Physics - Topic 4 - Atomic Structure (Isotopes and Nuclear Radiation…
Physics - Topic 4 - Atomic Structure
Developing the model of the atom
Which Developed into the Current Model of the Atom
Niels Bohr said that electrons orbiting the nucleus do so at certain distances called energy levels. His theoretical calculations agreed with experimental data.
Further experiments changed the model to have a nucleus made up of a group of particles (protons) which all had the same positive charge that added up to the overall charge of the nucleus.
The nuclear model that resulted from the alpha particle scattering experiment was a positively charged nucleus surrounded by a cloud of negative electrons.
About 20 years after the idea of a nucleus was accepted, in 1932, James Chadwick proved the existence of the neutron, which explained the imbalance between the atomic and mass numbers
Replacement of the plum pudding model with the nuclear model
In 1909, scientists in Rutherford's lab tried firing a beam of Alpha Particles at thin gold foil - this was the alpha scattering experiment.
From the plum pudding model , they expected the particles to pass straight through the gold sheet or only be slightly deflected.
Although most of the particles did go straight through the sheet, some were deflected more than expected, and a few were deflected back the way they had come.
Few alpha particles were deflected back, so scientists knew most of the mass of the atom must be concentrated at the centre in a tiny nucleus. This nucleus must also have a positive charge,
Nearly 100 years later JJ Thompson discovered particles called electrons that could be removed from atoms, Dalton's theory wasn't quite right.
They also realised that because nearly all the alpha particles passed straight through, most of an atom is just empty space. This was the first nuclear model of the atom.
In 1804 John Dalton agreed with Democritus that matter was made up of ting spheres that couldn't be broken up, he thought each element was made of a different type of atom.
The Current Model of the Atom
Most of the atom is empty space,
Negative electrons whizz round the outside of the nucleus really fast.
Its radius is about 10 000 times smaller than the radius of the atom.
number of protons = the number of electrons
The nucleus is tiny but it makes up most of the mass of the atom.
It contains protons and neutrons which give it a positive charge.
Electrons in energy levels can move within the atom.
If they gain energy by absorbing EM radiation they move to a higher energy level, further from the nucleus.
If they move to a higher energy level, further from the nucleus.
If one or more outer electrons leaves the atom, the atom becomes a positively charged ion.
Isotopes and Nuclear Radiation
Alpha Particles are Helium Nuclei
2) They don't penetrate very far into materials and are stopped quickly- they can only travel a few cm in air and are absorbed by a sheet of paper.
3)Because of their size they are strongly ionising.
1) Alpha radiation is when an alpha particle is emitted from the nucleus. An alpha particle is two neutrons and two protons (like a helium nucleus)
Beta Particles are High-Speed Electrons
1) A beta particle is simply a fast-moving electron released by the nucleus. Beta particles have virtually no mass and a charge of -1.
2) They are moderately ionising. They penetrate moderately far into materials before colliding and have a range in air of a few metres.
3) For every beta particle emitted, a neutron in the nucleus has turned into a proton.
Isotopes are Different Forms of the Same Element
4) All elements have different isotopes, but they are usually only 1 or 2 stable ones.
5) The other unstable isotopes tend to decay into other elements and give out radiation as they try to become more stable. This process is called radioactive decay.
3)Isotopes of an element are atoms with the same number of protons but they have a DIFFERENT NUMBER OF NEUTRONS(different mass number)
6) Radioactive substances spit out one or more types of ionising radiation from their nucleus- .
2) The mass number of an atom is the number of protons+ the number of neutrons in its nucleus.
7) They can also release neutrons when they decay, as they rebalance their atomic and mass numbers
1) Each nucleus has a positive charge, the number of protons is the atomic number.
8) Ionising radiation knocks electrons off atoms, creating positive ions.
Gamma Rays are EM Waves with a Short Wavelength
2) They penetrate far into materials without being stopped and will travel a long distance through air.
3) This means they are weakly ionising, they pass through atoms. Eventually they hit something and do damage.
1)Gamma rays are waves of electromagnetic radiation released by the nucleus.
4) They can be absorbed by thick sheets of lead or metres of concrete.
Mass and Atomic Numbers Have to Balance
2) They're written in the form:
atom before decay--> atom after decay + radiation emitted.
3) The total mass and atomic numbers must be equal on both sides
1) Nuclear equations are a way of showing radioactive decay by using element symbols.
Alpha Decay Decreases the Charge and Mass of the Nucleus
2) A proton is positively charged and a neutron is neutral, so the charge of the nucleus decreases.
3) In nuclear equations, an alpha particle can be written as a helium nucleus 4/2HE
1) Alpha particles are made up of two protons and two neutrons. So when an atom emits an alpha particle, its atomic number reduces by 2 and its mass number reduces by 4.
Beta Decay Increases the Charge of the Nucleus
2) The number of protons in the nucleus has increased by 1. This increases the positive charge of the nucleus(the atomic number)
3)Because the nucleus has lost a neutron and gained a proton, the mass of the nucleus doesn't change.
1)When beta decay occurs, a neutron in the nucleus turns into a proton and releases a fast-moving electron(the beta particle)
4) A beta particle is written as 0/-1e in nuclear equations
Gamma Rays Don’t Change the Charge or Mass of the Nucleus
1) Gamma rays are a way of getting rid of excess energy from a nucleus.
2) This means that there is no change to the atomic mass or atomic number of the atom
The Radioactivity of a Source Decreases Over Time
2) For some isotopes it takes just a few hours before nearly all the unstable nuclei have decayed, whilst others last for millions of years.
3) The problem with trying to measure this is that the activity never reaches zero, which is why we have to use the idea of half-life to measure how quickly the activity drops off.
1) Each time a radioactive nucleus decays to become a stable nucleus, the activity as a whole will decrease
The HALF-LIFE is the time taken for the number of radioactive nuclei in an isotope to halve.
4) It is also the time taken for the activity, and so count-rate, to halve.Source with a short half-life are dangerous because of the high amount of radiation they emit at the start, but they quickly become safe.
5) A long half-life means the activity falls more slowly because most of the nuclei don't decay for a long time- the source just sits there, releasing small amounts of radiation for a long time. This can be dangerous because nearby areas are exposed to radiation for millions of years.
Radioactivity is a Totally Random Process
3) Radioactive decay is entirely random
4) But you can find out the time it takes for the amount of radiation emitted by a source to halve, this is known as the half-life. It can be used to make predictions about radioactive sources, even though their decays are random.
2)This radiation can be measured with a Geiger-Muller tube and counter, which records the count-rate--the number of radiation counts reaching it per second.
5) Half-life can be used to find the rate which a source decays- its ACTIVITY. Activity is measured is Becquerels, Bq(where 1 Bq is 1 decay per second)
1) Radioactive substances give out radiation from the nuclei of their atoms-no matter what.
Background Radiation and Contamination
Exposure to Radiation is called Irradiation
2) Irradiating something does not make it radioactive.
3) Keeping sources in lead-lined boxes, standing behind barriers or being in a different room and using remote-controlled arms are all ways of reducing the effects or irradiation.
1) Objects near a radioactive source are irradiated by it. This is simply means they're exposed to it(we're always being irradiated by background radiation sources).
Contamination is Radioactive Particles Getting onto Objects
2) These contaminated atoms might then decay, releasing radiation which could cause you harm.
3) Contamination is especially dangerous because radioactive particles could ge inside your body.
1) If unwanted radioactive atoms get onto or into an object, the object is said to be contaminated.
4) Gloves and tongs should be used when handling sources. Some industrial workers wear protective suits to stop them breathing in particles.
Background Radiation Comes From Many Sources
2) Radiation from space, which is known as cosmic rays. Come mostly from the sun. Luckily, the earths atmosphere protects us from this radiation.
3) Radiation due to human activity, e.g fallout from nuclear explosions or nuclear waste. Represents a tiny proportion of the total background radiation.
1) Radioactivity of naturally occurring unstable isotopes which are all around us.
The radiation dose tells you the risk of harm to body tissues due to exposure to radiation. It's measured in Sieverts. The dose from background is small, so millisieverts are often used (1Sv=1000mSv). Your radiation dose varies depending on where you live or if you have a job that involves radiation.
Background radiation is the low-level radiation that's around us all the time. It comes from:
The Seriousness of Irradiation and Contamination Depends on the Source
2) Because of the short range of alpha radiation, contamination is the major concern over radiation.
The more we understand how radiation affects our bodies, the better we can protect ourselves when using it. The data is peer-reviewed and can quickly become accepted, leading to many improvements in our use of radioactive sources.
1) Alpha radiation does not travel as far as beta and gamma but is more ionising(changes the charge of the atom, can knock electrons out of their shells)
Uses and Risk
Gamma Sources are Usually Used in Medical Tracers
2) The use of iodine-123, absorbed by the thyroid gland, gives out radiation which can indicate whether the thyroid gland is taking in iodine as it should
3) Isotopes which are taken into the body like this are usually
gamma so that the radiation passes out of the body without causing much ionisation. They should have a short half-life so the radioactivity inside the patient quickly disappears.
1)Cetrain radioactive istopes can be injected/ swallowed and their progress around the body can be followed with an external detector. A computer reads this and detects where the strongest reading is coming from.
Radiotherapy — Treating Cancer with Radiation
Gamma rays are directed carefully and at just the right dosage to kill the cancer cells without damaging too many normal cells. Radiation-emitting implants can also be put next to or inside tumours.
However, a fair bit of damage is inevitably done to normal cells, which makes the patient feel very ill. But if the cancer is successfully killed off in the end.
Since high doses of ionising radiation will kill all living cells, it can be used to treat cancers
There are Risks to Using Radiation
2)Lower doses tend to cause minor damage without killing the cells. This can give rise to mutant cells which divide uncontrollably. This is cancer.
3)Higher doses tend to kill cells completely, causing radiation sickness if a lot of cells all get battled at once.
1) Radiation can enter living cells and ionise atoms and molecules within them. This can lead to tissue damage
You Have to Weigh Up the Risks and Benefits
Tracers can be used to diagnose life-threatening conditions,
while the risk of cancer from one use of a tracer is very small.
Whilst prolonged exposure to radiation poses future risks and causes many side effects, many people with cancer choose to have radiotherapy as it may get rid of their cancer entirely.
Perceived risk is how risky a person thinks something is. It’s not the same as the actual risk of a procedure and the perceived risk can vary from person to person.
Fission and Fusion
Nuclear Fission — Splitting a Large, Unstable Nucleus
The energy not transferred to the kinetic energy stores of the products is carried away by gamma rays.
The energy carried away by the gamma rays, and in the kinetic energy stores of the remaining free neutrons and the other decay products, can be used to heat water, making steam to turn turbines and generators
Two or three neutrons are also released when an atom splits. If any of these neutrons are moving slow enough to be absorbed by another nucleus,they can cause more fission to occur. This is a chain reaction.
The amount of energy produced by fission in a nuclear reactor is controlled by changing how quickly the chain reaction can occur. This is done using control rods, which are lowered and raised inside a nuclear reactor to absorb neutrons, slow down the chain reaction and control the amount of energy released.
When the atom splits it forms two new lighter elements that are roughly the same size.
Uncontrolled chain reactions quickly lead to lots of energy being released as an explosion — this is how nuclear weapons work.
Spontaneous (unforced) fission rarely happens. Usually, the nucleus has to absorb a neutron before it will split.
Nuclear Fusion — Joining Small Nuclei
The heavier nucleus produced by fusion does not have as much mass as the two separate, light nuclei did.
Fusion releases a lot of energy.
In nuclear fusion, two light nuclei collide at high speed and
join (fuse) to create a larger, heavier nucleus.
So far, scientists haven’t found a way of using fusion to generate energy for us to use. As it is hard and expensive to build.
Nuclear fusion is the opposite of nuclear fission.