Chapter 4: Atomic Structure

4.1-Developing the Model of the Atom

Rutherford replaced the plum pudding model with the nuclear model

• In 1804, John Dalton agreed with Democritus that matter was made up of tiny spheres that couldn’t be broken up, but he reckoned that each element was made up of a different type of atom
• However, 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.
• But 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-something the plum pudding model couldn’t explain
• Nearly 100 years later, J.J.Thompson discovered particles called electrons that could be removed from atoms.
  • So Dalton’s theory wasn’t quite right.
• Thomson suggested atoms were spheres of positive charge with tiny negative electrons stuck in them like fruit in a plum pudding
• the plum pudding model
• Because a few alpha particles were deflected back, the scientists realised that 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, since it repelled the positive alpha particles
• 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.

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Which developed into the current model of the atom

• The nuclear model that resulted from the alpha particle scattering experiment was a positively charged nucleus surrounded by a cloud of negative electrons
• Niels Bohr said that electrons orbiting the nucleus do so at certain distances called energy levels.
• His theoretical calculations agreed with experimental data
• Evidence from further experiments changed the model to have a nucleus made up of a group of particles which all had the same positive charge that added up to the overall charge of the nucleus
• 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

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Current model of the atom

• The model is constantly being changed, but currently
  • It contains protons and neutrons, which gives it an overall positive charge
  • The rest of mostly empty space, negative electrons more around the outside of the nucleus really fast
  • Radius of atom is 1x10(-10)
  • Number of protons = number of electrons
  • If they gain energy by absorbing EM radiation they move to a higher energy level

4.2-Isotopes and Nuclear Radiation

Isotopes are different forms of the same element

• All atoms of each element have a set number of protons.
• The number of protons in an atoms is its atomic number
  • The mass number is the number of protons+neutrons
• Isotopes are atoms with same number of protons different number of neutrons
  • All elements have different isotopes, but there are usually only one or two stable ones
• 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
• Radioactive substances spit out one or more types of ionising radiation from their nucleus-the ones you need to know are alpha, beta and gamma radiation
• They can also release neutrons when they decay, as they rebalance their atomic and mass numbers
  • Ionising radiation is radiation that knocks electrons off atoms, creating positive ions,
  • The ionising power of a radiation source is how easily it can do this

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Alpha particles are helium nuclei

• Alpha radiation is when an alpha particle is emitted from the nucleus.
• A a-particle is two neutrons and two protons
• 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
  • Because of their size they are strongly ionising

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Beta particles are high-speed electrons

• A beta particles, is simply a fast-moving electron released by the nucleus. Beta particles have virtually no mass and a charge of -1
  • They are moderately ionising.
  • They penetrate moderately far into materials before colliding and have a range in air of a few meters.
  • They are absorbed by a sheet of aluminium
• For every beta particle emitted, a neutron in the nucleus has turned into a proton

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Gamma rays are EM waves with a short wavelength

• Gamma rays are waves of electromagnetic radiation released by the nucleus
• They penetrate far into materials without being stopped and will travel a long distance through air
  • This means they are weakly ionising because they tend to pass through rather than collide with atoms.
  • Eventually they hit something and do damage
• They can be absorbed by thick sheets of lead or metres of concrete

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4.3-Nuclear Equations

Mass and atomic numbers have to balance

• Nuclear equations are a way of showing radioactive decay by using element symbols
• They’re written in the form:atom before decay - atom after decay+radiation emitted
• There is one golden rule to remember:
• the total mass and atomic number must be equal on both sides

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Alpha decay decreases the charge and mass of the nucleus

• Remember, alpha particles are made up of two protons and two neutrons.
• So when an atom emits an alpha particles, its atomic number reduces by 2 and its mass number reduces by 4
• A proton is positively charges and a neutron is neutral, so the charge of the nucleus decreases
• In nuclear equations, an alpha particles can be written as a helium nucleus

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Beta decay increases the charge of the of the nucleus

• When beta decay occurs, a neutron in the nucleus turns into a proton and releases a fast-moving electron
• The number of protons in the nucleus has increased by 1.
• This increases the positive charge of the nucleus
• Because the nucleus has lost a neutron and gained a proton during beta decay, the mass of the nucleus doesn’t charge
  • A beta particle is written as 0/-1e in nuclear equations

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Gamma rays don’t change the charge or mass of the nucleus

• Gammas rays are a way of getting rid of excess energy from a nucleus
• This means that there is no change to the atomic mass or atomic number of the atom

4.4-Half-life

Radioactivity is a totally random process

• Radioactive substances give out radiation from the nuclei of their atoms-no matter what
  • 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
• Radioactive decay is entirely random. So you can’t predict exactly which nucleus in a sample will decay next, or when any one of them will decay
  • 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
• Half-life can be used to find the rate at which a source decays-its ACTIVITY. Activity is measured in becquerels, Bq

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The radioactivity of a source decreases over time

• Each time a radioactive nucleus decays to become a stable nucleus, the activity as a whole will decrease
• For some isotopes it takes just a few hours before nearly all the unstable nuclei have decayed, whilst others last for millions of years
  • 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
• The half-life is the time taken for the number of radioactive nuclei in an isotope to halve
• It is also the time taken for the activity, and so count-rate, to halve.
• A short half-life means the activity falls quickly, because the nuclei are very unstable and rapidly decay.
• Sources with a short half-life are dangerous because of the high amount of radiation they emit at the start, but they quickly become safe
  • 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

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You can measure half-life using a graph

• If you plot a graph of activity against time, it will always be shaped like the one to the right

  • The half-life is found from the graph by finding the time interval on the bottom axis corresponding to a halving of the activity on the vertical axis

    

4.5-Background Radiation and Contamination

Background Radiation comes from many sources

Background radiation is the low-level radiation that’s around us all the time. You should always measure and subtract the background radiation from your results. It comes from:

• Radioactivity of naturally occuring unstable isotopes which are all around us-in the air, in food, in building materials and in the rocks under our feet
• Radiation from space, which is known as cosmic rays.
• These come mostly from the sun.
• Luckily, the earth’s atmosphere protects us from much of this radiation
  • Radiation due to human activity or nuclear waste exists.
  • But this represents a tiny proportion of the total background radiation
  • The radiation doss tells you the risk of harm to body tissues due to exposure to radiation. It’s measured in sieverts.
  • The dose from background radiation is small, so millisieverts are often used.
  • Your radiation dose varies depending on where you live or if you have a job that involves radiation

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Exposure to radiation is called irradiation

• Objects near a radioactive source are irradiated by it.
• This simply means they’re exposed to it
• Irradiating something does not make it radioactive
• Keeping sources in lead-in boxes, standing behind barriers or being in a different room and using remote-controlled arms are all ways of reducing the effects of irradiation

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Contamination is radioactive particles getting onto objects

• If unwanted radioactive atoms get onto or into an object, the object is said to be contaminated.
• These contaminating atoms might then decay, releasing radiation which could cause you harm
• Contamination is especially dangerous because radioactive particles could get inside your body
• Gloves and tongs should be used when handling sources, to avoid particles getting stuck to your skin or under your nails.
• Some industrial workers wear protective suits to stop them breathing in particles

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The seriousness of irradiation and contamination depends on the source

Contamination or irradiation can cause different amounts of harm

• Outside the body, beta and gamma sources are the most dangerous.
• This is because beta and gamma can penetrate the body and get to the delicate organs.
• Alpha is less dangerous because it can’t penetrate the skin and is easily blocked by a small air gap.
• High levels of irradiation from all sources are dangerous, but especially from ones that emit beta and gamma
• Inside the body, alpha sources are the most dangerous, because they do all their damage in a very localised area.
• So contamination, rather than irradiation, is the major concern when working with alpha sources.
• Beta sources are less damaging inside the body, as radiation is absorbed over a wider area, and some passes out of the body although.
• Gamma sources are the least dangerous inside the body, as they mostly pass straight out-they have the lowest ionising power
• The more we understand about how radiation affects our bodies, the better we can protect ourselves when using it.

4.6-Uses and Risk

There are risks to using radiation

• Radiation can enter living cells and ionise atoms and molecules within them, this can lead to tissue damage
  • Lower doses tend to cause minor damage without killing the cells.
  • This can give rise to mutant cells which divide uncontrollably.
  • This is cancer.
  • Higher doses tend to kill cells completely, causing radiation sickness if a lot of cells all get blatted at once

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Gamma sources are usually used in medical tracers

• Certain radioactive isotopes can be injected into people and their progress around the body can be followed using an external detector.
• A computer converts the reading to a display showing where the strongest reading is coming from
  • One example is the use of iodine-123, which is absorbed by the thyroid gland just like normal iodine-127, but it gives out radiation which can be detected to indicate whether the thyroid gland is taking in iodine as it should
• 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

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Radiotherapy-treating cancer with radiation

• Since high doses of ionising radiation will kill all living cells, it can be used to treat cancers
  • 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, then it’s worth it

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You have to weigh up the risks and benefits

• Risks: prolonged exposure to radiation poses future risks and causes side effects
  • Benefits: can get rid of cancer entirely
  • Perceived risk can vary from person to person

4.7-Fission and Fusion

Nuclear fission-splitting a large, unstable nucleus

• Nuclear fission is a type of nuclear reaction used to release energy from large and unstable atoms by splitting them into smaller atoms
  • Spontaneous fission rarely happens.
  • Usually, the nucleus has to absorb a neutron before it will split
  • When the atom splits it forms two new lighter elements that are roughly the same size
  • 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 cause more fission to occur, which is a chain reaction
• The energy not transferred to the kinetic energy is carried away by gamma rays
  • Energy carried away can be used to heat water making steam to turn turbines and generators
• The amount of energy produced by fission in a nuclear reactor is controlled by changing how quickly the chain reaction can occur, which is done by control rods
• Uncontrolled chain reactions quickly lead to lots of energy being released as an explosion-this is how nuclear weapons work

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Nuclear fusion-joining small nuclei

• Nuclear fusion is the opposite of nuclear fission
  • Two light nuclei collide at high speed and join to create larger, heavier nucleus
• Heavier nucleus produced does not have as much mass as the two separate light nuclei did.
• Some of the mass is converted to energy, which is released as radiation
• Fusion released a lot of energy
• Scientists haven’t found a way of using fusion to generate energy, and would be to hard and expensive to do at the moment

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