Transcript File - Loreto Science

The Atom – a history of the
smallest particle!
• Ancient Greeks were the first to imagine the
existence of tiny particles that could not be
divided or broken into anything smaller
• The Greek word for indivisible is “Atomos” this is
where we get the word “Atom”
• However the Greeks did not have the means to
prove the existence of atoms through
experiment
John Dalton
• The 12 year old
headmaster!
• In 1808 this English
Chemist released a book
on atomic theory to
explain experiments he
had done with gases
• He was widely praised
and honoured but was
very shy and shunned
glory
Summary of Dalton’s atomic theory
1. All matter is made up of very small particles
called atoms
2. All atoms are indivisible they cannot be broken
into smaller particles
3. Atoms cannot be created or destroyed
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•
Spot the incorrect point!?
Of course we know atoms can be broken down
into electrons, neutrons and protons we will
see now how we found this out!
Discovery of the electron
• William Crookes in
1875
• Proved that electrons
existed by showing
how cathode rays
behaved in a vacuum
tube
• A battery is connected at
its negative end called
the CATHODE to a
vacuum tube
• When the current is
turned on a glow comes
from the tube
• Crookes showed that this
was caused by some kind
of radiation by placing a
maltese cross in the path
of the rays and showing
that a shadow was
formed
To prove the rays were coming
from the cathode
Cathode
Anode
• When Crookes placed a paddle wheel in
between the cathode and the anode and
switched on the current he noticed that the
wheel turned away from the cathode this proved
that the vanes of the paddle were being struck
by particles coming from the cathode
• Remember CNAP Cathode Negative, Anode
Positive
J.J. Thomson
• Englishman in 1897
who showed that the
cathode rays were
made of small
charged particles
• If there was no charge on
the parallel plates in the
middle the beam came
through in a straight line
• If there was a charge on
the parallel plates in the
middle the beam would
move towards the
positively charged plate
Thomson’s conclusions
• Thomson concluded that since the cathode rays
were attracted to the positive plate they must be
made of negatively charged particles as
opposites attract
• These negatively charged particles were called
ELECTRONS
• CATHODE RAYS are streams of negatively
charged electrons
The Plum Pudding Model
• A very simple model of an
atom proposed by
Thomson
• Since atoms are neutral
and he knew electrons
were negative he
correctly said that there
must also be positive
charges
• He proposed that
electrons were dotted in a
positive cloud like raisins
in a plum pudding!
The tiniest of particles!
• In 1909 an American named Robert
Millikan performed the OIL DROP
EXPERIMENT to find the size of the
charge on an electron and he then
found the mass of an electron
• He found an electron has a mass of
9.1 x 10-39 kg! very very very small
How????
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•
•
•
•
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Spray tiny oil drops between 2
charged metal plates
Ionise the air with an xray to free
electrons
When oil drops fall they pick up
electrons and become charged
Use a microscope to view a
particular drop of oil and watch to
see when it will be attracted up to
the positive plate
Change the charge on the positive
palte until it is just enough to hold
the drop in mid air!
Do the maths and calculate the
charge on the electron!
Discovery of the nucleus
• Ernest Rutherford from
New Zealand 1909
• Used a radioactive
source to fire alpha
particles (positively
charged particles) at a
sheet of gold foil
• Expected that the
particles would be slightly
deflected as they passed
through the “plum
pudding” atoms in the
gold foil
• !
What would happen in the plum pudding
What actually happened
• Most alpha particles went straight through the
gold foil, some were slightly deflected but others
bounced right back and travelled back along
there own path like they had bounced off
something
• The only way to explain this was that the positive
part of the atoms in the gold foil was
concentrated in one small dense core – the
NUCLEUS was discovered!
Discovery of the Proton
• Rutherford and his team found that when
small atoms like oxygen and nitrogen were
bombarded they gave off positive particles
but heavier atoms such as gold didn’t
• Reason was that bombarding broke up the
nucleus in small atoms and released
protons but in bigger atoms there were too
many protons and the strong positive
charge repelled the alpha particles
Discovery of the Neutron
• James Chadwick
• If the nucleus was all protons
it would fall apart as they
would repel each other
• Chadwick a former student of
Rutherfords bombarded
beryllium with alpha particles
and found that neutral
particles were being knock
out of the beryllium
Importance of neutrons
• Stop nuclei coming apart
• Are used to split atoms of uranium to
release nuclear energy in the atomic bomb
and nuclear reactors
Properties of the sub atomic
particles
Relative
Charge
Relative
Mass
Location
Proton
+1
1
Nucleus
Neutron
0
1
Nucleus
Electron
-1
1
1838
Outside
Nucleus
2007 Question 11 (a)
(a) In 1910 Rutherford (pictured right) and his coworkers carried out an experiment in
which thin sheets of gold foil were bombarded
with alpha particles. The observations
made during the experiment led to the discovery
of the atomic nucleus.
(i) Describe the model of atomic structure which
existed immediately prior to this
experiment. (7)
(ii) In this experiment it was observed that most of the
alpha particles went straight
through the gold foil. Two other observations were
made. State these other
observations and explain how each helped
Rutherford deduce that the atom
has a nucleus. (12)