Chapter 4: Atoms and Elements
 Explain how the experiments of
Thomson, Rutherford, and Chadwick
led to the development of the nuclear
theory of the atom.
 Describe early atomic models.
 Solid Sphere Model:
 Democritus
 Dalton
 J. J. Thomson (1856–1940)
 In 1897, Thomson performed
his famous cathode-ray tube
experiments.

Electricity was passed through a vacuum
sealed tube.

When charged plates were brought close to
the tube, the beam of electricity reacted.
 Moved towards the positively charged plate
 Moved away from the negatively charged plate

Based on his experiments, Thomson
discovered the following:
 Electrons are negatively charged.
 Electrons are much smaller and lighter than
atoms.
 Electrons are uniformly present in many
different kinds of substances.
 He proposed that atoms must contain positive
charge that balances the negative charge of
electrons.
 Plum Pudding Model
 J.J. Thomson
 Negatively charged
electrons were held in a
sphere of positive charge

Ernest Rutherford (1871-1937)
 In 1909, Rutherford performed
his famous gold foil
experiment.
 Tiny particles called alpha-particles
were directed at a thin sheet of gold
foil.
 Most particles passed straight through the
foil, but some were deflected at large
angles.

Expected result of
Rutherford’s gold foil
experiment:
 If the plum pudding model
were correct, the alphaparticles would pass right
through the gold foil with
minimal deflection.
 Actual result of
Rutherford’s gold foil
experiment:
 A small number of alpha-
particles were deflected
or bounced back.
 Based on his experiments, Rutherford
discovered the following:
 Most of the atom’s mass and all of its positive
charge are contained in a small core called the
nucleus.
 Most of the volume of the atom is empty space
through which the tiny, negatively charged
electrons are dispersed.
 The number of negatively charged electrons
outside the nucleus is equal to the number of
positively charged particles (protons) inside the
nucleus, so that the atom is electrically neutral.
 Planetary Model
 Ernest Rutherford
 A small, dense
positively charged
nucleus is orbited by
electrons.

James Chadwick (1891-1974)
 In 1919, Chadwick and his
mentor, Rutherford, kept
finding that the atomic mass
of an atom was larger than
the atomic number of that
atom.
 Chadwick suggested that there could be
an additional particle in the nucleus that
had mass, but no charge.
 These particles were called neutrons

Nuclear Model
 Ernest Rutherford
 James Chadwick
▪ 99.9% of the atoms mass is
concentrated in a small dense
nucleus that contains protons
and neutrons.

The dense nucleus makes up
more than 99.9% of the mass
of the atom
 but the nucleus occupies only a
small fraction of an atoms
volume

The electrons are distributed
through a much larger region
but don’t have much mass.
 A single grain of sand composed of solid
atomic nuclei would have a mass of 5
million kg.
 Astronomers believe that black holes and
neutron stars are composed of this kind of
incredibly dense matter.