Rutherford's experiment involved shooting alpha particles at a thin gold foil. He expected the particles to be slightly deflected, but instead observed that most passed through undeflected, some were slightly deflected, and a few were scattered backwards. This showed that the atom is mostly empty space, with a small, dense positively charged nucleus at the center surrounded by orbiting electrons. Further experiments revealed protons and neutrons in the nucleus. Isotopes of an element have the same number of protons but different numbers of neutrons, resulting in different atomic masses but the same chemical properties.

1. Particles
Constituents of the atom
Rutherford’s experiment
Thursday, 10 November 2011

2. Thompson’s model
Thompson believed that the atom was made by:
• A distribution of positive charge spread over a relatively large
area
• Small negatively charged particles (Electrons) immersed in the
positive distribution
• These electrons are like raisin in a Christmas cake (plum
pudding) and are distributed in a way that the overall charge
is neutral

3. Rutherford’s Experiment
Rutherford shot a beam of -particles to a thin target of gold. He was
expecting the -particles to be slightly deflected by the positive
charge of the atoms of gold. Can you explain why he expected such
an outcome?
The problem was that the positive charge of the atoms of gold was too
spread out (in Thompson’s model) to produce a strong deflection, or
even the scattering backwards of -particles that was observed.
Possible
trajectory
Impossible
trajectory

4. Rutherford’s Model of the Atom
What did Rutherford observed?
1) Most α-particles passed straight through the foil undeflected
2) Some α-particles were slightly deflected (small angles)
3) Few α-particles were deflected at very large angles or even scattered
backwards

5. Rutherford’s Model of the Atom
What were the implications of Rutherford’s observations?
1) The atom is mainly made of empty space
2) The positive charge of an atom is concentrated inside a very small
nucleus in the centre of the atom.
3) The atom has a nucleus in its centre in which the positive charge and
most of the mass is concentrated

6. The Structure of the Atom
Rutherford’s experiment revealed that the atom was made of a positive
massive nucleus surrounded by electrons orbiting around it.
However, they didn’t know what was in the nucleus. More
experiments showed the whole structure of the atom.
Neutron: neutral particle inside the nucleus of
mass slightly bigger than a proton. Different no of
There is empty space between neutrons for the same element make different
nucleus and electrons. In scale the ISOTOPES of that element
nucleus is a pea at the centre of a
football field.
Nucleon no: given by the sum of no of
+ protons and no of neutrons. Using this no
+ + you can identify the different isotopes of
the same element.
Electron: atomic particle orbiting
around the nucleus with same and
opposite charge as the proton and
mass 1/1800 of a proton Proton: a +ve particle in
7
Li
the nucleus with mass
similar to neutron
Proton no: it tells the no of protons (and
3
also electrons) in the atom. This no is a
property of the element, so the atomic no
identifies what element the atom is.

7. Sub-atomic particles properties
Mass Relative Charge Relative
Particle
(kg) Mass (C) Charge
Proton 1.67x10-27 1 +1.6x10-19 +1
Neutron 1.68x10-27 About 1 0 0
Electron 9.11x10-31 1/2000 -1.6x10-19 -1

8. Isotopes
Thanks to the mass spectrometer Physicists were able to measure
the mass of the atoms of elements. In this way they made the
remarkable discovery that the atoms of a particular element do
not always have the same mass. What they all have in common
is their positive charge!

9. Isotopes
In other words, isotopes are atoms with the same atomic number,
but different mass numbers. This lead to the search for an
additional subatomic particle that would be responsible for the
extra mass, but have no charge. This additional particle is the
NEUTRON.
Match the terms with the correct definitions.
The number of protons in the nucleus.
Nuclide Tells what element the nuclide is.
Mass Number An atom of a particular nuclear structure.
Atomic Number A nuclear particle
Nucleon Total number of nucleons in the nucleus

10. Isotopes
Circle the isotopes of the same element with the same colour.
That’s right
Mass number A
=N+Z
Atomic number
Z = no of protons Chemical
symbol for the
element

11. Isotopes
Some elements have a large number of isotopes. Some isotopes
are unstable, which means that they will decay spontaneously
into more stable nuclei by the emission of particles and energy.
However, the percentage of isotopes of a particular element
mined on the Earth is remarkably constant no matter what part
of the World it has been extracted from.