Physics GCSE Combined Science: Radioactivity Topic
Physics GCSE Combined Science:
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 atom is its atomic number
The mass number of an atom is the number of protons + the number of neutrons
Isotopes of an element are atoms with the same number of protons (same atomic number) but a different number of neutrons (different mass number)
All elements have different isotopes, but there are usually only one or two stable ones
The unstable isotopes tend to decay into other elements and give out radiation as they try to become more stable (they try to balance the number of protons and neutrons in their nucleus or get rid of any excess energy). This process is called Radioactive Decay.
Radioactive substances spit out one or more types of ionising radiation from their nucleus - alpha, beta or gamma radiation.
They can also release neutrons when they decay to rebalance the number of protons and neutrons.
Ionising radiation is radiation that knocks electrons off atoms creating positive ions.
Alpha radiation is when an alpha particle (a) is emitted from the nucleus. An a-particle is two protons and two neutrons.
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.
A beta particle (B) 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 metres. They are absorbed by a sheet of aluminium (around 5mm).
For every beta particle emitted, a neutron in the nucleus has turned into a proton.
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 in 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.
Nuclear Equations show radioactive decay:
Mass and atomic numbers have to balance:
1) Nuclear equations are a way of showing radioactive decay by using element symbols.
2) They're written in the form: atom before decay = atom after decay + radiation emitted.
3) There is one rule to remember; the total mass and atomic numbers must be equal on both sides.
Alpha Decay decreases the charge and mass of the nucleus:
1) remember, 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.
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.
Beta Decay increases the charge of the nucleus:
1) When beta decay occurs, a neutron in the nucleus turns into a proton and releases a fast-moving electron (the beta particle).
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 during beta decay, the mass of the nucleus doesn't change (protons and neutrons have the same mass).
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.
Radioactivity is a totally random process:
1) Radioactive substances give out radiation from the nuclei of their atoms - no matter what
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.
3) 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.
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.
5) Half-life can be used to find the rate at which a source decays - its activity. Activity is measured in Becquerels. Bq (where 1 Bq is 1 decay per second).
The Radioactivity of a source decreases over time:
1) Each time a radioactive nucleus decays to become a stable nucleus, the activity as a whole will decrease. (older sources emit less radiation).
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.
The half-life is the time taken for the number of radioactive nuclei in an isotope to halve *
4) Half-life can also be described as the time taken for the activity (and so count-rate) to fall to half of its initial value.
You can measure half-life using a graph:
1) If you plot a graph of activity against time.
2) 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.
The count rate after n half-lives = the initial count rate divided by 2n.
Activity: The activity of a radioactive source is the number of unstable atoms in the source that decay per second.
Half-life: The half life of a radioactive material is the time (average) it takes for the number of nuclei of the isotope in a sample to halve.
Count Rate: The number of counts per second detected by the Geiger counter. This is proportional to the activity of the source as long as the distance between the tube and the source stays the same.
1) The nuclear model that resulted from the alpha particle scattering experiment was a positively charged nucleus surrounded by a cloud of negative electrons.
2) Niels Bohr said that electrons orbiting the nucleus do so at certain distances called energy levels. His theoretical calculations agreed with experiment data.
3) Evidence from 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.
4) 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.
The nucleus is tiny but makes up most of the mass of the atom
It contains protons (which are positively charged - they have a +1 relative charge) and neutrons (which are neutral, with a relative charge of 0) - which gives it an overall positive charge.
The rest of the atom is mostly empty space. Negative electrons (relative charge -1) whizz round the outside of the nucleus really fast
The number of protons = the number of electrons, as protons and electrons have an equal but opposite charge and atoms have no overall charge