nuclear reactions, the energies involved are so great that the changes
in mass become easily measurable. One no longer can assume that
mass and energy are conserved separately, but must take into account
their interconversion via Einstein's relationship, E = mc.
If mass is in grams and the velocity of light is expressed as c
= 3 x 10
then the energy is in units of g cm
or ergs. A useful conversion is from mass in amu to energy in million
electron volts (MeV):
1 amu = 931.4 MeV
What holds a nucleus together? If we attempt to bring two protons
and two neutrons together to form a helium nucleus, we might reasonably
expect the positively charged protons to repel one another violently.
Then what keeps them together in the
nucleus? The answer, as we mentioned in Chapter 2, is that a helium
atom is lighter than the sum of two protons, two neutrons, and two
electrons. Some of the mass of the separated particles is converted
into energy and dissipated when the nucleus is formed. Before the
helium nucleus can be torn apart into its component particles, this
dissipated energy must be restored and turned back into mass. Unless
this energy is provided, the nucleus cannot be taken apart. This energy is termed the binding
energy of the helium nucleus.