Week 2 Flashcards
as you go further down the periodic table…
the elements will become increasingly radioactive, nucleus is too big
Electrostatic (Coulomb) repulsion
The repulsion between protons acts to push these nucleons apart over a long range
Intended to make a nucleus radioactive, unstable
The strong nuclear force
a short range attraction between all nucleons
Make stable
Radioactivity 1: In nuclides with too few neutrons…
the electrostatic repulsion overwhelms the strong nuclear attraction
Radioactivity 2: As the nucleus gets larger…
the long-range electrostatic repulsion between protons accumulates and eventually overwhelms the strong nuclear attraction.
Nuclides with M > 208 (e.g. Uranium) are unstable
Radioactivity 3: When there are too many neutrons…
the nucleus is also unstable.
This is explained by a nuclear form of quantum theory, the Nuclear Shell Model
Noble gases are very stable because…
they have a complete shell
alpha decay
lose 2 protons and 2 neutrons
α particle is simply a helium nucleus with mass number 4 and charge 2+
beta decay
1 neutron lost and 1 proton gained
β(or β−) is an electron ejected from the nucleus. One neutron is changed into a proton in this nuclear reaction to balance the charge.
positron decay
1 neutron gained and 1 proton lost
β+ is a positron ejected from the nucleus. One proton is changed into a neutron in this nuclear reaction to balance the charge
electron capture
1 neutron gained, protons the same
Electrons fall into lower energy states to fill the vacancy left by the captured electron. A proton combines with the electron, forming a neutron.
Mass number stays the same
neutron emission
Simple emission of a neutron, which changes M but leaves Z unchanged.
gamma emission
No change in M or Z is associated with γ-emission on its own.
nuclear stability depends on…
size of nucleus
(there are no stable nuclei heavier than Pb w/ A=208 and Z=82)
N:Z ratio
(near to 1, but “bends” towards more neutrons per proton as the nucleus gets larger)
bad uses of radiation
Radiation sickness/burns
Cancer
Weapons
good uses of radiation
Cancer therapy
Medical imaging
radiation is…
high energy and produced by radioactive decay and causes ionisation of matter by ejecting electron from atoms
the ionisation of a single molecule needs
10 eV
how much energy in alpha radiation? (approx)
5 MeV
how much energy in beta radiation? (approx)
1 to 0.05 MeV
how much energy in gamma radiation? (approx)
1 MeV
Why is radiation so bad for the body?
because the body is 50-70% water and reactions begin with water.
Water ionised to a cation and an electron
Damage depends on 3 things…
- type of radiation
- length of exposure
- source of exposure
internal exposure to radiation
Ingestion or inhalation. Alpha and beta are most dangerous. Most gamma radiation escapes the body
external exposure to radiation
alpha and beta can’t penetrate through air and skin. Gamma radiation can penetrate skin - more dangerous
Sievert (Sv)
unit that measures biological effect of radiation
cancer therapy
Focusing ionising radiation onto the tumour
OR
Internal administration of a radiopharmaceutical
Radiation for imaging
Uses radiation (gamma) emitted from within the body to map the body
Computer-assisted tomography can give 3D reconstruction of the body
Technetium-99m imaging
Easily incorporated into many drugs
Easily prepared from Mo-99
Does not change its chemistry when it decays
Emits only highly-penetrating gamma rays, not harmful alpha/beta particles
PET imaging
Uses radionuclide that emits positrons: positron emitters are proton rich
Within the body, positron reacts with an electron, producing two high energy gamma rays, which are detected outside the body
FDG is used to observe parts of the body that use high levels of glucose (tumours, brain etc.)
Light (photoelectric effect)
Light is electromagnetic radiation that has both wave and particle (photons) nature.
The amount of energy (quantum) in each photon is determined by its frequency, ν (nu) or wavelength, λ (lambda).
Black-body radiation
Proposed that energy is quantised:
E = hν (ν = frequency of oscillation, h = Planck’s constant)
Spectroscopic lines
electrons in discrete orbits
Thus atom cannot lose energy continuously, but must do do in quantum jumps between different orbits
Light emitted by an excited atomic gas consists of discrete wavelengths, not a continuous band.
Bohr model
postulated a set of circular orbits for electrons with specific, discrete radii and energies and that electrons could move in each orbit without radiating energy
Energies for H
values between −ER (n = 1) and −ER/4 (n = 2) cannot be observed
as n increases, En approaches the energy of an unbound electron, or 0
Problems with Bohr model
- According to classical physics, revolving charged particles radiate energy
- Bohr’s model could only explain the emission spectra of
single-electronatoms. It failed to predict the spectra of multi-electron atoms. - Bohr could offer
no reasonwhy an electron should have discrete orbits or energies.
wavelength of a matter wave
λ=h/mv
m is mass
v is velocity
h is Planck’s constant
Planck’s constant
6.626 x 10^-34 J s
Mechanics of waves
Wave-behaviour + restricted motion lead automatically to discrete energy levels or frequencies.
Thus, the matter wave concept explains why electrons have discrete energy levels. No need for discrete orbits!
Classical vs. quantum
quantum
- has large scale (goes to subatomic objects)
- wave is particles (dual nature)
