Spontaneous disintegration of a radionuclide accompanied by the emission of alpha or beta particles or gamma rays. It can lead to the mutation of living cells.
Example: alpha, beta, gamma decay

i believe the answer is a hope it and is correct

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me

answerdmitry mendeleyev made a workable classification of the elements