1. Atomic Structure A level At The Sixth Form College Colchester

3. Early Ideas of Atoms Two fifth-century BC Greeks, –Said look at matter on smaller and smaller scales ultimately you would see individual atoms - objects that could not be divided further (the definition of atom). Galileo and Newton both believed in atoms Lavoisier, Dalton and Avogadro found support for atoms in Chemistry

4. Atomic Structure 1900 – state of atomic knowledge –Matter made of atoms –Atoms are electrically neutral –Atoms contain -ve electrons -- which were identified by JJ Thomson in 1897 –Atoms must contain +ve charge

5. Atomic Model Plum Pudding –Put forward by Kelvin in 1903 and supported by JJ Thomson –Uniform sphere of +ve charge with electrons embedded inside

6. Rutherford Scattering Alpha particle scattering –Rutherford aimed alpha particles at a thin foil and noted that some were deflected Manchester 1909 –Experiment performed just after Thomson (Rutherford’s old boss) published his “Plum Pudding” paper

7. Rutherford Scattering

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

9. Geiger and Marsden Found that while most alpha 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

10. Geiger Marsden Experiment

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

12. Disproof of the Pudding

13. 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

14. 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 it emits a single photon, its energy is exactly equal to the energy difference between the orbits

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

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

17. Atomic Particles ChargeMass Electron-e -1.6 x 10 -19 C 9.11 x 10 -31 kg Proton+e +1.6 x 10 -19 C 1836 m e NeutronNo charge1839 m e

18. 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

19. Describing the Nucleus indicates a helium NUCLEUS with its usual 2 protons and a total of 4 nucleons (2 neutrons) indicates a carbon NUCLEUS with its usual 6 protons and a total of 12 nucleons (6 neutrons)

20. 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