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Alpha rays are helium nuclei since they have two protons and no electrons. They are He 2+ ions. However we have learnt that helium has highest ionization energy and an extremely large amount of energy is needed for it to form an ion. According to this, it is not possible for a dipositive cation of helium to exist, however it does . What could be the reason?
First of all Alpha particle is not a doubly ionized Helium Atom. It is emitted from the nucleus of a discintegrating atom.
The energy required to peel off the two electrons is not too much as compared to the energy involved in the alpha particle emission which is a nuclear reaction. Alpha particles are normally emitted as a part of radio active disintegrations, emitted by naturally unstable nuclei or artificially instabilized nuclei.
The ionization energy of helium is discussed in the purview of the energy required to remove the electrons. Helium is having a stable duplet electronic configuration which makes it a stable atom. But the energy considerations in nuclear reactions are very much different and higher.
The first ionization energy of helium is 2372.3 kJ/mol and the second ionization energy is 5250.5 kJ/mol
16 MeV = 16×1.6×10-13 J per nucleus 2.56 x 10-12 J per nucleus
which makes of the order of 1010 kJ per mol of Helium. This is evidently a huge amount compared to that of the ionization energy.
Basically there are a number of differences between chemical reactions and nuclear reactions.
1. In chemical reactions the electrons are taking part in reactions whereas in nuclear reactions the nucleons (protons and neutrons) are involved
2. In nuclear reactions the energy involved is very much large as compared to that in chemical reactions
3. The identity of the element dioes not change in chemical reactions whereas the identity is changed in most of the nuclear reactions.