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(c) doc b(c) doc bDoc Brown's Chemistry KS4 science GCSE/IGCSE/AS Physics Revision Notes

8. Nuclear Fusion Reactions & formation of 'heavy elements'

What is nuclear fusion? Why do nuclear fusion reactions release so much energy? Why is it difficult to sustain a nuclear fusion reaction? What is an artificial or man-made element? How are heavy elements like the trans-uranium elements made-synthesised? What is an ion particle accelerator?

Radioactivity & nuclear physics Index 1a. The Structure of Atoms - 3 fundamental particles * 1b. What it is an atom like? * 2a. What is Radioactivity? Why does it happen? * 2b. How did they find out there were three types of atomic-ionising radiation? * 3a. Detection of Radioactivity and its measurement, units * 3b. Ionising Radiation sources * 4a. The properties of the three types of radioactive emission and symbols * 4b The dangers of radioactive emissions - beware of ionising radiation from radio-isotopes! * 5. The uses of radioactive Isotopes emitting alpha, beta or gamma radiation * 6a. The half-life of a radioisotope - how long does material remain radioactive? implications! * 6b. Uses of decay data and half-life values * 7a. What actually happens to the nucleus in alpha and beta radioactive decay? nuclear equations * 7b. The production of Radioisotopes - artificial sources * 8. Nuclear fusion reactions and the formation of 'heavy elements' * 9. Nuclear Fission Reactions, nuclear power energy resource

 

8. Nuclear Fusion Reactions and the formation of 'heavy elements'

  • At the extremely high temperatures in the 'heart' of stars the atomic nuclei have such enormous speeds and kinetic energies that on collision they can fuse together.

  • The extremely high energy is needed to overcome the natural and massive repulsion of the two positive nuclei involved.

  • The process by which a heavier atomic nucleus is made from two smaller atomic nuclei is called fusion and these changes also release enormous amounts of energy.

  • The smallest atom is hydrogen, this is converted to helium and gradually all the other elements up to uranium must have been formed in stars like the Sun.

    • Attempts are being made by nuclear scientists and engineers to build prototype nuclear fusion reactors BUT the task of maintaining nuclear fusion is proving extremely difficult. You have to maintain an extremely high temperature and confine and control the plasma of hydrogen atoms. So far, fusion has only been created for a fraction of a second?

  • Examples of fusion nuclear equations (get the balancing?) ....

  • (a) (c) doc b (initially a heavier isotope of hydrogen is formed and a positron)

  • (b)(c) doc b + (c) doc b

  • (c)(c) doc b

  • (d)(c) doc b

  • (e)(c) doc b

  • (f) (c) doc b (the most abundant helium isotope found today)

  • (g)(c) doc b

  • (h) helium nuclei fuse to form lithium, beryllium etc.

  • (i) then from carbon to oxygen etc. (c) doc b

  • (j) and lots of alternative fusions like (c) doc b... etc.

  • (k) gradually building up elements with increasing atomic and mass numbers, and finally the massive isotope of uranium, (c) doc b the biggest 'naturally' occurring atom!

  • (a), (b) and (f) are believed to be the main initial energy releasing fusion nuclear reactions in the Sun, they happen quite nicely at 15000000oC!

  • On 'Earth' super-heavy' elements are being made in nuclear reactors by bombarding elements like uranium (atomic number 92) with lighter particles (described below).

Advanced Chemistry Page Index and Links

The production of Trans-Uranium Elements

  • Heavy atomic nuclei tend to be naturally unstable and for example, many long lived isotopes of uranium (U92) finally decay via a series of relatively short-lived radioisotopes to produce stable isotopes of lead (82Pb).

  • No element higher than uranium (92U) is found in nature except for traces of neptunium (93Np) and plutonium (94Pu) isotopes. These are found in uranium ores but are produced by neutron-uranium collisions rather than from the Earth's origin. The neutrons come from the spontaneous fission of the unstable uranium isotope 235U and gives rise to heavy element 'synthesis' sequence.

    • eg 238U == + n ==> 239U == beta decay ==> 239Np == beta decay ==> 239Pu

  • Even heavier or 'trans-uranium' elements can be made by bombarding a heavy atomic nucleus with a smaller ionised atom particle, in an ion particle accelerator.

    • However many of the heaviest are only produced in minute quantities as little as a few hundred atoms in accelerator collisions.

    • In an accelerator the two atoms are ionised and accelerated in powerful electromagnetic fields to very high speeds eg close to speed of light, but in opposite directions and are then allowed to collide. The high kinetic energies are needed to overcome the repulsion of the two positive nuclei.

    • See examples 1. to 3. below.

  • The heavier elements are also made by neutron bombardment in a nuclear reactor.

    • Although most neutrons partake in nuclear fission reactions (see section 9.), in some cases this will create a bigger nucleus.

      •  e.g. Np and Pu 'natural' examples above and example 4. below.

  • So, from these two methods, a whole series of man-made or 'artificial' elements from atomic number 93 to 112 have been synthesised.

  • Where they are formed in nuclear reactors from neutron collision (e.g. plutonium), they can be chemically separated in quantities ranging from micrograms to kg in order study their physical and chemical properties.

  • Note again, the balancing of nuclear equations e.g.

(c) doc b

formation of einsteinium from uranium and nitrogen nuclei

(c) doc b

formation of californium from uranium and carbon nuclei

(c) doc b

formation of lawrencium from californium and boron nuclei

(c) doc b

formation of americium from plutonium and neutrons

Advanced Chemistry Page Index and Links

(c) doc b(c) doc bRADIOACTIVITY multiple choice QUIZZES and WORKSHEETS

Easier-Foundation Radioactivity Quiz

or Harder-Higher Radioactivity Quiz

 (c) doc b five word-fills on radioactivity * Q2 * Q3 * Q4 * Q5and ANSWERS!

crossword puzzle on radioactivity and ANSWERS!


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