Question

why some radioactive substance emit alpha particles of different energies​

Answer 1

In alpha decay, shown in Fig. 3-3, the nucleus emits a 4He nucleus, an alpha particle. Alpha decay occurs most often in massive nuclei that have too large a proton to neutron ratio. ... Alpha radiation reduces the ratio of protons to neutrons in the parent nucleus, bringing it to a more stable configuration.

Answer 2

because of the unstability of particle.

The radioactive substance which emits alpha,beta, and gamma rays simultaneously.

suppose a radioactive substance Uranium emitting these rays then one of the ray is directed towards the negative potential continuously and this what we called alpha rays which are nuclei of helium having 2 times charge of proton and 4 times mass of proton.

In the emitting of an radioactive substance of Uranium it goes into the nuclear fission which produces barium and krypton and different energies of different substances which can include alpha particle of different energies due to excitation of electrons.