Alpha particles have two protons and two neutrons, wich is the configuration of an Helium Nucleus.
The atomic number of an element it is the number of protons that the element has in its nucleus, this number is uniqe to every element. Helium is the second element in the periodic table so Its atomic number is 2, This means it has two protons and the same amount of neutrons, wich makes alpha particles identical to a helium nucleus, not to a Helium atom, because a Helium atom has electrons and alpha particles don't have electrons.
