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Year 10 Radioactivity 2
Students should understand the following:
- Atoms are very small, having a radius of about 1 × 10-10 metres.
- The radius of a nucleus is less than 1/10 000 of the radius of an atom.
- New experimental evidence may lead to a scientific model being changed or replaced.
- Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.
- The discovery of the electron led to the plum pudding model of the atom. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.
- The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. This nuclear model replaced the plum pudding model.
- Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations.
- Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles.
- The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.
- Students should be able to describe
- why the new evidence from the scattering experiment led to a change in the atomic model
- the difference between the plum pudding model of the atom and the nuclear model of the atom.
- Details of experimental work supporting the Bohr model are not required.
- Details of Chadwick’s experimental work are not required.
- Nuclear equations are used to represent radioactive decay.
- In a nuclear equation an alpha particle may be represented by the symbol:
- and a beta particle by the symbol:
- The emission of the different types of nuclear radiation may cause a change in the mass and /or the charge of the nucleus.
- So alpha decay causes both the mass and charge of the nucleus to decrease.
- So beta decay does not cause the mass of the nucleus to change but does cause the charge of the nucleus to increase.
- Students should be able to use the names and symbols of common nuclei and particles to write balanced equations that show single alpha (α) and beta (β) decay. This is limited to balancing the atomic numbers and mass numbers. The identification of daughter elements from such decays is not required.
- The emission of a gamma ray does not cause the mass or the charge of the nucleus to change.
- Background radiation is around us all of the time.
- It comes from:
- natural sources such as rocks and cosmic rays from space
- man-made sources such as the fallout from nuclear weapons testing and nuclear accidents.
- The level of background radiation and radiation dose may be affected by occupation and/or location.
- Radiation dose is measured in sieverts (Sv)
- 1000 millisieverts (mSv) = 1 sievert (Sv)
- Students will not need to recall the unit of radiation dose.
- Nuclear fission is the splitting of a large and unstable nucleus (eg uranium or plutonium).
- Spontaneous fission is rare. Usually, for fission to occur the unstable nucleus must first absorb a neutron.
- The nucleus undergoing fission splits into two smaller nuclei, roughly equal in size, and emits two or three neutrons plus gamma rays. Energy is released by the fission reaction.
- All of the fission products have kinetic energy.
- The neutrons may go on to start a chain reaction.
- The chain reaction is controlled in a nuclear reactor to control the energy released. The explosion caused by a nuclear weapon is caused by an uncontrolled chain reaction.
- Students should be able to draw/interpret diagrams representing nuclear fission and how a chain reaction may occur.
- Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. In this process some of the mass may be converted into the energy of radiation.
- The Sun was formed from a cloud of dust and gas (nebula) pulled together by gravitational attraction.
- Students should be able to explain:
- how, at the start of a star’s life cycle, the dust and gas drawn together by gravity causes fusion reactions
- that fusion reactions lead to an equilibrium between the gravitational collapse of a star and the expansion of a star due to fusion energy.
- A star goes through a life cycle. The life cycle is determined by the size of the star.
- Students should be able to describe the life cycle of a star:
- the size of the Sun
- much more massive than the Sun.
- Fusion processes in stars produce all of the naturally occurring elements. Elements heavier than iron are produced in a supernova.
- The explosion of a massive star (supernova) distributes the elements throughout the universe.
- Students should be able to explain how fusion processes lead to the formation of new elements.
- LOGON SCIENCE CODES:
188.8.131.52, 184.108.40.206, 220.127.116.11, 18.104.22.168, 22.214.171.124, 126.96.36.199, 188.8.131.52, 184.108.40.206, 220.127.116.11, 18.104.22.168,
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