Lecture 3- introduction to isotopes, radioactivity and radioactive decay

Question 1

What is a neutron and it’s role?

Answer 1

A neutron is a sub-atomic particle with a similar mass to a proton.
- it is uncharged.
- the neutron binds protons together via nuclear force which overcomes the repulsion between protons and stabilises the nuclei.

Question 2

Neutrons are a source of ….

Answer 2

beta radiation

Question 3

give the equation for the decay of a neutron

Answer 3

neutron – proton + electron (beta particle) + neutrino

Question 4

radioactive elements can decay into …., what does this give off?

Answer 4

different elements, giving off radiation.

Question 5

Becquerel showed different sources of radiation produce beams which behave differently when exposed to a magnetic field. They deflect in different directions or not at all. Three classes of radioactivity were shown. What are these three classes?

Answer 5

alpha, beta and gamma.

Question 6

Alpha particles

Answer 6

stopped by air, postive

Question 7

beta particles

Answer 7

stopped by an aluminium sheet, negative

Question 8

gamma rays

Answer 8

only stopped by lead several cm thick OR a metre of concrete. Did not deflect.

Question 9

What is the best known source of alpha particles?

Answer 9

• Alpha decay of heavy atoms
• Alpha particles are shown to be helium atoms
• Parent atomic nucleus decays into different atomic nucleus (with a mass number reduced by 4 and atomic number reduced by 2), plus a helium/alpha particle

Question 10

Give equation for the alpha decay of uranium

Answer 10

238/92 U —- 4/2 He + 234/90 Th

Question 11

How are beta particles produced?

Answer 11

• produced by neutrons decaying.
• a neutron will decay to produce a proton, electron (the beta particle) and a neutrino.

Question 12

Write the equation for the beta decay of carbon- 14

Answer 12

14/6 C —– 14/7N + e- + neutrino

Question 13

How are gamma rays produced?

Answer 13

• after alpha or beta decay.
• the daughter nucleus produced is in an excited state which will decay to a lower energy state by emitting gamma ray photon.

Question 14

Decay constant λ

Answer 14

the probability that a given radioactive nucleus will decay per unit time is a constant, the decay constant.

Question 15

A simpler way to view radioactive decay is half life. What is half life?

Answer 15

Half life is the time taken for half the radioactive nuclei in a given sample to undergo decay.

Question 15

Equation for radioactive decay law

Answer 15

N(t) = N0e-λt

Question 16

give the equation for half life (plus the equation for it rearranged)

Answer 16

t½ = 0.693/λ
λ = 0.693/ t½

Question 17

18F decays to 18O, with a half life of 109.7 minutes. What is the decay constant for 18F? (in min)

Answer 17

0.00631 min^-1

Question 18

half life can also be calculated using…

Answer 18

half life curves.

Question 19

What is an isotope?

Answer 19

Isotopes are atoms with the same number of protons and electrons but a different number of neutrons (different mass number)

Question 20

Why do isotopes have the same chemical properties?

Answer 20

They have the same chemical properties due to having the same number of electrons and the same electron configurations.

Question 21

Isotopes have different ………. properties

Answer 21

physical

Question 22

There is an exception to isotopes having the same chemical properties. The kinetic isotope effect. Describe this effect.

Answer 22

• bond energies are effected by the mass of atoms that form the bond
• heavier isotopes will therefore react slower than lighter isotopes of the same element because the bond enthalpy is higher so more energy is required to break the bond.
• eg. deuterium (H-2) has twice the mass of a proton (H-1) so there is a big difference in chemical properties between these two.

Question 23

When does the kinetic isotopic effect not really have much of an effect.

Answer 23

heavier atoms. The difference in mass between isotopes is less so the effects is less.

Question 24

How do neutrons stabilise the nucleus?

Answer 24

1. nuclear force which binds the protons and neutrons
2. neutrons reduce the electrostatic repulsion of protons by spreading them out.

Question 25

As the number of protons increases, the ratio of neutrons to protons needed to stabilise the nucleus also increases. Too many or too few neutrons can cause a nucleotide to become unstable. How will unstable nuclei try to become stable?

Answer 25

try to reach a lower energy level by nuclear decay.

Question 26

elements have how many stable isotopes? give the stable isotopes of hydrogen.

Answer 26

usually at least two.
proton, deuterium and tritium

Question 27

isotopes can be used in drug metabolism/ to investigate drug metabolism. Explain (HINT: isotopic labelling)

Answer 27

isotopic labelling:
- a normal nucleotide in a drug can be replaced by an isotope. this will have no chemical effect.
- this allows us to investigate how the drug is metabolised in the body.
- the isotope will be introduced to the drug during chemical synthesis.
- the drug is administered.
- studies are done to see where the label ends up. eg. c-13 may end up in 13CO2.
- to identify the labelled metabolites we can use mass spec.

Question 28

What is radiolabelling?

Answer 28

drugs or radioactive tracers can be labelled with an unstable radionucleotide. The unstable radionucleotide will release radiation to become more stable, allowing us to follow the tracer around the body.

Question 29

Name the most common application of radiolabelling.

Answer 29

Positron emission tomography (PET scans)

Question 30

PET usually uses Fluorine- 18. Explain.

Answer 30

• Fluorine- 18 is a positron emitting radionuclide.
• In FDG, the 2-hydroxyl group is replaced by a fluorine- 18. FDG= fludeoxyglucose (similar structure to glucose)
• FDG is rapidly taken up by high-glucose using cells. These are usually in the brain, kidneys and cancer cells.
• Because FDG has a fluorine-18 in place of 2-hydroxyl group, FDG does not go through glycolysis like glucose. FDG therefore just sits in the cell.
• FDG emits positron emissions.
• FDG distribution therefore tells us where glucose is taken in, in the body. This is why PET scans are used to look for cancers- cancer cells take up more glucose than normal.

Question 31

The radioactivity of 18F-FDG splits into two main routes. What are they?

Answer 31

• 20% is excreted via renal system.
• the remainder undergoes nuclear decay via positron emission converting into 18O which is not radioactive.

Question 32

18F has a relatively short half- life. Those injected, however, should avoid contact with….. for how long?

Answer 32

• radiation sensitive people- like pregnant women, for 12 hours.