1. Chpt. 2: The Atom

2. - first proposed that matter was composed of minute particles
History of the Atom
Greek Philosophers (400BC):
- first proposed that matter was composed of minute particles
- believed that the tiny particles of which all matter was composed were so small that nothing smaller was possible
‘Atomos’ Greek word for indivisible - ATOM

3. 2. John Dalton (1808):
Dalton’s Atomic Theory
- All matter is made up of
very small particles called
atoms
All atoms are indivisible. They cannot be broken down into simpler particles
Atoms cannot be created or destroyed

4. What is inside the atom???

5. Discovery of the Electron
3. William Crookes – cathode ray tube (1875):
- passed electric current through gases
at low pressure
invisible radiation that caused the
glass to glow came from *cathode (-)
called cathode rays
showed existence of this radiation by
placing Maltese Cross inside the tube
*Note:
Cathode = plate connected to negative end of battery
Anode = plate connected to positive end of battery ,

7. 4. J.J Thomson – cathode ray tube experiments (1897):
devised experiment to
investigate if cathode rays
consisted of charged
particles
cathode rays attracted up
towards positive plate
(anode) => consisted of
negatively charged particles
hence cathodes are streams
of negatively charged
particles called electrons
Definition: cathode rays are streams of negatively charged particles called electrons

8. + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative to the positive end

9. Thomson’s Experiment Voltage source+ -
By adding an electric field he found that the moving pieces were negative

10. Further experiment:
- he found electrons were also deflected in magnetic
field
found ratio of charge to mass of the electron (e/m):
(electrical charge of electron)
(mass of electron)
= 1.76 x 108 coulombs = 1 gram of electrons
*Note: In 1891 George Stoney proposed that the smallest amount of electric charge be called an electron.

12. Dough = positive charge
Thomson's ‘Plum Pudding Model’ of the Atom (1898):
Proposed that since atoms are neutral each one consists of:
- a sphere of positive
charge
- electrons embedded
randomly
Dough = positive charge
Raisins = electrons

13. 5. Robert Millikan (1909):
- Experiment to measure size of
charge on electron – Oil Drop
Experiment
- Charge of one electron =
1.6 x coulomb
THUS….
Mass of e- = x 10-31g

14. Discovery of radiation led to the use of alpha particles in experiments
Alpha particles are positively charged particles produced by certain radioactive substances

15. Rutherford discovered the nucleus and the proton
6. Ernest Rutherford (1909):
Rutherford discovered the nucleus and the proton

16. Rutherford's Gold Foil Experiment
- bombarded tin foils of gold with alpha (α) particles
- If plum pudding model was correct he expected:
The alpha particles to pass through without changing direction very much
Fluorescent
Screen
Gold Foil
Lead block
Uranium

17. What He Got!!!!!
*Note: Detector flashes - of light produced when α particles strike zinc/sulphite screen

18. Rutherford’s Gold Foil Experiment
Results
Most alpha particles
passed straight through
undeflected
Some were deflected at
wide angles
Few deflected back
along own path
Explanation/Conclusion
Atom mainly empty space
occupied by electrons (negative)
Both the mass and positive
charge were concentrated in
a small dense core which he
called the nucleus

19. Rutherford – discovery of protons (1924):
Light atoms (oxygen, nitrogen) were bombarded with
alpha particles - small POSITIVE charged particles
were given off
This did not occur with heavier metals e.g. gold
Explanation – alpha particles were breaking up the
nuclei of the lighter atoms to release positively
charged particles
referred to these small positive particles as protons

20. 7. James Chadwick (1932):
Search for a neutral particle
to cement the nucleus
Bombarded beryllium with
alpha particles
Produced neutral particles –
neutron

21. Properties of Sub-Atomic Particles
Name
Relative Charge
Relative Mass
Location
Proton +1 1
nucleus
Electron -1
1/1836 (no mass)
outside nucleus
Neutron

22. Dalton Model of the Atom
Small, indivisible spheres

23. J.J. Thompson’s Model of Atom
Plum Pudding Model, 1896
Thought an atom was like plum pudding

24. Rutherford’s Model of the Atom
Rutherford Model, 1911
Thought atom was mostly empty space:
- Nucleus
- Electrons (negatively
charged) revolving
around nucleus

25. Bohr’s Model of the Atom
Neils Bohr, 1913
Similar to Rutherford’s model
Thought atom was mostly empty space:
- Nucleus in center is dense, positively charge
- Electrons in orbits around nucleus

26. (Modern) Quantum Mechanical Model of the Atom
Heisenberg, Schrodinger, many others, ~1926
Think atom is mostly empty space:
Nucleus in center is dense, positively charge
- Electrons cannot locate

27. Evidence for the existence of small particles!!!
Why is it possible to smell the perfume that someone is wearing from several metres away?

28. Diffusion The process by which molecules of a substance
spread through a solid, liquid or gas.
Some examples which can be demonstrated in the
lab:-
Gas Jar
full of air

29. Demonstration Diffusion of ink in water Diffusion of NH3 and HCl
Diffusion of smoke in air

30. Diffusion of NH3 and HCl

31. Ammonia + Hydrogen chloride = Ammonium chloride
Diffusion of NH3 and HCl
Word Equation:
Ammonia + Hydrogen chloride = Ammonium chloride
(Gas) (Gas) (White powdered ring)
Chemical Equation:
NH3 + HCl = NH4Cl