How does atomic radius change from top to bottom in a group in the periodic table?

The atomic radius increases as you would go down a particular group on the periodic table of elements. This is because along with a greater number of protons, there would also be electrons as well, and thus the need of electron shells surrounding the atom would also be required, to compensate for the more electrons, as according to the bohr model, each shell contains 8 electrons in its electron shell. Thus the distance from the nucleus to the outermost shell increases, the atomic radius.

No. an alpha particle is a pair of protons and a pair of neutrons, i.e. a helium