles in the nucleus). Some atoms of the same element have different A values. This means they have different numbers of neutrons. They are called isotopes.

22. Isotope notation Isotope notation is used to represent an isotope. Z, A and symbol of element are represented. Isotope notation can be used to determine the number of protons, neutrons and electrons in an isotope.