SP6 Flashcards
1
Q
What is the average radius of an atom?
A
10 x 10(-10)
2
Q
What is the modern view of the atom?
A
A positively- charged nucleus containing protons and neutrons with smaller electrons orbiting around the nucleus.
3
Q
What is the relative charge of a proton?
A
+1
4
Q
What is the relative mass of a proton?
A
1
5
Q
What is the relative charge of a neutron?
A
0
6
Q
What is the relative mass of a neutron?
A
1
7
Q
What is the relative charge of an electron?
A
-1
8
Q
What is the relative mass of an electron?
A
1/1835
9
Q
What are the heaviest particles in the atom?
A
Protons and neutrons so they make up most of the mass of the atom. Electrons have such a light mass that they are considered insignificant.
10
Q
What does the number of protons do?
A
Defines the element.
11
Q
What is mass number?
A
The total number of protons and neutrons in an atom’s nucleus.
12
Q
What is atomic number?
A
The total number of protons in an atom.
13
Q
What is an ion?
A
An ion is an atom that has lost or gained one or more electrons.
14
Q
What symbol notation can be used to represent an atom?
A
Z- mass number
A- atomic number
X- the symbol
Eg, chlorine is 35,CL,17
15
Q
What did most people in ancient Greece believe?
A
That matter was made up of combinations of 4 elements:
earth, air, fire and water.
16
Q
What did the ancient Greek philosopher, Demokritus , think of matter?
A
He thought that matter was made up of millions of tiny, uncuttable pieces of that same matter.
17
Q
After discovering the electron in 1897, what did JJ Thompson propose that the atom looked like?
A
A plum pudding.
18
Q
What was the evidence that supported this at the time?
A
Solids cannot be squashed, therefore the atoms which make them up must be solid throughout.
Rubbing 2 solids together often results in static charge so there must be something (electrons) on the outsides of atoms which can be transferred as atoms collide.
19
Q
In 1905, what did Ernest Rutherford do an experiment on?
A
The plum pudding model.
20
Q
What did his 2 students, Hans Geiger and Ernest Marsden, do?
A
Direct a beam of alpha particles at a very thin gold leaf suspended in a vacuum.
21
Q
Why is the vacuum important?
A
Because any deflection of the alpha particles would only be because of collisions with the gold foil and not due to deflections off anything else.
22
Q
What was originally thought would happen?
A
That the alpha particles would pass straight through the thin foil, or possibly puncture it.
23
Q
What other things happened during their alpha particle investigation?
A
Most of the alpha particles did pass straight through the foil.
A small number of alpha particles were deflected by large angles (>40°) as they passed through the foil.
A very small number of alpha particles came straight back off the foil.
24
Q
What did Rutherford conclude?
A
The fact that most alpha particles went straight through the foil is evidence for the atom being mostly empty space.
A small number of alpha particles being deflected at large angles suggested that there is a concentration of positive charge in the atom. Like charges repel, so the positive alpha particles being repelled by positive charges.
The very small number of alpha particles coming straight back suggested that the positive charge and mass are concentrated in a tiny volume in the atom (the nucleus). The tiny number doing this means the chance of being on that exact collision course was very small, and so the ‘target’ being aimed at has to be equally tiny.
25
What had Rutherford discovered?
The nuclear atom - a small, positively charged nucleus surrounded by empty space and then a layer of electrons to form the outside of the atom.
26
What does an element's atomic number do?
Defines it.
27
However, what part of an element can vary>
Mass numbers.
28
What is an isotope?
An element with the same number of protons but a different number of neutrons.
29
What are atoms normally?
Neutral.
30
What can atoms lose or gain?
Electrons- this is due to collisions or other interactions.
31
What is it called when an atoms loses or gains an electron?
An ion.
32
If an atom loses one or more electrons, what does it become?
A positively- charged ion.
33
In 1913, what did Niels Bohr do?
Revise Rutherford's model by suggesting that the electrons orbited the nucleus in different energy levels or at specific distances from the nucleus.
34
What was Bohr able to explain by doing this?
That since particular chemicals burn with certain- coloured flames, the pattern of energy released by electrons in the chemical reaction must be the same for every single atom of that element.
35
Therefore, what did this mean?
Electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom.
36
What happens when atoms absorb energy?
The electrons gain energy and so move up a shell. As they lose energy, they return to that shell.
37
How can an atom's nucleus be stable?
An atoms nucleus can only be stable if it has a certain number of neutrons for the number of protons it has.
38
What happens if an element has fewer protons?
It can still be stable if they have the same number of neutrons and protons.
39
What needs to increase as the number of protons increase?
The number of neutrons needs to increase in order to keep the nucleus stable.
40
What happens to nuclei with too many or too few neutrons?
It will decay, as it's unstable, in a random process, emitting radiation.
41
How can an unstable nucleus decay?
By emitting an alpha particle, a ß- (beta minus) particle, a ß+ (positron), a gamma ray or in some cases a single neutron.
42
What happens if the nucleus is unstably large?
It will emit a 'package' of two protons and two neutrons called an alpha particle.
43
What does an alpha decay cause?
The mass number of the nucleus to decrease by 4 and the atomic number of the nucleus to decrease by two.
44
What happens if the nucleus has too many neutrons?
A neutron will turn into a proton and emit a fast-moving electron. This electron is called a beta minus (β-) particle - this process is known as beta radiation.
45
What is the relative mass of a beta particle?
0, so its mass number is 0.
46
The beta particle is an electron but where has it come from?
The nucleus, not the outside of the atom.
47
What does beta decays do to the atomic number of the nucleus?
It causes it to increase by 1. The mass number remains the same.
48
What happens if the nucleus has too few neutrons>
A proton will turn into a neutron and emit a fast- moving positron.
49
What can this positron be called?
A beta plus (β+) particle - this process is known as positron emission.
50
What is a positron?
The antimatter version of an electron.
51
What is a positron's relative mass?
0
52
What is a positron's mass number?
0
53
What is a positron's relative charge?
+1
54
What does beta plus decay- positron emission- cause?
It causes the atomic number of the nucleus to decrease by one and the mass number remains the same.
55
What can a re-arrangement of particles in the nucleus do?
Move the nucleus to a lower energy state.
56
What very high frequency electromagnetic ray is emitted due to the re-arrangement of the particles in a nucleus?
A gamma ray.
57
What happens after an atom emits an alpha or beta particle?
The nucleus will often still have excess energy and will again lose energy.
58
What will a nuclear re-arrangement do?
It will emit the excess energy as a gamma ray.
59
Occasionally. what is possible to be emitted by radioactive decay?
A neutron- this can occur naturally i.e by absorption of cosmic rays high up in the atmosphere can result in neutron emission, although this is rare at the earth's surface.
60
What does neutron emission do to the mass number of the nucleus?
It causes the mass number of the nucleus to decrease by one and the atomic number stays the same.
61
In what terms are different types of radiation often compared?
1) Penetrating power - the power of the radiation that demonstrates how far into the material the radiation will go.
2) Ionising power - to ionise is to convert an unchanged atom or molecule into a charged particle by adding or removing electrons.
3) How far they can travel in air
62
What is an alpha particle's penetrating power, ionising power and range in air?
Skin/paper
High
<5cm
63
What is a Beta particle's penetrating power, ionising power and range in air?
3mm aluminium foil
Low
≈ 1 metre (m)
64
What is a gamma ray's penetrating power, ionising power and range in air?
Lead/concrete
Very low
> 1 kilometre (km)
65
How can all types of radioactive decay be detected?
By photographic film or a Geiger- Muller tube (G-M tube). The photographic film is chemically changed by the radiations so it can be developed to see if there has been exposure. In a G-M tube, the radiations ionise the gas inside and the resulting charged particles move across the chamber and get counted as charges rather like an ammeter.
66
What is radioactive decay in terms of a process?
A random process.
67
What will a block of radioactive material contain?
Many trillions of nuclei and not all nuclei are likely to decay at the same time so it is impossible to tell when a particular nucleus will decay.
68
What is possible to predict in terms of nuclei decaying?
It's possible to say that a certain amount of nuclei will decay in a certain time. Scientists cannot tell when a particular nucleus will decay but they can use statistical methods to tell when half the unstable nuclei in a sample will have decayed.
69
What is this called?
The half-life.
70
What is the half-life?
The time it takes for the number of nuclei of a radioactive isotope in a sample to halve. Also defined as the time it takes for the count rate from a sample containing a radioactive isotope to fall to half its starting level.
71
How can the GM tube be used to calculate half-life?
The GM tube is a device that detects radiation. It gives an electrical signal each time radiation is detected. These signals can be converted into clicking sounds, giving a count rate in clicks per second or per minute.
72
What is the activity of a radioactive substance measured in?
Becquerel (Bq). One Becquerel is equal to one nuclear decay per second.
73
What should also be possible to state?
How much of a sample remains or what the activity or could should become after a given length of time. This could be stated as a fraction, decimal or ratio.
74
For example, how could the amount of a sample remaining after 4 half-lives be expressed as?
A fraction- a ½ of a ½ of a ½ of a ½ remains which is ½ x ½ x ½ x ½ = 1/16 of the original sample.
A decimal- 1/16 = 0.0625 of the original sample.
75
So if the half- life is 2 days, how many days is 4 half-lives?
8 days.
76
If a half- life of a sample has a count rate of 3200 Becquerel (Bq) at the start, what is its count rate after 8 days?
1/16th of 3,200 Bq = 200 Bq.
77
The half life of cobalt-60 is 5 years. If there are 100g of cobalt-60 in a sample, how much will be left after 15 years?
1/8th of 100= 12.5g
78
What is the half-life of a sample where the activity drops from 1,200 Bq to 300 Bq in 10 days?
Half of 1,200 is 600, half of 600 is 300. So it takes 2 half-lives to drop from 1,200 Bq to 300 Bq, which is 10 days. So one half-life is 5 days.
79
How does a nucleus change into a new element?
By emitting nuclear radiations; these changes are described using nuclear equations.
80
What does Alpha decay (two nuclei and two neutrons) do?
Changes the mass number of the element -4 and the atomic number by -2. An alpha particle is the same as a helium -4 nucleus.
81
What does beta decay do?
Changes the atomic number by +1 (the nucleus gains a proton) but the mass number remains unchanged (it gains a proton but loses a neutron by ejecting an electron, so a beta particle is an electron).
82
What is gamma? (In terms of atoms)
| Gamma = A type of ionising radiation that is also part of the EM spectrum. It has no mass.
Gamma is pure energy and will not change the structure of the nucleus in any way.
83
Uranium 238 emits an alpha particle to become what nucleus?
Positron emission changes the atomic number by -1 (the nucleus loses a proton) but the mass number stays the same. Neutron emission does not change the atomic number but the mass number reduces by 1.
Alpha decay changes the mass number of the element by minus 4 so the and the atomic number by -2 so the remaining nucleus will be mass number= 234 and atomic number = 90.
84
What does shining visible radiation from a torch beam onto a hand do?
Lights the hand up because the hand has been exposed to light?
85
What I exposing objects to beams of radiation called?
Irradiation- the term applies to all types of radiation including from the nuclei of atoms.
86
What does irradiation from radioactive decay do?
Damage living cells. This can be put to good use as well as being a hazard.
87
What can irradiation be used for?
Sterilisation - Irradiation can be used to preserve fruit sold in supermarkets by exposing the fruit to a radioactive source- typically cobalt- 60. The gamma rays emitted by the cobalt will destroy any bacteria on the fruit but will not change the fruit in any significant way. The process of irradiation does not cause the irradiated object to become radioactive.
88
How can doctors use irradiation for medical reasons?
Doctors use radioactive reasons for a number of reasons:
- Sterilisation of surgical instruments
- Beams of gamma rays, called a gamma knife, can be used to kill cancerous tumours deep inside the body. The beams are aimed at the tumour from many different directions to maximise the doe on the tumour but to minimise the dose on the surrounding soft tissue. This technique can damage healthy tissue, so careful calculations are done to establish the best dose- enough to kill the tumour but not so much so that the healthy tissue is damaged.
89
In medical applications that involve using radioactive sources, efforts are made to ensure that irradiation does not cause any long-term effects. This is done by considering what?
- The nature of decay( alpha, beta or gamma)
- The half - life (long enough for the isotope to produce useful measurements, but short enough for the radioactive sources to decay to safe levels soon after use)
- Toxicity (poison level)
90
What happens if the Half-life is too long?
The damaging effects of the radiation would last for too of the radiation would last for too long and the dose received would continue to rise.
91
What are the advantages of irradiation?
- Sterilisation can be done without high temperatures.
| - It can be used to kill bacteria on things that would melt.
92
What are the disadvantages of irradiation?
- It many not kill all bacteria on an object.
- It can be very harmful- standing in the environment where objects are being treated by irradiation could expose people's cells to damage and mutation.
93
How does contamination occur?
If an object has a radioactive material introduced into it.
94
How is contamination useful in medicine?
| Highlight 3 points
- Cancer tumours can be treated by injecting them with a radioactive material. The cancer cells absorb radiation from the material and receive a high dose of energy. Doctors must work out the danger to nearby healthy tissue before giving this treatment.
- In some cases, injected radioactive sources (such as technetium -99) can be used as tracers to make soft tissues, such as blood vessels or the kidneys show up through medical imaging processes. An isotope emits gamma rays that easily pass through the body to a detector outside the body, for example an x-ray machine or 'gamma camera'. In this way, the radioactive isotope can be followed as it flows through a particular process in the body.
- Changes in the amount of gamma emitted from different parts would indicate how well the isotopes are flowing, or if there is a blockage, perhaps caused by a cancer.
95
In medical applications that involve injecting radioactive sources, efforts are made to ensure that contamination does not cause any long- terms effects. How is this done?
By choosing isotopes that:
- Have a very short half-life- sources used typically have half-lives of hours so after a couple of days there will be hardly any radioactive material left in a person's body.
- Aren't poisonous.
96
What does a Positron emission tomography (PET) scan do?
Uses a positron emitter as the contamination tracer. These tracer materials, such as fluorine- 18, have very short half-lives, and need to be manufactured in the hospital and injected into the patient's blood soon after manufacture.
The positrons emitted will react with electrons in the patient's body and produce gamma rays which can detected outside the body. Multiple detections can build up a picture of the movement of the tracer inside the body.
97
How can water supplies find leaks in pipes?
Water supplies can be contaminated with a gamma- emitting radioactive isotope to find leaks in pipes.
98
How does it do this?
Where there is a leak, contaminated water seeps into the ground, causing a build-up of gamma emission in that area. The build-up of gamma emissions can ve found using a Geiger- Muller tube, this make makes it easier to decide where to dig to fin the leak.
99
The isotope used for this purpose must...?
- Be a gamma emitter
- Have a half-life of at least several days to allow the emissions to build up in the soil.
- Not be poisonous to humans as it will form part of the water supply.
100
What are the advantages of contamination?
Radioactive isotopes can be used as medical and industrial tracers.
Use of isotopes with a short half-life means exposure can be limited.
Imaging processes can replace some invasive surgical procedures.
101
What are the disadvantages of contamination?
Radioactive isotopes may not go where they are wanted.
It can be difficult to ensure that the contamination is fully removed so small amounts of radioisotope may still be left behind.
Exposure to radioactive materials can potentially damage healthy cells.
102
What are the differences between irradiation and contamination?
- Irradiations occurs when an object is exposed to a source of radiation outside the object where as contamination occurs if the radioactive source is on or in the object.
- Irradiation doesn't cause the object to become radioactive whereas a contaminated object will be radioactive for as long as the source is on or in it.
- Irradiation can be blocked with suitable shielding whereas when an object is contaminated the radiation can't be blocked.
- Irradiation stops as soon as the source is removed whereas it can be very difficult to remove all of the contamination.
103
What is one other use of radiation?
Smoke alarms- alpha particles are easily absorbed. A household smoke alarm measures the movement of alpha particles across a small gap. If smoke enters the detector, it will absorb the alphas and the detector will measure a drop in the number getting across the gap. This drop in measurement will trigger the alarm to sound.
Americium-241 is the most common material used as the alpha emitter in a domestic smoke alarm.
104
What is another use of radiation?
Thickness monitoring - Radiation is absorbed as it passes through materials. This allows it to be used to gauge the thickness of a material by measuring the amount of radiation that passes through.
This is commonly used with beta minus emitters in industry, for paper milling and the production of aluminium foil.
If the foil is too thick it absorbs more beta particles. The detector receives less beta particles and then send s a signal to the rollers to increase the force on the foil, making it thinner.
If the foil is too thin it absorbs less beta particles. The detector receives more beta particles and then sends a signal to the rollers to decrease the force on the foil, making it thicker.
105
What are radioactive materials?
Hazardous.
106
What can nuclear radiation do?
Ionise chemicals within a body, which changes the way the cells behave. It can also deposit large amounts of energy into the body, which can mutate DNA and damage or destroy cells completely.
107
What are some of the effects of radiation on the human body?
Eyes- high doses can cause cataracts.
Thyroid- radioactive iodine can build up and cause cancer.
Lungs- breathing in radioisotopes can damage DNA.
Reproductive organs- high doses can cause sterility or mutations.
Skin- radiation can burn skin or cause cancer.
Bone marrow- radiation can cause leukaemia and other diseases of the blood.
108
Given that radioactive materials are hazardous, certain precautions can be taken to reduce the risk of using radioactive sources. What do these include?
- Keep radioactive sources like technetium-99 shielded (preferably in a lead-lined box) when not in use.
- Wear protective clothing to prevent the body becoming contaminated should radioactive isotopes leak out.
- Avoid contact with bare skin and not attempt to taste the sources.
- Wear face masks to avoid breathing in materials.
- Limit exposure time- so less time is spent around radioactive materials.
- Handle radioactive materials with tongs in order to keep a safer distance from sources.
- Monitor exposure using detector badges etc.
109
How do radioactive materials occur?
Naturally, and, as a result, everyone is exposed to a low-level of radiation every day. This exposure comes from a mixture of natural and manmade sources.
110
What does the actual amount of radiation that a person is exposed to depend on?
Where they live, what job they do and many other things.
111
What must scientists always take into consideration?
The amount of background radiation when working or experimenting with the radioactive sources and discount from their results.
112
Who and how does background radiation effect?
Everyone mainly by irradiation but a small amount is from being contaminated by radioisotopes in the food and drink that is consumed.
113
What is the activity of a source that emits one particle per second?
1 Bq.
114
However, this particle could be alpha or beta, and so what effect would each one have on a person's body?
Beta- a beta particle has a lot of energy but may not cause a lot of damage because of its low ionisation power.
Alpha- an alpha particle will have less energy but will cause more damage in a shorter distance because it is bigger.
115
How do you measure radiation dose?
```
The Sievert (Sv) is used to measure the radiation dose and is the amount of damage that would be caused by the absorption of 1 joule (J) of energy in each kilogram of body mass.
Typically, absorption is less than 1 Sv, so milliSieverts (mSv) are often used instead. 1000 mSv - 1Sv
```
116
What are some example doses of this?
Eating a banana that contains radioactive potassium - 0.000000098Sv or 0.000098mSv.
Exposure for cabin crew on airliners (per year)- 0.0016Sv or 1.6mSv
6 months on the International Space Station (ISS)- 0.08Sv or 80mSv.
Highest dose to a worker during Fukushima disaster- 0.67Sv or 670mSv.
Typical fatal dose- 10Sv or 10000mSv
117
What is nuclear fission?
The splitting of a large atomic nucleus into smaller nuclei.
118
What happens in a nuclear reactor?
A slow-moving neutron is absorbed into a nucleus (typically uranium- 235). This causes the nucleus to become uranium-236, which is unstable.
The entire nucleus splits into two large fragments called 'daughter nuclei'. In addition to the 'daughter' products, two or three nuclei also explode out of the fission reaction and these can collide with other uranium nuclei to cause further fission reactions. This is known as a chain reaction.
119
What do fast-moving neutrons do?
Carry most of the energy from the reaction with them (99%) but before the neutrons can collide with fresh uranium nuclei, they need to be slowed down.
Their energy is passed on to the other components in the nuclear reactor, which is used to heat water to drive the turbines that turn the generator.
120
What are the parts that a fission reactor contains?
- Nuclear fuel- the uranium or plutonium isotope that will split when triggered by an incoming neutron. The fuel is held in rods so that the neutrons released will fly out and cause nuclear fission in other rods.
- Moderator- Graphite core- a graphite core, for example, slows the neutrons down so that they are more likely to be absorbed into a nearby fuel rod.
- Control rods- these are raised and lowered to stop neutrons from travelling between fuel rods and therefore change the speed of the chain reaction.
- Coolant- this is heated up by the energy released from the fission reactions and is used to boil water to drive turbines in the power station.
- Concrete shield- the daughter products of the fission reaction are radioactive and can be a hazard.
121
What are many of the feature of the reactor designed to do?
Control the speed of the reaction and the temperature inside the shielding. An uncontrolled fission reaction is the basis of an atomic bomb.
122
What are the advantages of fission reactors?
- Produces no polluting gases.
- Doesn't contribute to global warming.
- Very low fuel costs.7-Low fuel quantity reduces mining and transportation effects on environment.
- High technology research required benefits other industries.
- Power station has very long lifetime.
123
What are the disadvantages of fission reactors?
- Waste is radioactive and safe disposal is very difficult and expensive.
- Local thermal pollution from wastewater affects marine life.
- Large-scale accidents can be catastrophic.
- Public perception of nuclear power is negative.
- Costs of building and safely decommissioning are very high.
- Cannot react quickly to changes in electricity demand.
124
What is nuclear fusion?
When two small, light nuclei join together to make one heavier nucleus. Fusion reacts occur in stars, were for example, two hydrogen nuclei fuse together under high temperatures and pressure to form a nucleus of a helium isotope.
125
What is the issue with nuclear fusion?
Is that it requires the fusing of nuclei, which are positive particles. As 2 nuclei approach each other, they will repel because they have the same charge. The fusion of the nuclei has to happen under intense pressure and very high temperatures in order to force the nuclei together and overcome this electrostatic repulsion.
126
Where does the energy come from in fusion reactions?
Some of the mass converts to energy.
127
Why does fusion only happen at very high temperatures?
To overcome the repulsion of the positive charges.
128
Why is the reactor filled with graphite?
To slow down the neutrons.
129
How is the chain reaction slowed in a nuclear reactor?
By inserting the control rods, neutrons are blocked from getting to the other uranium nuclei which stops the reaction. Eventually reactor would cool down after this.
130
What is a chain reaction?
When neutrons from one reaction fire off to cause further reactions. This is what happens in a fission reactor.
