1. Atomic Structure A level at The Sixth Form College Colchester Adapted from: An example of How Science Works: Development of the Model of Atomic Structure

3. Early Ideas of Matter Polystyrene foam?

4. Early Ideas of Atoms Two Ancient Greeks (5 th Century BC): Look at matter on smaller and smaller scales: Ultimately you will see individual atoms - objects that cannot be divided further (the definition of atom or  ). Galileo and Newton both believed in atoms Lavoisier, Dalton and Avogadro found support for atoms in Chemistry

5. Billiard Ball Atoms The theory of atoms only became widely recognised when scientists could measure accurately the formulas of compounds. Knowing that water was formed from gases: 2 volumes of hydrogen and 1 volume of oxygen suggested that H 2 O was the formula and lead Dalton to hypothesise that water was made from 2 atoms of hydrogen joined to one atom of oxygen.

6. Atomic Structure – 1808 state of atomic knowledge (Dalton) –Matter made of atoms like billiard balls –Atoms are electrically neutral – Atoms react in simple whole number ratios e.g. H 2 O CO 2 MgSO 4

7. Electrical Matter Davy and Faraday realised that matter is electrical in nature as a result of experiments involving electrolysis Now the billiard ball did not look so good Where were the electrical charges? More investigations were carried out to find out what was electrically charged

8. Crookes 1875 Crookes passed an electric current through an almost-evacuated tube. He found that a beam travelled from the cathode towards the anode and made the glass fluoresce

10. JJ Thompson - 1897 He found that the beam of cathode rays was deflected towards the positive electrode when passed through an electric field. The cathode rays must be: Negatively charged. He used the term “electron” (first used a few years earlier) for these negative particles

11. Thompson’s experiment:

12. Plum Pudding Thompson (Nobel Prize 1906) came up with the plum pudding atomic model: Uniform sphere of +ve charge with electrons embedded inside

13. Goldstein - 1886 Discovered that positive rays were emitted behind the cathode in the cathode ray tube.

14. Positive Rays The mass of the particles in the positive rays depended on which element was in the Crookes tube The smallest positive rays were present when hydrogen was the initial gas. This smallest positive particle was eventually named the PROTON by Rutherford in 1914

15. Becquerel- Radioactivity 1896 One type of radiation discovered by Becquerel was Alpha particles These were found to be particles much smaller than atoms with a 2+ charge Rutherford and his co-workers decided to use a stream of alpha particles like bullets to probe the inner structure of atoms

16. Rutherford Scattering Alpha particle scattering –Rutherford aimed alpha particles at a thin foil –He expected all to go straight through. –But noted that some were deflected Manchester 1909 –Experiment performed just after Thomson (Rutherford’s old boss) published his “Plum Pudding” paper

17. Rutherford Scattering

18. Geiger and Marsden Continued with Rutherford’s work Scattered alpha particles with heavy metal foils, particularly gold

19. Geiger and Marsden Found that: Most alpha particles went straight through with very little deflection A few were deflected by large angles About 1 in 8000 was reflected This image is taken from a Java Applet at: http://www.scri.fsu.edu/~jac/Java/rutherford.html

20. Geiger Marsden Experiment

22. Disproof of the Pudding Rutherford calculated from the results – 1911: To reflect alpha particles the +ve charge (and most of the mass) has to be in a very small diameter About 1 x 10 -15 m compared to 1 x 10 -10 m for the diameter of the atom

23. Disproof of the Pudding

24. New Model: To explain the large size of the atom and the very dense nucleus the next model had both protons and electrons in the central nucleus and orbiting electrons.

25. Chadwick 1932 It took Chadwick’s discovery of the neutron to produce the more modern version of the atom: A nucleus containing protons and neutrons with electrons orbiting in shells. Even this is not the full story ………

26. Solar System Model Positively charged nucleus at centre Negatively charged electrons in orbit Problem – –Orbiting electrons are accelerating – –Will give off energy – –Will spiral in to centre Model not stable

27. Bohr Atom 1913 Bohr presented his theory (Nobel Prize 1922) Electrons in atoms can exist ONLY in certain discrete orbits, and they do not radiate energy When an electron jumps from one orbit to another its energy is exactly equal to the energy difference between the orbits Quantum theory was then the focus of research to explain the structure in more accurate detail

28. Atomic Structure Atoms are approx 1 x 10 -10 m in diameter Atoms consist of a positively charged nucleus surrounded by orbiting electrons The nucleus is approx 1 x 10 -15 m in diameter Most of the atom is empty space

29. Nuclear Structure The nucleus is made up of two particles (collectively called NUCLEONS) Protons and neutrons –Protons are +ve in charge –Neutrons are neutral

30. Atomic Particles Relative ChargeRelative Mass Electron1/1840 Proton+11 NeutronNo charge1

31. Describing the Nucleus We describe the nucleus by noting its Chemical Symbol along with –The number of protons Z –The number of nucleons A Each chemical symbol always has the same number of protons –Hydrogen – 1 –Helium -- 4 –Carbon – 6

32. Describing the Nucleus indicates a lithium NUCLEUS with its usual 3 protons and a total of 7 nucleons (4 neutrons) indicates a carbon NUCLEUS with its usual 6 protons and a total of 12 nucleons (6 neutrons)

33. Isotopes These two represent isotopes of carbon –Chemically they would behave identically (if they are combined with 6 electrons to make atoms) –To a physicist they are different Different masses Different behaviour –They can be separated by physics, not by chemistry