How has our theory of atomic structure changed over time?
John Dalton first proposed ideas of atoms in the 18th-19th century
JJ Thompson developed the plum pudding model at the end of the 19th century
Ernest Rutherford found that electrons had a small nucleus in 1909
Niels Bohr said that electrons orbited the nucleus in fixed orbits
What were JJ Thompson’s ideas about the atom?
He discovered that electrons could be removed from atoms, so they must be made up of smaller parts. He proposed the ‘plum pudding’ model - atoms were spheres of positive charge with tiny negative electrons stuck in them.
What were Rutherford’s ideas about the atom?
He conducted a gold foil experiment in 1909 alongside Marsden, firing alpha particles at a thin sheet of gold foil. Most particles went through the foil while others bounced back or were deflected, showing that atoms are mostly empty space and that there is a small positive nucleus.
What were Bohr’s ideas about the atom?
He said the atom is like the solar system, with electrons orbiting the nucleus like planets orbit the sun, at energy levels. Different energy levels hold different amounts of electrons, and the energy of electrons increases at every level. Electrons can move between them if the gain or lose energy.
What is the size of an atom in standard form?
About 1 x 10^-10 m
What is the size of a nucleus in standard form?
About 1 x 10^-15 m
What are the particles in an atom and their relative charges and masses?
Proton : +1, 1
Neutron: 0, 1
Electron: -1, 0.0005
What is an isotope?
An isotope is an element that has the same number of protons and different numbers of neutrons
How are electrons arranged in an atom?
They orbit at particular, fixed distances away from the nucleus in shells because they have discrete amounts of energy. The first shell contains 2 electrons, then 8, then 8. Shells are actually energy levels, with the lowest closest to the nucleus. Electrons can only exist on energy levels.
How can electrons move between energy levels?
If an electron absorbs EM radiation with the right amount of energy, it can move up to a higher energy level. This is called excitation. The electron will then quickly fall back down to its original energy level, emitting the energy it absorbed as EM radiation. This is called de-excitation. The energy absorbed from the EM spectrum creates an absorption spectrum, and the energy emitted creates an emission spectrum.
What is ionisation?
When an atom gains so much energy an electron may become so excited it leaves the atom completely. The atom becomes an ion. The energy required is called ionisation energy,
Why does ionising radiation exist?
When the nucleus of an atom is unstable, it tries to throw out some of the particles or energy that is making it unstable. This radiation is very high energy and so is ionising to other atoms and cells.
What are alpha particles?
Alpha radiation is when an alpha particle is emitted from the nucleus. It is two neutrons and two protons, like a helium nucleus. They only travel a few cm in air and are absorbed by a thin sheet of paper because of their size, but this also makes them strongly ionising.
What are beta particles?
They can be electrons or positrons. Beta minus particles are fast moving electrons released by the nucleus after a neutron turns into a proton. Beta plus particles are fast moving positrons, emitted after a proton turns into a neutron, the antiparticle of the electron - it has the same mass but a positive charge. They are both moderately ionising. Beta minus particles have a range in air of a few metres and are absorbed by a sheet of aluminium around 5mm thick. Beta plus particles have a smaller range as they are annihilated by electrons, releasing gamma rays.
What are gamma rays?
Gamma rays are waves of EM radiation released by the nucleus after nuclear rearrangement to carry away energy. They penetrate very far into materials and travel long distances through air, often kilometres. They are weakly ionising as they tend to pass through rather than collide with atoms. They can be absorbed by thick sheets of lead or metres of concrete.
How can we detect radiation?
Using a Geiger counter, which has a Geiger-Müller tube that clicks each time it detects radiation. This can be attached to a counter, which displays the number of clicks per second (the count rate)
What is background radiation and where does it come from?
Background radiation is low-level radiation that is around us all the time. It comes from: radioactivity of naturally occurring unstable isotopes e.g. rocks, foods, the air, and building materials; radiation from space (cosmic rays), mostly from the Sun; and radiation from human activity e.g. medical tests
What is the decay equation for alpha radiation?
a,zX —> a-4,z-2Y + 4,2He
E.g. 238,92U –> 234,90Th + 4,2He
What is the decay equation for beta minus radiation?
a,zX –> a,z+1Y + 0,-1e
E.g. 137,55Cs –> 137,56Ba + 0,-1e
What is the decay equation for beta plus radiation?
a,zX –>a,z-1Y + 0,+1e+
E.g. 37,20Ca –> 37,19K + 0,+1e+
What is the decay equation for gamma radiation?
a,zX –> a,zX + 0,0gamma
E.g. 226,88Ra –> 226,88Ra + 0,0gamma
What is activity?
The number of decays per second (Becquerels, Bq)
What is halflife?
While radioactive decay is a random process, the amount of time it takes for half the nucleus to decay is always the same. This is called halflife.
Or the average time taken for the number of radioactive nuclei in an isotope to halve.
To calculate it from a graph, choose a value and its time, then find half the value and its time, and find the difference in time.
How is ionising radiation used in smoke detectors?
A weak source of alpha radiation is placed close to two electrodes. It causes ionisation and so a current flows. Smoke absorbs the radiation, stopping the current and sounding the alarm.
How is ionising radiation used in irradiating food and sterilising equipment?
A high dose of gamma radiation is fired at the food or equipment, killing all microbes. It means high heat is not needed, which could spoil the food, and allows it to keep for longer. With medical equipment isotope with a long half life is used so the process can be fairly infrequent.
How is ionising radiation used in measuring the thickness of a material?
Beta radiation is fired through a material at a detector. When the amount detected changes, the thickness is different, so rollers can correct it.
How is ionising radiation used in medical tracers?
Beta or gamma radiation, but never alpha as it cannot pass out of the body and does lots of damaged, is injected or swallowed and emits radiation that passes out of the body and is tracked by a monitor. They detect medical conditions that cause blockages. Isotopes with a short half-life are used.
How is ionising radiation used in cancer treatment?
Alpha or beta emitters are injected near to the tumour, damaging cancer cells and normal cells around it. Gamma rays are aimed carefully at the tumour externally, sometimes with shielding on the body to protect healthy cells. They have long half lives so it does not have to be replaced often.
What are some precautions when using ionising radiation?
Keeping sources in lead lined boxes - prevents gamma rays from escaping
Standing behind barriers - absorbs some radiation to stop workers from being irradiated
Wearing photographic film badges - allows the monitoring of the dose of radiation
Wearing gloves/ protective suits - stops particles getting stuck to skin of under nails, or being breathed in
What is the reactor vessel?
It is the part of a nuclear fission plant where nuclear fission takes place. It is made of thick lead and concrete and contains two things: fuel (uranium-235) and a moderator (water)
What is fission?
The splitting of a nucleus into two daughter nuclei with the release of energy
How does nuclear fission work?
A slow moving neutron is fired at a nucleus e.g. uranium-235, making the nucleus unstable and causing it to split and release energy. This then releases some more neutrons, causing a chain reaction to start.
What is a moderator?
It is water, used in nuclear fission to slow down neutrons, allowing them to split nuclei.
What are control rods?
They are rods, usually made of boron. They can be raised or lowered into a reactor vessel to control the reaction rate. They absorb some neutrons to stop the reaction getting out of hand and causing a meltdown.
What is nuclear fusion?
The joining of two separate nuclei into one nucleus with the release of energy
How does nuclear fusion work?
Two lighter nuclei collide at high speed and fuse to create a larger, heavier nucleus (e.g. hydrogen to helium). The heavier nucleus does not have as much mass as the two smaller ones did in total; some of their mass is converted to energy, which is released as radiation.
What conditions are needed for nuclear fusion? Why?
Very high temperatures and pressures (~10,000,000 degrees) The protons are positive so repel each other. We need to put in huge amounts of energy to overcome this electrostatic repulsion.
What are the cons of using nuclear power?
Public perception is very poor
Waste products have very long half-lives so will be radioactive for hundreds of thousands of years - there is the danger of pollution and leakages
Nuclear meltdowns cause lots of damage
The overall cost is high due to the initial and decommissioning costs of plants
What effect can ionising radiation have on living cells?
Mutation - damaging genetic material leading to mutation or cancer
Damage cells - preventing cell division and causing tissue damage
What is the difference between irradiation and contamination?
Irradiation occurs when an object is exposed to a source outside the object. It does not cause the object to become radioactive, stops as soon as the source is removed, and can be blocked from the object with suitable shielding.
Contamination occurs when the radioactive source is on or in the object. A contaminated object will be radioactive for as long as the source is on or in it, it can be very difficult to remove all contamination, and once an object is contaminated the radiation cannot be blocked from the object.
How does PET scanning work?
A patient is injected with a substance (e.g. glucose) containing a positron emitting isotope with a short half-life - it acts as a tracer
The positrons meet electrons in an organ and annihilate, emitting gamma rays in opposite directions. Detectors detect each pair; the tumour lies along this path, and with another location, it can be triangulated.
The distribution of radioactivity matches up with the rate of metabolic activity, as more of the glucose is used by cells there.
What are the pros of using nuclear power?
It is quite safe overall
It is very reliable and reduces the need for fossil fuels
It does not produce carbon dioxide or sulphur dioxide
Lots of energy can be generated from a small amount of nuclear material