Question

why is it that only alpha particles are emitted by radioactive nuclei ,while protons and neutrons are not?

Answer 1

In big nuclei which decays though alpha-decaynucleons (protons and neutrons) inside nucleus are clusterised - they are merged into pairs and into bigger clusters. The alpha particle has the biggest binding energies of possible merges. So nulceons merged in alpha particle have more energy to break the attraction potential of others and leave the nucleus.