In alpha decay, the nucleus emits an alpha particle; an alpha particle is essentially a helium nucleus, so it's a group of two protons and two neutrons. A helium nucleus is very stable. Alpha particles do not travel far in air before being absorbed; this makes them safe for use in smoke detectors.
The process of transforming one element to another is known as transmutation.
An example of an alpha decay involves uranium-238:
Use data from Appendix F in the textbook to examine this nuclear reaction:
Atomic mass of U-238 is 238.050786 u
Atomic mass of Th-234 is 234.043596 u
Atomic mass of He-4 is 4.002603 u
The total mass on the left side of the equation is 238.050786 u. The total mass on the right side of the equation is
234.043596 u + 4.002603 u = 238.046199 u
The left side of the equation has more mass, by 0.004587 u, than the right side. Where did the extra mass go?
The missing mass was converted to 0.004587 u * 931.5 MeV/u = 4.273 MeV of energy. This shows up in the kinetic energy of the two atoms after the reaction.
This reaction can be treated as a super-elastic collision between a light object (the alpha particle) and a relatively heavy particle (the remaining atom). How is the kinetic energy shared between the two particles?
The kinetic energies of the two are:
alpha particle: 1/2mv2 = 1/2 (mv) * v
heavy atom: 1/2MV2 = 1/2 (MV) * V
To conserve momentum mv = MV, so v, the speed of the alpha particle afterwards, is larger than V, the speed of the heavy atom, by a factor of M/m. The kinetic energy of the alpha particle is also larger than that of the heavy atom by the same factor.
In other words, the alpha particle takes most of the kinetic energy.
Reactions occur spontaneously when the total mass afterwards is less than the total mass before. Such reactions give off energy. Any reactions where there is more mass afterwards than there is before require an input of energy.