Transcript Lesson 1 Atomic structure and Geiger

Do now
1. Can you write
the title
Radioactivity
in your books?
2. Draw a diagram
of an atom
Radioactivity
Today’s lesson
• describe the structure of an atom in terms of
protons, neutrons and electrons and use
symbols to describe particular nuclei
• understand the terms atomic (proton) number,
mass (nucleon) number and isotope
• What is the evidence?
The atom
orbiting electrons
Nucleus (protons
and neutrons)
Nuclide notation
Atomic mass (mass number)
= number of protons and
neutrons
7
Li
3
Atomic number (proton number)
= number of protons
Isotopes
It is possible for the nuclei of the same element
to have different numbers of neutrons in the
nucleus (but it must have the same number of
protons)
7
6
3
3
Li
Li
Isotopes
For example, Lithium atoms occur in two forms,
Lithium-6 and Lithium-7
4 neutrons
3 neutrons
7
6
3
3
Li
Li
Relative atomic mass
On average, lithium atoms have a mass of 6.941
(relative to Carbon 12)
6.941
3
Li
Isotopes of Hydrogen
The three isotopes of Hydrogen even have their
own names!
Hi! I’m
hydrogen
Hola! Mi
nombre es
tritium y yo
soy de
Madrid!
They call
me
deuterium
1
2
3
1
1
1
H
H
H
Questions!
Element
Chemical
symbol
Atomic
number
Hydrogen
H
1
Helium
He
2
Lithium
Li
3
Beryllium
Be
4
Boron
B
5
Carbon
C
6
Nitrogen
N
7
Oxygen
O
8
Radium
Ra
88
Thorium
Th
90
Uranium
U
92
Plutonium
Pu
94
Particles in the modern model
11 of 40
© Boardworks Ltd 2007
Atomic structure – key words
12 of 40
© Boardworks Ltd 2007
How do we know the structure
of the atom?
The Plum Pudding Atomic Model
Before about 1910
many scientists
believed that an atom
consisted of:
Positively charged
matter spread out like a
pudding embedded by
negatively charged
electrons (like plums in
a pudding).
The ‘Plum Pudding’ Model
Rutherford’s Atomic Model
In 1909 Ernest Rutherford suggested that an atom
consists of a a tiny positively charged nucleus
surrounded by negatively charged electrons.
Lord Rutherford
1871 - 1937
Types of radiation
Unstable
nucleus
New
nucleus
Alpha
particle
Beta
Unstable
nucleus
Unstable
nucleus
New
nucleus
New
nucleus
particle
Gamma
radiation
10/04/2016
1) Alpha () – an atom decays into a
new atom and emits an alpha particle
(2 protons and 2 neutrons – the
nucleus of a helium atom)
2) Beta () – an atom decays into a
new atom by changing a neutron into
a proton and electron. The fast
moving, high energy electron is called
a beta particle.
3) Gamma – after  or  decay
surplus energy is sometimes emitted.
This is called gamma radiation and
has a very high frequency with short
wavelength. The atom is not
changed.
Geiger & Marsden’s alpha particle
scattering experiment
In 1909 Hans Geiger
and Ernest Marsden
performed an
experiment using alpha
particles to determine
which of the two
models was the better
in describing the
structure of an atom.
Geiger and Marsden
The apparatus
2
5
4
3
1
What was observed
alpha
source
thin metal foil
1. Virtually all of the alpha particles went straight through the metal foil.
2. A few alpha particles were deflected through a small angle.
3. About 1 in 10 000 were deflected backwards.
How their results supported
Rutherford’s atomic model
1. The relatively small number of
deflections indicates that most of
the atom is empty space with only
a very small nucleus.
2. The backward deflections can
only occur if the nucleus is
positively charged and contains
most of the atom’s mass.
3. The ‘plum pudding’ model would
not produce backward
deflections.
How the results can be explained
atom
1. Deflections occur because there is
a force between the charged
nucleus and the positively charged
alpha particles.
2. Most of the alpha particles do not
go near enough to the nucleus to
be deflected.
3. Backwards deflections occur when
the alpha particles make near
head on collisions with the
positively charged nucleus.
nucleus (highly enlarged)
Rutherford did the calculations!
Rutherford (their supervisor) calculated
theoretically the number of alpha particles
that should be scattered at different angles.
He found agreement with the experimental
results if he assumed the atomic nucleus
was confined to a diameter of about 10-15
metres.
Rutherford did the calculations!
That’s 100 000 times smaller than the size of
an atom (about 10-10 metres).
Stadium as atom
If the nucleus of an atom was a ping-pong
ball, the atom would be the size of a football
stadium (and mostly full of nothing)!
Nucleus
(pingpong ball
Choose appropriate words to fill in the gaps below:
Rutherford an atom consists of a tiny,
According to __________
positively
nucleus
___________
charged __________
surrounded by a cloud of
________
negative electrons. The nucleus also contains most of the
______
mass of an atom.
alpha particle scattering
This model was supported by the ______
experiment in 1909. In this experiment most alpha particles
straight through a thin metal foil with only about 1
passed ________
backwards
in 10000 being deflected _________.
WORD SELECTION:
Rutherford mass backwards negative
straight positively alpha nucleus
Unstable nuclei
Some nuclei are unstable, for example
Uranium 235
Hi! I’m uranium-235 and I’m
unstable. I really need to lose
some particles from my
nucleus to become more
stable.
Unstable nuclei
To become stable, an unstable nuclei
emits a particle
Weeeeeeeeeeeeee!
Unstable nuclei
We say the atom has decayed
Weeeeeeeeeeeeee!
Unstable nuclei
The decay of an unstable nucleus is random. We know it’s
going to happen, but we can’t say when! It cannot be
affected by temperature/pressure etc.
Weeeeeeeeeeeeee!
Becquerels (Bq)
• The amount of radioactivity given out by a
substance is measured in Becquerels.
One becquerel is one particle emitted per
second.
Detection
• Particles can be detected by photographic
film
• Particles can also be detected (and
counted) by a Geiger-Müller tube (GM
tube) connected to a counter
Background radiation
There are small amounts radioactive
particles around us all the time. This is
called background radioactivity. The
amount varies depending on location.
Background radiation
•
•
•
•
•
Background radiation
comes from
Cosmic rays from
space
Radioactive rocks in
the ground
Nuclear tests
Nuclear bombs
Nuclear accidents
Radiation Safety
Radiation Safety
• Run away!
Mr
Porter
Radiation Safety
• Run away!
• In other words keep the distance between
you and a radioactive source as big as
possible!
Mr
Porter
Radiation Safety
• Don’t waste time!
Radiation Safety
• Don’t waste time!
• In other words limit the time you are exposed
to radiation.
Radiation Safety
• If you can’t run away, hide behind something!
Radiation Safety
• If you can’t run away, hide behind something!
• Put a barrier between you and the radiation
source that can absorb the radioactive
particles
Let’s try some questions.
Let’s try some
questions.