2. Dalton Model
• Modern atomic theory
began to take shape in the
early 1800s
• John Dalton (1766-1844)
considered the idea that
each different kind of
element is composed of a
different kind of atom
3. Dalton Model
• Dalton imagined that all
atoms were small spheres
that could vary in size, mass
or colour
• Different atoms would have
different properties
• All the atoms of an element
have identical properties
such as size
and mass
4. Thomson Model
• J.J. Thomson (1856-1940)
researched the idea that
atoms might be made from
a combination of particles
• Thomson used cathode ray
tubes to produce streams of
negatively charged particles
(later named electrons)
5. Thomson Model
• Electrons are now
understood to be negatively
charged particles in atoms.
• Because all of the elements
that Thomson tested
produced electrons, he
reasoned that atoms of all
elements must
contain electrons
6. Thomson Model
• Because Thomson knew that
atoms have no overall charge,
he reasoned that atoms must
also contain positive charges.
• Thomson’s model is commonly
referred to as the “plum
pudding” model
7. Rutherford Model
• Rutherford (1871-1937)
conducted an experiment
where he shot positively
charged particles at a very
thin sheet of gold foil
9. Rutherford Model
• Most of the positive particles
went right through the foil,
however, about 1 in 10,000
positive particles bounced back
• These particles had been
deflected by something very
massive and positively charged
• Rutherford had discovered the
nucleus - the centre of the atom
10. Rutherford Model
• The nucleus is a tiny positively
charged part of the atom that
contains most of the atom’s mass
• Rutherford calculated that the size
of the nucleus compared to the
rest of the atom was like the size
of a single green pea compared to
an entire football field!
11. Rutherford Model
• Based on his gold foil
experiment, Rutherford revised
the atomic model
12. Chadwick Model
• Chadwick (1871-1974) was a
student of Rutherford who
refined the concept of the
nucleus
• He discovered that the nucleus
contains neutral particles as
well as positive particles
13. Chadwick Model
• The neutral particles in the
nucleus of the atom are
called neutrons
• The positively charged
particles are called protons
• Protons and neutrons have
approximately the same
mass, however neutrons do
not have an electrical charge
14. Bohr Model
• Bohr (1885-1962)
transformed the model of
the atom into one that is
commonly used today
• His model shows how
electrons are arranged
in different elements
15. Bohr Model
• Bohr suggested that
electrons surround the
nucleus in specific energy
levels called “shells”
• He discovered that electrons
jump between these shells by
gaining or losing energy
16. Bohr Model
• Each shell can only contain a
specific number of electrons
• The first shell can contain a
maximum of 2 electrons
• The second and third shells
can contain a maximum of 8
electrons each
17. Quantum Mechanical Model
• The quantum mechanical
model is the most advanced
and accurate model of the
atom, used today by chemists
and physicists
• In this model, electrons do
not exist as tiny points inside
the atom, but instead
surround the nucleus in a
form resembling a cloud
