1. Atomic Structure A level AtThe 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

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