(a) Briefly explain what would happen to a stable element if it is bombarded by \(\alpha\)-particles (b) Explain how the bombardment of Uranium with neutron...

(a) A stable element bombarded by alpha-particles

The alpha-particle (a helium nucleus) may penetrate the nucleus of the stable atom and be captured, forming a new, heavier and usually unstable (radioactive) nucleus, often with the emission of another particle such as a proton or a neutron. This is artificial transmutation: the target element is changed into a different element (a new nuclide), which may then be radioactive. (For example, alpha bombardment of nitrogen produces oxygen and a proton.)

(b) Nuclear fission chain reaction

When a slow (thermal) neutron strikes a nucleus of uranium-235, the nucleus captures it, becomes highly unstable and splits (fissions) into two lighter nuclei, releasing a large amount of energy. Crucially, each fission also releases two or three fresh neutrons. If enough fissile material is present (a critical mass), these new neutrons go on to split further uranium nuclei, each releasing yet more neutrons, so the number of fissions multiplies rapidly. This self-sustaining, ever-growing sequence is a chain reaction. If it is uncontrolled, an enormous amount of energy is released in a very short time, producing a nuclear explosion. (In a reactor the reaction is controlled by absorbing surplus neutrons.)

(c) Three characteristics of nuclear activity

• It is spontaneous and random, and unaffected by temperature, pressure or chemical combination.
• It releases very large amounts of energy compared with chemical reactions.
• It involves changes within the nucleus, producing new elements and emitting radiation (\(\alpha\), \(\beta\), \(\gamma\)).

(d) Three applications of atomic (nuclear) energy

• Generation of electricity in nuclear power stations.
• Use of radioisotopes in medicine (diagnosis, treatment of cancer, sterilization).
• Use as tracers in agriculture/industry, in carbon dating, and in propulsion (nuclear submarines).

(e) Bohr's model

Two postulates:

• An electron moves round the nucleus only in certain allowed circular orbits (stationary states) in which it does not radiate energy; each orbit has a fixed energy.
• Energy is emitted or absorbed only when an electron jumps from one allowed orbit to another, the photon energy being \(hf = E_2 - E_1\).

Two limitations:

• It successfully explains only the hydrogen (one-electron) atom and fails for atoms with more than one electron.
• It cannot account for the fine structure (splitting) of spectral lines or their relative intensities, and it violates later quantum ideas (the uncertainty principle).