Question

why it is said that alpha particles are of helium

Answer 1

it is said that Alpha particle are Helium because it is lightweight and and it is a noble gas so it cannot be seen so it is called Alpha particle

Answer 2

Helium itself is a noble gas. Alpha particles are not helium atoms, but rather helium nuclei. Compounds with covalent or ionic bonds to helium have not yet been prepared, because elemental Helium is inert.

Another way to answer your question is to consider the opposite. Usually, formation of an ion is stabilizing, right?

For example:

Na ---> Na+ (Na+ is more stable)

Cl2 ---> Cl- (Cl- is more stable)

but if you try to do that to a compound that already has a full valence shell, it is destabilizing:

Ag --> Ag+ (Forms a reactive ion)

So alpha particles can't be placed with traditional ions, because they attempt to become stable by ceasing to be ions.

I hope that explains it better.