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1.2 Atomic Structure (Time needed: 6 class periods)

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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.

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

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

11 DALTON 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

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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 EXPT Go 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.

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

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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

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45 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. Learning Outcomes

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

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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.

51 Mass number Mass Number = (Number of Protons) + (Number of Neutrons)

52 Isotope Atoms that have the same number of protons but different numbers of neutrons are called isotopes

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54 Hydrogen isotopes The element hydrogen for example, has three commonly known isotopes: protium, deuterium and tritium

55 Deuterium an atom of deuterium consists of one proton one neutron and one electron

56 Tritium An atom of tritium consists of one proton two neutrons and one electrons

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58 Relative Atomic Mass The relative atomic mass of an element the mass of one of the element's atoms -- relative to the mass of an atom of Carbon 12,

59 Learning Outcomes Calculation of approximate relative atomic masses from abundance of isotopes of given mass number (e.g. Calculation of approximate relative atomic mass of chlorine).

60 Chlorine Chlorine-35 and Chlorine-37 are both isotopes of chlorine

61 Relative mass of chlorine Chlorine consists of roughly 75% Chlorine-35 and roughly 25% Chlorine-37. We take an average of the two figures The relative atomic mass of chlorine is usually quoted as 35.5.

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63 Learning outcomes Use of the mass spectrometer in determining relative atomic mass. Fundamental processes that occur in a mass spectrometer: vaporisation of substance, production of positive ions, acceleration, separation, detection (mathematical treatment excluded).

64 THE MASS SPECTROMETER Atoms can be deflected by magnetic fields - provided the atom is first turned into an ion.

65 Stage 1: Ionisation The atom is ionised by knocking one or more electrons off to give a positive ion.

66 Stage 2: Acceleration The ions are accelerated so that they all have the same kinetic energy.

67 Stage 3: Deflection The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.

68 Stage 4: Detection The beam of ions passing through the machine is detected electrically.

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