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

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

11. DISCOVERY OF THE ELECTRON
• CROOKES CONDUCTED EXPERIMENTS WITH A
GLASS TUBE

12. CROOKES TUBE

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

14. 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

15. TUBES

16. CROOKES PADDLE TUBE

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

18. 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.

19. JJ THOMPSONS APPARATUS

20. JJ THOMPSON
• USED A MAGNETIC FIELD FROM AN
ELECTROMAGNET TO DEFLECT THE
ELECTRONS
• CALCULATED THE RATIO OF CHARGE TO MASS
FOR ELECTRON

21. GEORGE STONEY
• NAMED PARTICLES ELECTRONS

22. 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.

23. ROBERT MILLIKAN

24. ROBERT MILLIKAN

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

26. ERNEST RUTHERFORD
• FIRED THIN ALPHA PARTICLES AT A TIN GOLD
FOIL
• THOMPSONS PLUM PUDDING MODEL
PREDICTED THAT THEY WOULD PASS THRU’
WITH LITTLE DEFLECTION

27. RUTHERFORD’S EXPT
•

28. RUTHERFORD’S EXPT

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

30. ACTUAL RESULT
• MOST PASS THROUGH UNDEFLECTED
• SOME BOUNCED RIGHT BACK!

31. EXPLANATION
• HARD DENSE CORE OF POSITIVE MATTER IN
THE CENTER OF EACH ATOM-NUCLEUS
• ATOMS ARE MOSTLY EMPTY SPACE.

32. THE PROTON
• RUTHERFORD CONTINUED TO BOMBARD
DIFFERENT ELEMENTS SUCH AS NITROGEN
AND OXYGEN
• SMALL POSITIVE PARTICLES WERE GIVEN OFF-- PROTONS

33. 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.

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

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

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

41. 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.

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

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

46. 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.

47. Mass number
•Mass Number =
(Number of Protons) +
(Number of Neutrons)

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

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

51. Deuterium
•an atom of deuterium
consists of one proton
one neutron and one
electron

52. Tritium
• An atom of tritium consists
of one proton two
neutrons and one electrons

54. 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,

55. 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).

56. Chlorine
•Chlorine-35 and
Chlorine-37 are both
isotopes of chlorine

57. 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.

59. 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).

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

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

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

63. 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.

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