Structure Of The Atom
Important Discoveries
Particle Scientist
Electron J.J. Thomson
Proton Ernest Rutherford
Neutron Chadwick
X Rays W k Roentgen
Atomic Nucleus Rutherford
Model Of An Atom Neils Bohr
Radioactivity Henry Becquerrel, Marie Curie
Atomic Number Moseley
THOMSON’S MODEL OF AN ATOM
Thomson proposed the model of an atom to be
similar to that of a Christmas pudding. The electrons,
in a sphere of positive charge, were like currants (dry
fruits) in a spherical Christmas pudding.
We can also
think of a
watermelon, the
positive charge in
the atom is spread
all over like the red edible part of the watermelon,
while the electrons are studded in the positively
charged sphere, like the seeds in the watermelon.
1
, Structure Of The Atom
Thomson proposed that:
(i) An atom consists of a positively charged sphere and
the electrons are embedded in it.
(ii) The negative and positive charges are equal in
magnitude. So, the atom as a whole is electrically
neutral.
Although Thomson’s model explained that atoms are
electrically neutral, the results of experiments carried
out by other scientists could not be explained by this
model, as we will see below.
RUTHERFORD’S MODEL OF AN ATOM
Ernest Rutherford was interested in knowing how
the electrons are arranged within an atom.
In this experiment, fast moving alpha (α)-particles
were made to fall on a thin gold foil.
• He selected a gold foil because he wanted as thin
a layer as possible. This gold foil was about 1000
atoms thick.
• α-particles are doubly-charged helium ions. Since
they have a mass of 4 u, the fast-moving α-particles
have a considerable amount of energy.
• It was expected that α-particles would be
deflected by the sub-atomic particles in the gold
2
Important Discoveries
Particle Scientist
Electron J.J. Thomson
Proton Ernest Rutherford
Neutron Chadwick
X Rays W k Roentgen
Atomic Nucleus Rutherford
Model Of An Atom Neils Bohr
Radioactivity Henry Becquerrel, Marie Curie
Atomic Number Moseley
THOMSON’S MODEL OF AN ATOM
Thomson proposed the model of an atom to be
similar to that of a Christmas pudding. The electrons,
in a sphere of positive charge, were like currants (dry
fruits) in a spherical Christmas pudding.
We can also
think of a
watermelon, the
positive charge in
the atom is spread
all over like the red edible part of the watermelon,
while the electrons are studded in the positively
charged sphere, like the seeds in the watermelon.
1
, Structure Of The Atom
Thomson proposed that:
(i) An atom consists of a positively charged sphere and
the electrons are embedded in it.
(ii) The negative and positive charges are equal in
magnitude. So, the atom as a whole is electrically
neutral.
Although Thomson’s model explained that atoms are
electrically neutral, the results of experiments carried
out by other scientists could not be explained by this
model, as we will see below.
RUTHERFORD’S MODEL OF AN ATOM
Ernest Rutherford was interested in knowing how
the electrons are arranged within an atom.
In this experiment, fast moving alpha (α)-particles
were made to fall on a thin gold foil.
• He selected a gold foil because he wanted as thin
a layer as possible. This gold foil was about 1000
atoms thick.
• α-particles are doubly-charged helium ions. Since
they have a mass of 4 u, the fast-moving α-particles
have a considerable amount of energy.
• It was expected that α-particles would be
deflected by the sub-atomic particles in the gold
2
