Physics (Atomic structure) GCSE 9-1 AQA COPY
Atoms contain a positively charged nucleus made up of protons and neutrons.Which is surrounded by negatively charges electrons.
How may the energy level of an electron change?
– Absorbing electromagnetic radiation moves electrons to a higher energy level, further from the nucleus.
– Electromagnetic radiation is emitted when an electron drops to a lower energy level.
All isotopes of an element contain the same number of protons.It is the number of neutrons that is different.
The plum pudding model.
The ‘plum pudding’ model of the atom was proposed by JJ Thomson, who had also discovered the electron. It was put forth before the discovery of the nucleus. According to this model, the atom is a sphere of positive charge, and negatively charged electrons are embedded in it to balance the total positive charge
Rutherford and Marsden
A scientist called Rutherford designed an experiment to test the plum pudding model. It was carried out by his assistants Geiger and Marsden. A beam of alpha particles was aimed at very thin gold foil and their passage through the foil detected. The scientists expected the alpha particles to pass straight through the foil, but something else also happened.
Some of the alpha particles emerged from the foil at different angles, and some even came straight back. The scientists realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom. As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.
How do unstable atoms become stable?
Some atomic nuclei are unstable and give out radiation in order to become more stable.
Alpha radiation is the least penetrating. It can be stopped (or absorbed) by a sheet of paper.
Highly likely to be absorbed and cause damage if passing through living cells.
Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.
Likely to cause damage if absorbed by living cells.
can penetrate the body to inner organs.
Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead, or many metres of concrete.
Likely to pass through living s without being absorbed and causing ionization.
Radioactive contamination is the unwanted presence of materials containing raidoactive atoms on other materials.
Irradiation is the process of exposing an object to nuclear radiation.
– can be delebriate or accidental
– Does not cause the object to become radioactive.
the time taken for the radioactivity of a specified isotope to fall to half its original value.
Uses of Nuclear radiation in medicine
Treatment of tumors
Fission is another word for splitting:the nucleus splits into two smaller nuclei, which are radioactive
two or three more neutrons are released
some energy is released
The additional neutrons released may also hit other uranium or plutonium nuclei and cause them to split. Even more neutrons are then released, which in turn can split more nuclei. This is called a chain reaction. The chain reaction in nuclear reactors is controlled to stop it going too fast.
Nuclear fusion involves two atomic nuclei joining to make a large nucleus. Energy is released when this happens.