atomic structure (p4)

Question 1

what is the radius of an atom compared to the nucleus?

Answer 1

atom: 1 x 10^-10m
nucleus: less than 1/10,000th the radius of the atom.

• most of the mass of the atom is concentrated in the nucleus.

Question 2

describe the basic structure of an atom:

Answer 2

positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons.

• electrons are arranged at different energy levels away from the nucleus.

Question 3

how can electromagnetic radiation affect electrons?

Answer 3

by absorbing electromagnetic radiation, electrons can move up an electron shell, to a higher energy level. it can then release the electromagnetic radiation back out, and then descend back to its original electron shell.

Question 4

describe the numbers of protons and neutrons in an atom:

Answer 4

• the number of electrons is equal to the number of protons. atoms therefore have no overall electric charge.
• all atoms of a particular element have the same number of protons: atomic number (on the bottom).
• total number of protons and neutrons in an atom is called the mass number (on the top).

Question 5

what is a mass number and what is an atomic number?

Answer 5

mass number = mass of nucleus
atomic number = charge of nucleus

Question 6

what are isotopes?

Answer 6

• atoms of the same element that have different numbers of neutrons.
• these atoms are called isotopes of that element.

Question 7

describe ionisation:

Answer 7

ionisation is where ionising radiation is so strong that it’s able to knock one of the outermost electrons off the atom. this leaves more protons than electrons, making it a positive ion.

Question 8

describe the development of the model of the atom:

Answer 8

• democritus thought that everything was made up of small molecules
• john dalton believed everything was made up of small, solid spheres, that could not be divided, and that different spheres made up different elements
• j.j. thompson believed that atoms where general balls of positive charge, with small, discrete spheres of negative charge inside (plum pudding model)
• ernest rutherford created the nuclear model, showing a nucleus of positive charge, surrounded by a cloud of negative charge (this would collapse in on itself) - from the alpha particle scattering experiment.
• niels bohr discovered that electrons orbited the nucleus on electron shells
• later experiments led to the idea that the positive charge of the nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge: protons.
• james chadwick discovered neutrons in the nucleus. this was about 20 years after the nucleus became an accepted scientific idea.

Question 9

describe the alpha particle scattering experiment in more detail:

Answer 9

• scientists took a piece of gold foil (can be hammered until it’s just a few atoms thick)
• fired tiny alpha particles at the gold foil.
• alpha particles have a positive charge
• most of the alpha particles passed straight through the gold foil, without changing direction.
• sometimes, an alpha particle was deflected (changed direction), as it passed through, and sometimes, an alpha particle would simply bounce straight back off the foil.
• the fact that most of the alpha particles went through the gold foil told scientists that atoms are mainly empty space.
• immediately, they knew the plum pudding model must be wrong.
• because some of the alpha particles were deflected, they knew the centre of the atom must have positive charge (any alpha particle that comes close to the positive
  centre of an atom is repelled).
• because some alpha particles bounced straight back, the scientists were told that the centre of an atom must contain a great deal of mass.

Question 10

describe radioactive decay:

Answer 10

when an isotope’s nucleus is unstable, it can emit radiation in order to become more stable. this radioactive decay is totally random - the activity is the rate at which it decays (measured in becquerels Bq).
- 1 Bq = 1 decay per second

Question 11

how can we measure the radioactive decay at a source?

Answer 11

we can do this by using a geiger-muller tube - the count-rate is the number of decays recorded each second by the detector

Question 12

what is the difference between activity and the count-rate?

Answer 12

activity is the rate at which the unstable nucleus decays (measured in becquerels), whereas the count-rate is the rate at which radioactive emissions are detected by a detector (e.g. a Geiger-Muller tube).

Question 13

describe alpha radiation:

Answer 13

• the same as a helium nucleus (2 neutrons, 2 protons).
• large, travel 5cm through the air before they stop.
• easy to stop (single sheet of paper)
• very strongly ionising

Question 14

describe beta radiation:

Answer 14

• an electron which is ejected from the nucleus at a very high speed (formed inside the nucleus when a neutron changes into a proton and an electron).
• travel 15cm in air before stopping.
• stopped by a few mm of aluminium
• quite strongly ionising

Question 15

describe gamma radiation:

Answer 15

• a type of electromagnetic radiation from the nucleus.
• travels several metres in air before stopping.
• stopped by several cm of lead
• weakly ionising

Question 16

what are nuclear equations, and what can they be used for?

Answer 16

represent radioactive decay.

Question 17

how are alpha particles represented in a nuclear equation?

Answer 17

4 He
2

Question 18

how are beta particles represented in a nuclear equation?

Answer 18

0 e
-1

Question 19

what does radon form when it undergoes alpha decay?

Answer 19

219 radon = 215 polonium + 4 He
86 84 2

• when radon decays, both its mass number and atomic number decreases.

Question 20

what does carbon form when it undergoes beta decay?

Answer 20

14 carbon = 14 nitrogen + 0 e
6 7 -1

• beta decay does not cause the mass number to change, but causes the atomic number to increase.

Question 21

do gamma rays cause decay?

Answer 21

the emission of a gamma ray does not cause the mass or charge of a nucleus to change.

Question 22

what is the half-life of an isotope?

Answer 22

• the time it takes for the number of nuclei of the isotope in a sample to halve.
• the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level.

Question 23

how do you work out the half-life of a substance based off of a graph showing its count rate?

Answer 23

1. half life is the amount of time taken for half of the nuclei in a sample to decay, or for the count rate to halve.
2. find the count rate at the start on the graph, and halve it.
3. find the half count rate value on the graph, then read across and down, to the years, usually.
4. the number of years is the half-life of the substance.

Question 24

how do you calculate a decline in emission?

‘the initial activity of a sample is 350Bq. what is the final activity as a percentage after 2 half lives?’

Answer 24

1. work out the activity after 2 half lives.

• after 1 half life, the activity is 350 / 2 = 175
• after 2 half lives, the activity is 175 / 2 = 87.5

2. express the activity as a percentage of the original activity.

• (87.5 / 350) x 100 = 25%
• the final activity is 25% of the initial activity.

Question 25

what is irradiation?

Answer 25

exposing an object to nuclear radiation (e.g. alpha, beta, gamma). some medical equipment is sterilised using gamma radiation. this does not make the object radioactive, as it only comes in contact with the radiation, not the radioactive isotope

• most medical objects are sterilised through heating, but some objects cannot be heated, so they must be exposed to irradiation instead.

Question 26

describe the process of sterilising syringes through irradiation:

Answer 26

• place syringe in plastic wrapper - stops bacteria from entering after sterilisation.
• place object in a lead box that acts as a shield for workers from the radiation. object is near radioactive isotope that emits gamma radiation.
• removing the internal lead shield allows gamma radiation to irradiate the object. this gamma radiation kills any bacteria present.

Question 27

what is radioactive contamination?

Answer 27

this is where unwanted radioactive isotopes end up on other materials. this is hazardous, as the radioactive atoms decay and release ionising radiation.
- the type of radiation emitted affects the level of hazard.

Question 28

what are the different levels of danger when a person comes into contact with different types of radiation?

Answer 28

alpha radiation: strongly ionising, but easily stopped by dead skin cells. dangerous if inhaled or swallowed. can damage cells badly once inside.

beta radiation: quite ionising and can penetrate the skin and into the body.

gamma radiation: weakly ionising. can penetrate the body but likely to pass right through it.

Question 29

what precautions must you take around ionising radiation?

Answer 29

• can increase risk of cancer.

alpha radiation: use gloves.
beta and gamma radiation: use a lead apron.

• with high levels of radiation, a lead apron may not be enough. when working with nuclear fuel, for example, you must be protected by lead walls and a lead-glass screen.

Question 30

what is another method of reducing radiation exposure?

Answer 30

• use a radiation monitor: measures how much radiation has been received.
• doesn’t stop radiation, but means that we can identify if a person has worked with too much radiation, and so can stop them from working near any more radioactive isotopes.

Question 31

what is the importance of peer review, and what is it?

Answer 31

• over the years, scientists have explored the effects of radiation on humans.
• it’s important that these studies are published and then shared with other scientists.
• this allows the findings to be checked (peer review).

Question 32

what are two natural sources of background radiation?

Answer 32

1. radioactive rock (e.g. granite). in cornwall, for example, this is a major source of background radiation.
2. cosmic rays from space (very high energy particles which travel through space and crash into the Earth’s atmosphere). this can be created by a supernova.

Question 33

what are two human sources of background radiation?

Answer 33

1. fall out from nuclear weapons testing - has released radioactive isotopes into the environment for decades.
2. nuclear accidents - radioactive isotopes are released by nuclear accidents, which takes decades to clear up.

Question 34

what two factors can affect the levels of background radiation, or you radiation dose?

Answer 34

• occupation
• location

Question 35

what is radiation dose measured in?

Answer 35

• radiation dose is measured in sieverts (Sv)

Question 36

describe the half-life range between radioactive isotopes, and the associated hazards:

Answer 36

• half lives can vary between isotopes - some radioative isotopes have long half-lives, some have very short.
• however, the radioactivity of all isotopes will decrease over time.
• if a radioactive substance has a long half-life, we know it will remain radioactive for a long time.
• the longer a substance remains radioactive, the more dangerous it is.

Question 37

how can gamma rays be used to treat cancer?

Answer 37

• radiotherapy.
• a strong beam of gamma rays is aimed at the cancerous tissue.
• the cancerous tissue is killed due to the radiation exposure.

Question 38

how can gamma isotopes be used as tracers?

Answer 38

• a different form of gamma rays than in radiotherapy.
• patients given an injection/drink containing the gamma isotope.
• using a special gamma camera, the isotope can be traced as it moves through the body.

Question 39

how can iodine be used in the thyroid gland?

Answer 39

• a radioactive isotope of iodine can be used to check the function of the thyroid gland.
• the radioactive isotope will be absorbed and traced as it moves into the thyroid gland.

Question 40

evaluate the use of radiation in medicine:

Answer 40

• radiotherapy can be harmful. doctors must be careful during radiotherapy, as gamma rays can kill normal tissue too. therefore, they need to strike a balance between killing cancer cells and not killing too many normal cells.
• gamma rays are good for exploration of internal organs, due to their weak ionising power. they allow life-threatening conditions to be diagnosed, whilst not ionising too much tissue.

Question 41

what are the risks of radiation?

Answer 41

• radiotherapy has a lot of side effects, e.g. radiation sickness.
• although radiation sickness can be seen as negative, there are more benefits than risks with this treatment. radiotherapy can kill cancer cells and remove tumours, which is obviously a huge benefit.

Question 42

what is nuclear fission?

Answer 42

• splitting up of large and unstable nuclei into smaller nuclei, which releases lots of energy.
• either spontaneous (rare), or by absorbing a neutron which splits the nucleus.
• when the nucleus splits, it forms two smaller, equal daughter nuclei, along with two or three neutrons, energy, and gamma rays.
• all of the fission products have kinetic energy.
• these neutrons can then be absorbed by more nuclei, triggering fission again.
• chain reaction. this can be stopped by control rods, which absorb the neutrons released by the split nuclei, and slow down the reaction.

Question 43

give examples of when nuclear fission is controlled, and when it’s not controlled:

Answer 43

• a controlled chain reaction is used to release energy in a nuclear reactor.
• the explosion in a nuclear weapon is caused by an uncontrolled fission chain reaction.

Question 44

what is nuclear fusion?

Answer 44

• when two smaller, lighter nuclei (e.g. hydrogen) fuse together to make a larger, heavier nucleus.
• however, some of the mass of the nuclei can be converted into a massive amount of energy, which is released as radiation.
• nuclear fusion is NOT a chain reaction.
• releases lots of energy (creates stars).
• not possible to do on Earth yet.