The document summarizes the development of atomic models over time from Dalton's model to the current quantum mechanical model. Dalton imagined atoms as spheres that differed in size and mass. Thomson discovered the electron and proposed the "plum pudding" model. Rutherford's gold foil experiment revealed the tiny, dense nucleus. Chadwick discovered neutrons in the nucleus. Bohr incorporated electron shells. The current quantum model depicts electrons as clouds surrounding the nucleus.

1. Models of the Atom
SNC1D Grade 9 Science

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

8. Rutherford Model

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

18. Quantum Mechanical Model