Nuclear Chemistry Notes
Nuclear reactions involve changes inside the nucleus of an atom. When the nucleus
of an atom is unstable, it tends to decay, or fall apart. When a nucleus decays, it
gives off radiation in the form of particles and energy. Generally, the atoms with the
largest nuclei are the most unstable. When an atom’s nucleus decays it often
changes the number of protons in the nucleus. Therefore, it also changes the
identity of the atom.
The half-life is the time it takes for half of a sample of radioactive material to decay.
Study the graph at the top of page 688. The red dots represent the original
radioactive element. The grey dots represent the element it is changing into as it
decays. Why does the graph have a curved shape?
When a nucleus decays it can give off many types of radiation:
Alpha particle—two protons and two neutrons (a helium nucleus). Mass of 4 amu.
Charge of + 2. Low energy—cannot penetrate a piece of paper.
Beta particle—an electron ( Mass of zero. Charge of 1-. Can penetrate paper and
skin, but not lead or glass.) Beta particles come from the decay of a neutron into a
proton and an electron. The proton generally stays in the nucleus and the electron
is emitted.
Gamma radiation—high energy radiation, very short wavelength, very high
frequency. No mass, no charge. Can be blocked only by thick layers of lead,
concrete, or both.
The force that holds protons to protons, protons to neutrons, and neutrons to
neutrons inside a nucleus is called the strong nuclear force. This force of attraction
operates only within a distance of a few proton diameters, but it is a very powerful
force. (If it wasn’t so strong, the positively charged protons would all repel each
other because like charges repel.) The strong nuclear force is what gives a nuclear
reaction its energy. A nuclear reaction releases thousands of times more energy
than a chemical reaction. A chemical reaction involves only the electrons around
atoms. Chemical reactions do not affect the nucleus. In a nuclear reaction, some of
the matter inside the nucleus is changed into energy. In the famous equation E=mc2,
the E is energy and the m is mass. In the chemical reactions that happen in the
world around us everyday, matter doesn’t change to energy and energy doesn’t
change to matter, but in a nuclear reaction matter can change to energy and energy
can change to matter.
Fission and fusion are two different types of nuclear reactions. Which one is
happening in the sun? Which one happens in a nuclear power plant? (see pages
697—699)