Chpt. 2: The Atom
History of the Atom
1. 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
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
What is inside the atom???
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
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
Thomson’s Experiment
Voltage source

Passing an electric current makes a beam
appear to move from the negative to the
positive end
Thomson’s Experiment
Voltage source

By adding an electric field he found that the
moving pieces were negative
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.
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
5. Robert Millikan (1909):
- Experiment to measure size of
charge on electron – Oil Drop
Experiment
- Charge of one electron =
1.6 x 10-19 coulomb
THUS….
Mass of e- = 9.11 x 10-31g
Discovery of radiation led to the use of
alpha particles in experiments
Alpha particles are positively charged particles
produced by certain radioactive substances
6. Ernest Rutherford (1909):
Rutherford discovered the nucleus and the proton
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
What He Got!!!!!
*Note: Detector flashes - of light produced when α particles
strike zinc/sulphite screen
Rutherford’s Gold Foil Experiment
Results
• Most alpha particles
passed straight through
undeflected
Explanation/Conclusion
• Atom mainly empty space
occupied by electrons
(negative)
• Some were deflected at
wide angles
• Both the mass and positive
charge were concentrated in
a small dense core which he
called the nucleus
• Few deflected back
along own path
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
7. James Chadwick (1932):
• Search for a neutral particle
to cement the nucleus
• Bombarded beryllium with
alpha particles
• Produced neutral particles –
neutron
Properties of Sub-Atomic Particles
Name
Relative
Charge
Relative
Mass
Location
Proton
+1
1
nucleus
Electron
-1
1/1836
(no mass)
outside
nucleus
Neutron
0
1
nucleus
Dalton Model of the Atom
• Small, indivisible spheres
J.J. Thompson’s Model of Atom
• Plum Pudding Model,
1896
•Thought an atom was like
plum pudding
Rutherford’s Model of the Atom
• Rutherford Model, 1911
• Thought atom was mostly
empty space:
- Nucleus
- Electrons (negatively
charged) revolving
around nucleus
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
(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
Evidence for the existence of
small particles!!!
Why is it possible to
smell the perfume
that someone is
wearing from several
metres away?
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
Demonstration
• Diffusion of ink in water
• Diffusion of NH3 and HCl
• Diffusion of smoke in air
Diffusion of NH3 and HCl
Diffusion of NH3 and HCl
Word Equation:
Ammonia + Hydrogen chloride = Ammonium
chloride
(Gas)
(Gas)
Chemical Equation:
NH3 +
HCl
(White powdered ring)
= NH4Cl