1.2 Atomic Structure (Time needed: 6 class periods)

1. 1.2 Atomic Structure(Time needed: 6 class periods)

2. Learning outcomes • Matter is composed of particles, which may be atoms, molecules or ions. • Atoms. Minute size of atoms. • Law of conservation of mass.

3. DIFFUSION- evidence for the existence of small particles • SPREADING OUT OF GASES • COLOUR OF INK SREADING OUT WHEN MIXED WITH WATER • HYDROGEN CHLORIDE AND AMMONIA SOLUTION

4. AMMONIUM CHLORIDE

5. law of conservation of mass/matter • The law of conservation of mass/matter, also known as law of mass/matter conservation says that the mass of a closed system will remain constant, regardless of the processes acting inside the system. • Matter cannot be created/destroyed, although it may be rearranged. • For any chemical process in a closed system, the mass of the reactants must equal the mass of the products.

8. Learning Outcomes • Very brief outline of the historical development of atomic theory (outline principles only; mathematical treatment not required): Dalton: atomic theory; • Crookes: vacuum tubes, cathode rays; • Stoney: naming of the electron; • Thomson: negative charge of the electron; e/m for electrons (experimental details not required); • Millikan: magnitude of charge of electrons as shown by oil drop experiment (experimental details not required); • Rutherford: discovery of the nucleus as shown by the α−particle scattering experiment; • discovery of protons in nuclei of various atoms; • Bohr: model of the atom; • Chadwick: discovery of the neutron.

9. - e + - e + e - e + + + + e - + e e - e + e + e Models of the Atom "In science, a wrong theory can be valuable and better than no theory at all." - Sir William L. Bragg Greek model (400 B.C.) Dalton’s model (1803) Thomson’s plum-pudding model (1897) Rutherford’s model (1909) Charge-cloud model (present) Bohr’s model (1913) Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

10. HISTORY OF THE ATOM • GREEKS – MATTER MADE OF TINY INDIVISIBLE PARTICLES

11. DALTON 1766-1844 • ALL MATTER MADE OF SMALL PARTICLES CALLED ATOMS • ATOMS ARE INDIVISIBLE • ATOMS CANNOT BE CREATED OR DESTROYED

12. Dalton’s Symbols John Dalton 1808

13. DISCOVERY OF THE ELECTRON • CROOKES CONDUCTED EXPERIMENTS WITH A GLASS TUBE • Go to video clip

14. CROOKES TUBE

15. CROOKES TUBES • CATHODE CONNECTED TO NEGATIVE ELECTRODE • ANODE CONNECTED TO THE POSITIVE ELECTRODE • CNAP

16. VACUUM TUBES • GAS AT LOW PRESSURE • ELECTRIC CURRENT PASSED THROUGH • RADIATION CAME FROM THE END OF THE TUBE CONNECTED TO THE NEGATIVE(CATHODE) END OF THE BATTERY • CATHODE RAYS

17. TUBES

18. CROOKES PADDLE TUBE

19. CATHODE RAYS • CAST SHADOWS • CAUSE GLASS TO GLOW • TURN A PADDLE WHEEL • RAYS ARE MADE OF PARTICLES

20. JJ THOMPSON • HOLE IN ANODE TO ALLOW BEAM OF RAYS TO PASS THROUGH. • BEAM COULD BE DEFLECTED BY ELECTRIC PLATES. • THEREFORE BEAM IS MADE OF NEGATIVE PARTICLES.

21. JJ THOMPSONS APPARATUS

22. JJ THOMPSON • Used a magnetic field from an electromagnet to deflect the electrons • Calculated the ratio of charge to mass for electron

23. GEORGE STONEY • NAMED PARTICLES ELECTRONS

24. ROBERT MILLIKAN • Famous oil-drop experiment • It measured the charge on the electron • X-rays ionised air molecules by striping electrons off their atoms. • Oil droplets picked up electrons became negative • Increased the + charge until the droplet hovered. • Took measurements and calculated the charge on the electron.

25. ROBERT MILLIKAN

26. ROBERT MILLIKAN

27. THOMPSON’S ATOM • ATOM A SPHERE OF POSITIVE CHARGES WITH NEGATIVE ELECTONS EMBEDDED

28. ERNEST RUTHERFORD • Fired thin alpha particles at a tin gold foil • Thompsons plum pudding model predicted that they would pass thru’ with little deflection

29. RUTHERFORD’S EXPTGo to Atom video

30. RUTHERFORD’S EXPT

31. EXPECTED RESULT • ALPHA PARTICLES SHOULD PASS THROUGH WITH LITTLE DEFLECTION + + +

32. ACTUAL RESULT • Most pass through undeflected • Some were deflected at large angles • Some bounced right back!

33. EXPLANATION • Observation 1 • Most pass through undeflected • Deduction • Atoms are mostly empty space.

34. EXPLANATION • Observation 2 • Some were deflected at large angles • Deduction • The positive alpha particles had hit something positive

35. EXPLANATION • Observation 3 • Some bounced right back! • Deduction • Hard dense core of positive matter in the center of each atom-nucleus

36. THE PROTON • Rutherford continued to bombard different elements such as nitrogen and oxygen • Small positive particles were given off--- PROTONS

37. THE NEUTRON • James Chadwick bombarded beryllium with alpha particles. • Small particles were given off which were neutral and had the same mass as the proton—the neutron.

39. Bohr’s atom • Electrons travel in orbits around the nucleus

41. Learning Outcomes • Properties of electrons, protons and neutrons (relative mass, relative charge, location within atom).

42. Proton • Protons are positively charged particles found within atomic nucleus

45. Learning Outcomes Atomic number (Z ), mass number (A), isotopes; hydrogen and carbon as examples of isotopes. Relative atomic mass (A r). The 12C scale for relative atomic masses.

46. Atomic number Also called proton number, this is the number of protons the atom has

49. Atomic number Also called proton number, this is the number of protons the atom has

50. The Number of Electrons • Atoms must have equal numbers of protons and electrons. In our example, an atom of krypton must contain 36 electrons since it contains 36 protons.