Lecture 3- introduction to isotopes, radioactivity and radioactive decay
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What is a neutron and it’s role?
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.
Neutrons are a source of ….
beta radiation
give the equation for the decay of a neutron
neutron – proton + electron (beta particle) + neutrino
radioactive elements can decay into …., what does this give off?
different elements, giving off radiation.
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?
alpha, beta and gamma.
Alpha particles
stopped by air, postive
beta particles
stopped by an aluminium sheet, negative
gamma rays
only stopped by lead several cm thick OR a metre of concrete. Did not deflect.
What is the best known source of alpha particles?
• 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
Give equation for the alpha decay of uranium
238/92 U —- 4/2 He + 234/90 Th
How are beta particles produced?
• produced by neutrons decaying. • a neutron will decay to produce a proton, electron (the beta particle) and a neutrino.
Write the equation for the beta decay of carbon- 14
14/6 C —– 14/7N + e- + neutrino
How are gamma rays produced?
• 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.
Decay constant λ
the probability that a given radioactive nucleus will decay per unit time is a constant, the decay constant.
A simpler way to view radioactive decay is half life. What is half life?
Half life is the time taken for half the radioactive nuclei in a given sample to undergo decay.
Equation for radioactive decay law
N(t) = N0e-λt
give the equation for half life (plus the equation for it rearranged)
t½ = 0.693/λ λ = 0.693/ t½
18F decays to 18O, with a half life of 109.7 minutes. What is the decay constant for 18F? (in min)
0.00631 min^-1
half life can also be calculated using…
half life curves.
What is an isotope?
Isotopes are atoms with the same number of protons and electrons but a different number of neutrons (different mass number)
Why do isotopes have the same chemical properties?
They have the same chemical properties due to having the same number of electrons and the same electron configurations.
Isotopes have different ………. properties
physical
There is an exception to isotopes having the same chemical properties. The kinetic isotope effect. Describe this effect.
• 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.
When does the kinetic isotopic effect not really have much of an effect.
heavier atoms. The difference in mass between isotopes is less so the effects is less.