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## Doubt on Alpha Rays

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.

## Ionizing Power of alpha and beta particles

Which has greater ionization power alpha or beta?

(Question received via speak pipe – a facility for the visitors to ask questions orally, if you have a mic)

Alpha particles have greater ionization power as they have two units of positive charge and 4 units of mass, whereas beta has only one unit of negative charge and negligible mass as compared to alpha.

## What is impact parameter?

Impact parameter is the perpendicular distance between the velocity vectors of two objects (considered to take part in a collision) when they are at infinite distance (at such a distance that there is no interaction between the two particles)

In Rutherford’s alpha scattering experiment, the gold nucleus is at rest and therefore, in this case

Impact parameter is the perpendicular distance between the gold nucleus and the the direction of the velocity vector of alpha particle when the alpha particle is at infinite distance (i.e; the force of interaction between the gold nucleus and the alpha particle is negligibly small)

## de Broglie Wavelength

Alpha particle and a proton are accelerated from rest by the same potential. Find the ratio of their de- broglie wavelength

Charge of alpha particle = 2e

Mass of alpha particle = 4 u

Charge of proton = e

mass of proton = u

The energy acquired by proton when accelerated through a pd of V,

E=eV

The momentum acquired by proton=${\sqrt{2ueV}}$

The de Broglie wavelength is given by $\lambda =\frac{h}{mv}$

Therefore, de Broglie wavelength of Proton, $\lambda _{proton}=\frac{h}{\sqrt{2ueV}}$

Similarly,$\lambda _{alpha}=\frac{h}{\sqrt{2\times 4u\times 2e\times V}}$

$\frac{\lambda _{alpha}}{\lambda _{proton}}=\frac{\sqrt{2ueV}}{\sqrt{2\times 4u\times 2e\times V}}=\frac{1}{2\sqrt{2}}$

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