Chemistry - Nuclear fusion
Nuclear fusion differs from fission in that the nuclei of small atoms
combine to form nuclei of larger elements. For fusion to occur, the nuclei
must be moving at very high speeds to overcome electric repulsion forces
and come close enough to fuse together.
You should recall the accepted definition of relative atomic mass:
'The weighted mean (average according to percentage composition) of the
relative isotopic masses of the atoms of the element on a scale where a
carbon-12 atom has a mass of 12 exactly.'
An interesting observation associated with the determination of relative
atomic mass is that the relative isotopic mass of a particular isotope is
So to all intents and purposes, the relative mass of helium is the same
as the relative mass of helium-4, RIM (He-4).
A He-4 atom contains two protons, two neutrons and two electrons. On the
standard scale (carbon-12 = 12) of relative atomic masses:
* a has a relative mass of
* a a relative mass of
* an a relative mass of
So the relative mass of two protons and two neutrons is:
2x1.00728 + 2x1.00866 = 4.03188. Add in two electrons and the total
relative mass is 4.03298.
99.9999 per cent of He atoms contain two protons, two neutrons and two
electrons. But the figures above indicate that the relative mass of a
helium atom (4.00260) is less than the sum of the relative masses of two
protons, two neutrons and two electrons (4.03298).
The answer to this question is tied up in the concept of nuclear binding
or the packing effect. The forces acting within the nucleus of an atom are
The obvious electrostatic repulsion between protons is counteracted by
Consider two protons being brought closer together. When the distance
between the protons decreases, the potential energy of the system increases
as the electrostatic repulsion increases.
This diagram shows the changes in potential as two protons are brought
together. Note that you have to move from right to left along the
horizontal axis for decreasing distance between the protons.
The energy released when two protons fuse together corresponds to the of
the new nucleus.
It should follow that if energy is released when a new nucleus is formed
from the basic particles, i.e. protons and neutrons, then energy, would
also be required to separate a particular nucleus into its constituent
Earlier we stated that the total relative mass of the nucleons in a
helium-4 nucleus is 4.03298 but the relative atomic mass of helium is
This difference of 4.03288 - 4.00260 = 0.03028 mass units must presumably
be the which was during the formation of the nucleus.
At higher levels of Chemistry, nuclear binding energies are associated
with stability of nuclei. However because bigger nuclei, with more
nucleons, will have higher nuclear binding energies, the reference quantity
is i.e. total binding energy divided by the total number of nucleons in the
Interestingly it is the transition element Fe - more specifically the
Fe-56 nucleus -which has the highest binding energy per nucleon.
An interesting aspect of iron's nuclear stability is that energy is when
nuclei, but energy is when are formed.
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