 |
 |
| Energy and matter are
equivalent. |
 |
 |
| The building blocks of nuclei. |
| |
 |
| The helium nucleus has two protons and two neutrons. |
|
Introduction – Energy from Matter
E=mc2
Einstein's famous equation from 1905 E=mc2 demonstrates
that energy (E) is equivalent with matter (mass m). It suggests
that the concept of mass is indeed, less basic than what can be
believed from everyday experiences with massive bodies. In fact,
energy can be transformed into massive particles, and mass can
be transformed into energy. Energy in all its different appearances
is a key concept in physics.
The Nucleus
The atomic nucleus is made up of protons (charge +1) and
neutrons (neutral). A light nucleus like helium (He) consists of
two protons and two neutrons. A typical medium-sized nucleus is tin,
with 50 protons and 69 neutrons. Uranium is a heavy nucleus with 92
protons. One of the different uranium nuclei has 143 neutrons. This
nucleus is called 235U with
235 being the sum of the numbers of protons and neutrons.
 |
A light,
medium and heavy nucleus.
|
|
 |
The mass of the helium
nucleus is less than the sum of the masses of its building blocks.
|
|
Binding Energy
A peculiar and very important
property of the nucleus is that its mass is not the sum of the masses
of the protons and neutrons that make up the nucleus, it is less!
This is because the strong nuclear force binds neutrons and protons
in the nucleus together and energy is required to
separate them.
This energy is called the binding energy, and by E=mc2 gives
the nucleus less mass than the sum of the masses of its constituents.
This is also the reason why many nuclei are stable. The binding energy
per proton or neutron is largest for medium-sized nuclei and less
for very heavy and for very light nuclei.
|
