The document summarizes atomic structure and the development of atomic theory. It discusses key scientists and experiments that led to discoveries such as electrons, protons, neutrons, and the nuclear model of the atom. These include Thomson's work on cathode rays, Rutherford's gold foil experiment, and Chadwick's discovery of the neutron. The document also covers atomic number, mass number, isotopes, relative atomic mass, and how mass spectrometry is used to determine relative atomic masses.

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.