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3. Exothermic nuclear reactions
The elements in nature consist of molecules formed from atoms. The atoms
are characterised by a positively charged nucleus and by peripheral negative
electrons, which determine their species.
The nucleus in turn is formed of
neutrons and protons – collectively known as nucleons – held together by
extremely intense forces within a short range of action. The mass of the
nucleus is less than the sum of the masses of the nucleons (protons and neutrons)
that form it. The mass difference (Δm), which
is related to the binding energy according to the mass-energy equivalence
principle ΔE=Δmc2 , is called
the mass defect.
What about nuclear fuels? The nuclei that have the lowest binding energy per nucleon provide nuclear energy by fusion if they have low atomic mass and by fission if their atomic mass is high.
Nuclear
binding energy vs. mass
Let us now look at the nuclear reactions that are accompanied
by the production of energy:
- The fusion reaction: two light nuclei fuse together and form a heavier
nucleus. Here the energy is released because the mass of the resulting
nucleus is lower than the mass of the two reacting nuclei.
- The fission reaction: a very heavy nucleus splits into two lighter nuclei. In this case, too, the total mass of the components is less than the mass of the original nucleus.
For the same amount of reactants, the energy released during a nuclear reaction is millions of times the energy released during chemical reactions (combustion).
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