Transcript Radioactivity

Do now!
Can you also look
through the tests
and make sure that
Mr Porter has
added the marks up
correctly?
Can you look through
your books and read the
comments?
Homework
• Set 13th November
• Due 19th November
• Use pages 202 to 223 to make REVISION
CARDS for the electromagnetism topic.
• OPTIONAL for David and Harsh
Radioactivity
Last 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
• understand that ionising radiations can be
detected using a photographic film or a GeigerMuller detector
• recall the sources of background radiation
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
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
• 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!
• In other words limit the time you are exposed
to radiation.
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
Today’s lesson
• 7.4 understand that alpha and beta particles and gamma
rays are ionising radiations emitted from unstable nuclei
in a random process
• 7.5 describe the nature of alpha and beta particles and
gamma rays and recall that they may be distinguished in
terms of penetrating power
• 7.6 describe the effects on the atomic and mass
numbers of a nucleus of the emission of each of the
three main types of radiation
• 7.7 understand how to complete balanced nuclear
equations
You’ll learn
about some
really weird
ones in year 13!
3 types of particle
There are 3 (at least
in IGCSE!) types of
particles that can be
ejected from an
unstable nuclei.
Alpha particles
Hi!
α
Alpha particles
•
•
•
•
•
2 protons and 2 neutrons joined together
The same as the nucleus of a helium atom
Stopped by paper or a few cm of air
Highly ionising
Deflected by electric and strong magnetic
fields
2+
4
2
He
Alpha Decay
Atomic mass goes down by 4
235
231
92
90
U
Th +
Atomic number goes down by 2
α
Beta particles
Yo!
β
Beta particles
•
•
•
•
Fast moving electrons
Stopped by about 3 mm of aluminium
Weakly ionising
Deflected by electric and magnetic fields
0
e
-1
Beta decay
• In the nucleus a neutron changes into an
electron (the beta particle which is ejected)
and a proton (which stays in the nucleus)
• During beta decay the mass number stays
the same but the proton number goes up
by 1.
231
Th
90
231
0
Pa + -1e
91
Gamma rays
Hola!
Gamma rays
•
•
•
•
High frequency electromagnetic radiation
Stopped by several cm of lead
Very weakly ionising
NOT affected by electric or magnetic fields
Gamma rays
Associated with alpha decay
235
231
92
90
U
Th +
α
Stick it in!
Questions!
Rayneritis!