To become stable, it undergoes a change and releases radiation. Alpha particles are heavy and cannot travel very far. They are also not very penetrating. They can't even get through the dead layer on the surface of the skin. Beta particles are very energetic electrons.
Many radioisotopes used in labs are beta emitters.
High energy beta particles are capable of penetrating living tissue in both your skin and eyes. Plastic shielding is required for protection. Gamma rays are on the high energy, or high frequency, end of the spectrum. Gamma rays are capable of deep penetration and are a serious hazard. Lead shielding is required.