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Chemistry-Physics Notes: 8. Nuclear fusion and synthesising heavy elements

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

Doc Brown's Chemistry KS4 science GCSE Physics Revision Notes

(c) doc bWhat 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? Can we build a nuclear fusion reactor power station? These revision notes on nuclear fusion and synthesising heavy elements should help with IGCSE/GCSE/ chemistry or physics courses and A/AS advanced level chemistry or physics courses.


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8a. Nuclear Fusion Reactions and the formation of 'heavier elements'

Know and understand that nuclear fusion is the joining together of atomic nuclei and is the process by which energy is released in stars.

Compare the uses of nuclear fusion and nuclear fission, but limited to the generation of electricity

See also Energy resources and comparison of methods of generating electricity

and Energy resources & uses, general survey & trends, comparing sources of renewables, non-renewables

  • At the extremely high temperatures (107 oC = 10 million degrees!) in the 'heart' of stars the atomic nuclei have such enormous speeds and kinetic energies that on collision they can fuse together - the nuclear process of fusion.

  • Extremely high temperatures (and pressures) are needed to give the particles sufficiently high kinetic energy to overcome the natural and massive repulsion forces of the two positive nuclei involved e.g. two positive hydrogen nuclei (+) <==> (+).

  • 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.

    • In a nuclear fusion process two smaller atomic nuclei may fuse into one larger nucleus or a larger nucleus that either of the starting nuclei plus a smaller nucleus.

    • Either way a heavier nucleus is created.

    • One advantage of nuclear fusion over nuclear fission is that little radioactive waste is produced, BUT, technically, nuclear fusion has proved technically very difficult to produce a continuous energy output, and we are a very long way from a nuclear fusion power station generating electricity.

  • The lightest 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, but it takes a massive star undergoing a supernova explosion to make the heaviest elements.

    • We would like to be able to reproduce this fusion process e.g. converting hydrogen into helium as an energy resource to generate electricity.

    • 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 fusing into helium atoms with powerful magnetic fields and this is proving technically very difficult, since you can't use a physical container.

    • So far, in a few experimental fusion reactors, fusion has only been created for a fraction of a second but cannot be controlled and sustained yet!

    • In fact its taking far more power to create the fusion than any energy released, not a good deal for the consumer at the moment!

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

  • (a) two hydrogen-1  ===>  hydogen-2  +  positron (beta plus particle)

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

    • -

  • (b) hydrogen-1  +  hydrogen-2  ===> helium-3

    • (c) doc b  + (c) doc b

    • -

  • (c) two hydrogen-2  ===> hydrogen-3  +  hydrogen-1

    • (c) doc b

    • -

  • (d) two hydrogen-2  ===>  helium-3  +  neutron

    • (c) doc b

    • -

  • (e) two hydrogen-2  ===>  helium-3  +  neutron

    • (c) doc b

    • -

  • (f)  two helium-3  ===>  helium-4  +  two hydrogen-1

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

    • -

  • (g) hydrogen-3  +  hydrogen-2  ===>  helium-4  +  neutron

    • (c) doc b

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

  • (i) then from carbon to oxygen etc.

    • carbon-13  +  helium-4  ===>  oxygen-16  +  neutron

    • (c) doc b

    • -

  • (j) and lots of alternative fusions like

    • nitrogen-14  +  helium-4  ===>  fluorine-17  +  neutron

    • (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 15 000 000oC!

  • From main sequence stars, like our own sun, converting hydrogen to helium in nuclear fusion, and also in red giants the elements from lithium (3Li) to iron (26Fe) are formed from fusing heavier nuclei, BUT ...

    • Elements heavier than iron, (cobalt to uranium, atomic numbers 27 to 92), are only formed in a supernova explosion of a red supergiant in a truly 'cosmic' scale explosion, where the temperatures are much higher than the 15 million degrees of the Sun!

    • The positive nuclei of heavier elements need enormous kinetic energies to overcome the massive repulsion forces between the positively charged nuclei, and fuse to make an even larger nucleus.

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

  • The hydrogen atomic bomb - the ultimate nuclear weapon uses fusion, not fission

    • The hydrogen atomic bomb was developed after WW2 in which fission bombs were dropped on the Japanese cities of Hiroshima and Nagasaki.

    • As already mentioned, an extremely high temperature is needed.

    • This is provided uses the energy from uncontrolled nuclear fission to produce the necessary temperature for fusion.

    • So, the hydrogen bomb is triggered to reproduce the energy releasing nuclear physics of a sun.

    • The hydrogen fusion bomb is many times more powerful than a fission bomb.


Cold fusion is nuclear fusion at low temperatures e.g. room temperature (NOT millions of degrees!).

In 1989 two scientists called Stanley Pons and Martin Fleischmann reported in scientific research paper that using a simple electrolysis cell system they had caused hydrogen atoms to fuse at room temperature.

They reported that much more heat energy was evolved compared to the electrical energy passed into the cell.

However their paper had NOT been peer reviewed, that is, read and their work validated by other independent scientists.

Many scientists were sceptical about their work, and since 1989, few scientists, if any?, have reliably reproduced their results.

Therefore, the scientific community does not officially accept that cold fusion is possible at the moment.

This is how science works, results must be reproducible in laboratories all around the world and so cold fusion theory is not accepted as a viable scientific concept.

8b. The production of Trans-Uranium Elements - very heavy 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 e.g.

    • 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.

uranium-238  +  nitrogen-14  ===> einsteinium-248  +  four neutrons

(c) doc b

formation of einsteinium from uranium and nitrogen nuclei


uranium-238  +  carbon-12  ===> californium-246  +  four neutrons

(c) doc b

formation of californium from uranium and carbon nuclei


californium-252  +  boron-11  ===>  lawrencium-257  +  six neutrons

(c) doc b

formation of lawrencium from californium and boron nuclei


plutonium-239  +  two neutrons  ===>  americium-241  +  beta minus particle

(c) doc b

formation of americium from plutonium and neutrons

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See also Electromagnetic radiation, types, properties, uses and dangers

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Atomic structure, radioactivity and nuclear physics revision notes index

Atomic structure, history, definitions, examples and explanations including isotopes

1. Atomic structure and fundamental particle knowledge needed to understand radioactivity

2. What is Radioactivity? Why does it happen? Three types of atomic-nuclear-ionising radiation

3. Detection of radioactivity, its measurement and radiation dose units, ionising radiation sources - radioactive materials, background radiation

4. Alpha, beta & gamma radiation - properties of 3 types of radioactive nuclear emission & symbols ,dangers of radioactive emissions - health and safety issues and ionising radiation

5. Uses of radioactive isotopes emitting alpha, beta (+/) or gamma radiation in industry and medicine

6. The half-life of a radioisotope - how long does material remain radioactive? implications!, uses of decay data and half-life values - archaeological radiocarbon dating, dating ancient rocks

7. What actually happens to the nucleus in alpha and beta radioactive decay and why? nuclear equations!, the production of radioisotopes - artificial sources of radioactive-isotopes, cyclotron

8. Nuclear fusion reactions and the formation of 'heavy elements' by bombardment techniques

9. Nuclear Fission Reactions, nuclear power as an energy resource

(c) doc b


Easier Foundation Tier Radioactivity multiple choice QUIZ

Harder Higher Tier Radioactivity multiple choice QUIZ

Worksheet QUIZ Question 1 on RADIOACTIVITY - absorption of alpha, beta and gamma radiation

Worksheet QUIZ Question 2 on RADIOACTIVITY - dangers & monitoring ionising radiation levels

Worksheet QUIZ Question 3 on RADIOACTIVITY - revision of atomic structure

Worksheet QUIZ Question 4 on RADIOACTIVITY - what happens to atoms in radioactive decay?

Worksheet QUIZ Question 5 on RADIOACTIVITY - uses of radioisotope and half-life data


Crossword puzzle on radioactivity and ANSWERS!

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