Starter
What are the 3 subatomic particles in
atoms?
What are each of their charges and
relative masses?
What is the centre of the atom called?
C1.5 History of the Atom
• State how the atomic model has changed over
time
• Understand why the atomic model has
changed
Back to the Greeks 430 BCE
Demokritos– a Greek philosopher
In 400 B.C. he did a mind experiment in an
attempt to explain the nature of matter.
His question was “If I take an object and
repeatedly cut it in half, will there be a point that
it cannot be cut any smaller?”
His theory
1. All matter consists of invisible particles called atoms (from the
Greek word “Atomos” meaning something that cant be cut).
2. Atoms are indestructible.
3. Atoms are solid but invisible.
4. Atoms differ in size, shape, mass, position, and arrangement.
John Dalton
Agreed with Demokritos concerning the
structure of the atom (solid and
indivisible)
From his experiments he suggested that
substances were made up of atoms that
are like tiny hard spheres. These hard
spheres (atoms) were the building blocks
of nature.
John Dalton’s Theory
1.
2.
3.
4.
All matter is made up of atoms
All atoms of the same element are identical.
All atoms of different elements are different
Atoms can not be created nor destroyed only
rearranged. (law of conservation of matter)
He presented his theory to the world in 1803.
J.J. Thomson
At the end of the 1800’s Thomson
Discovered the existence of what he
called electrons (negatively charged
particles).
He experimented by applying high
voltages to gases at low pressure.
Described the atom as being like
“plum pudding”.
J.J. Thomson’s Experiment
Using a cathode ray tube, he
projected a beam of
particles through a magnetic
field and observed how they
were deflected.
He conducted his experiment on the beams of particles, they
were attracted to a positive charge showing they must be
negatively charge themselves. He then went on to name the
negatively charged particles electrons.
The experiment showed the electrons must have come from
inside the tube.
So Daltons idea that atoms cant be divided or split had to be
revised.
His discovery was announced in 1897.
Still More to Find
With Thomson’s discovery, it was realized there
were more to an atom than was first thought.
If electrons had a negative charge, why are
atoms electrically neutral?
Ernest Rutherford
Was not satisfied with the current
theory of the atom.
His 2 students Hans Geiger and
Ernest Marsden conducted an
experiment; they were firing dense,
positively charged particles (alpha
particles) at a thin sheet of gold.
They expected the particles to pass
through the gold atoms with a
cloud of positive charge however
their results showed something
different.
Ernest Rutherford
Based on his results, Rutherford announced the atom
was mostly empty space with a dense positively charged
central core.
The positive charge must be concentrated to a spot in the
centre of the atom
It was also said that electrons must be orbiting around
the nucleus (centre of the atom) which contains the
dense positively charged protons
His results were announced in 1911.
Niels Bohr
In 1914, Niels Bohr proposed an atom was
like a tiny solar system (the nucleus was
like the sun and the electrons like the
planets).
Bohr noticed that the lights
given out by atoms when they
were heated only had specific
amounts of energy
He suggested that electrons must
be orbiting the nucleus at set
distances, in certain fixed energy
levels (shells)
Energy must be given out when excited electrons fall
from a high to a low energy level.
Recap
Our view of the structure of the atom has changed
over time as our ability to examine it has improved.