Download Revision of Atomic Structure and Nuclide Notations Nuclide

Topic 13 – National 5 Chemistry Summary Notes
Nuclear Chemistry
In this topic you will learn about radioactive elements whose atoms are so unstable that
they have to release radiation in order to become more stable. This release of radiation
often results in the formation of new substances. We will also learn about the
different types of radiation and some of the important uses of radioactive elements.
LI 1
Revision of Atomic Structure and Nuclide Notations
Atoms consist of a dense central nucleus containing protons and neutrons with
electrons orbiting around the outside:
Each element has its own unique number of protons (atomic number) but the mass
number can vary depending on how many neutrons there are.
Isotopes are atoms with the same atomic number but different mass numbers.
Nuclide Notations
We can use a special diagram to illustrate the different isotopes. It is called a nuclide
notation:
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LI 2
Radioactive Isotopes and Radioactive Emissions
The atoms of most elements have isotopes. Some of these isotopes have nuclei which
are unstable and give out different types of particles or rays. This release (emission)
of particles and rays is what we call radiation and helps to make the atom more stable.
Radioactivity is all around us. We are constantly being bombarded by particles and rays,
this is often described as background radiation.
Types of Radiation
There are three types of nuclear radiation that we will be looking at:
1. Alpha Particles,
These are the heaviest particles. They are made up of a Helium nuclei, He
2+,
containing
two protons and two neutrons and have their own nuclide notation:
2. Beta Particles,β
These light weight particles are made up of a single, very high energy (fast) electron.
Nuclide notation:
2
3. Gamma Waves,
These are not particles. They are a form of electromagnetic radiation, “waves” and
they come from the nucleus. These very high energy rays can travel very long distances
and penetrate deep into even very dense substances.
LI 3
Properties of alpha, beta and gamma radiation
When the three different types of radiation are passed through an electric field they
are affected in different ways:
1. Beta particles are negatively charged so they are attracted (deflected) to the positive
side.
2. Alpha particles have a positive charge so they are attracted (deflected) to the
negative side.
3. Gamma ways are not charged so pass through unaffected.
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The radioactive emissions also have different penetrating powers:
1. Alpha particles are the biggest and they can be stopped by a thin piece of paper.
2. The smaller beta particles can travel a few metres through the air but will be stopped
by a few millimetres of aluminium.
3. Gamma ways have no mass and charge and can travel long distances. They are only
stopped by thick lead and even thicker concrete.
LI4
Nuclear Equations
Another word to describe the breakdown of unstable nuclei is decay. An atom can often
go through a series of changes by emission of radiation in order to become stable.
We can represent radioactive decay using nuclear equations:
Alpha Emission
mass number
atomic number
Nuclear equations show the mass number, atomic number and symbols of all the
particles involved. In other words the nuclide notations we revised earlier.
When an atom loses an alpha particle it loses 4 mass units and turns into a new element
with an atomic number 2 less than the original.
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Beta Emission
When an atom decays by losing a beta particle it gains a proton but its mass is
unchanged.
A neutron has been changed into a proton and an electron.
Gamma emission often happens at the same time but has no effect on the mass number
or atomic number. It is not included in nuclear equations.
Balancing the Numbers!
In nuclear equations the top numbers (mass numbers) must add up to the same number
on both sides of the equation. The bottom numbers, the atomic numbers must also add
up to the same number on both sides of the equation. This means the nuclear equation
is balanced. If you know what sort of particle an atom is emitting then you should be
able to work out the identity of the new atom produced:
Example 1: What new atom is formed when an atom of Bismuth-211 loses an alpha
particle?
+ ?
The numbers must balance so the new atom must have an atomic number of 83 - 2=81
and a mass number of 211 - 4=207:
Answer:
Example 2: What new atom is formed when an atom of Sodium-24 decays by beta
emission?
+
?
24 - 0= 24 so the mass number of the new atom stays at 24,
11 – (-1) = 12 so the atomic number of the new atom is now 12 (a neutron has changed to
a proton)
Answer:
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LI 5
Half Life
You have probably heard that substances can remain radioactive for a long time. It can
in fact take anything from seconds to millions of years for different elements to
completely decay and lose their radioactivity. Scientists use the amount of time it
takes for an element to lose half of its radioactivity as a useful measure. Each
radioisotope has a unique half-life:
Radioisotope
Half-life
Cobalt-60
5.27 years
Americium-241 433 years
Iodine-131
8.02 days
A graph can be drawn showing the curve you always get when plotting radioactivity
(often in counts per minute) against time. In the graph below you can see that the
sample halves its radioactivity every five days so its half-life is 5 days.
Half-life Calculations
1. Strontium-90 has a half-life of 28 years. How many years would it take for a sample of
it to reduce to 1/16th of its original radioactivity?
Answer – 1/16th is a “half of a half of a half of a half” i.e. 4 half-lives so it must have
taken 4 x 28 = 112 years.
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2. The half-life of Caesium-137 is 30 years. What mass of caesium-137 would be left
after 90 years if 100g was there at the start?
Answer – 90 years is 90/30 = 3 half-lives.
100
50
25
12.5g left
The following table helps you to match fractions and percentages of a sample left to
numbers of half-lives that have passed:
% of sample left Fraction of sample left Number of Half-lives
100
0
LI6
50
1
25
2
12.5
3
6.25
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Uses of Radioisotopes
1. Dating
Since every radioisotope has a unique and constant half-life that fact can be used to
find out how old an object containing that substance is. The radioactive element
Carbon-14 is found in any object made of once living things. Carbon-14 is made in the
upper atmosphere at a constant rate due to a type of cosmic radiation (high speed
neutrons) colliding with nitrogen atoms:
This radioactive form of carbon is quickly oxidised to carbon dioxide, trapped by plants
in photosynthesis and then passed into the food chain. All livings contain a small but
constant amount of Carbon-14 until they die. At that point no more new carbon-14 can
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be trapped so the amount of carbon-14 will drop, the older the object made of a dead
thing the less radioactive it will be.
The half-life of carbon-14 is 5730 years and very old objects up to around 50,000
years can be dated accurately this way.
2. Other Uses of Radioisotopes
There are many radioisotopes used in medicine and industry. They
can be used to help diagnose diseases, kill tumours and even
sterilise medical equipment. They can be used to test metals and
welds for cracks, calculate (gauge) the thickness of materials to
name just a few. The following table shows some examples with
space left to add your own:
Radioisotope
Use
Cobalt-60
Caesium-137
Produces gamma radiation to kill cancerous
cells
Thickness gauging
Iodine-131
Diagnosing and treating thyroid problems
Americium-241
Smoke detectors
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Topic 13 Pupil Self Evaluation
Number
1
Learning Intention
Nuclear Chemistry Nat 5
Success Criteria
I will revise atomic structure and
nuclide notations
I will find out most elements have
isotopes and some of these are
unstable and produce different types
of radiation
I can draw nuclide notations given the name and mass of an atom.
3
I will find out about the properties of
alpha, beta and gamma radiation
4
I will find out about how to write
nuclear equations
I can describe how and why the three different types of radiation are
affected by an electric field.
I can describe the penetrating powers of alpha, beta and gamma
radiation.
I can write equations for beta and alpha emission
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I will find out that “Half Life” is a
useful term used to measure how long
an isotope takes to lose half its
radioactivity
I will find out some of the uses of
radioisotopes
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I can:
 Describe the three different types of radiation
 State that we are being constantly bombarded by particles and
rays known as background radiation
I can interpret Half Life graphs and do simple calculations involving
half life and radioactivity
I can give example and describe some of the uses as:
 Radiocarbon dating
 Medical uses
 Industrial uses
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