When the universe came into being, the elements formed by a coming together of protons
and neutrons, accompanied by an enormous emission of energy. Our sun and other stars
continue to fuel nuclear reactions and emit their associated energy.
The nuclei of atoms contain protons and neutrons held together by strong binding
forces. Over one hundred kinds of subatomic particles of nuclear origin have been
identified. The so-called fundamental particles, protons and neutrons, are made up
of more basic particles called quarks.

Chemical reactions take place as a result of changes in the higher electron energy
levels in the atom. If these changes are spontaneous they lead to greater stability of
products with respect to reactants. Nuclear reactions are the result of changes within
the nucleus of unstable atoms. An unstable nucleus, with a very high or very low
neutron to proton ratio, may lead to natural radioactivity with alpha, beta, or gamma
emission. This emission cannot be delayed or stopped by any external activity nor
does it depend upon whether the element is free or in a compound. Radioactivity
results in transmutation of an unstable nucleus to a stable nucleus. Stable nuclei
with a favorable neutron to proton ratio do not spontaneously decay. (See Decay
Pattern of  ²³⁵₉₂U in Appendix.)

The splitting of very heavy nuclei is called nuclear fission. When light nuclei
combine, the reaction is referred to as nuclear fusion. These changes lead to more
stable nuclei with the release of great amounts of energy. We make use of natural and
induced nuclear reactions in power plants, nuclear weapons, medical treatment, and
consumer products.

1. The nucleus is composed of protons (with a +1 charge) and neutrons (no
    charge), which are in turn composed of smaller particles called quarks.

2. There is a critical relationship between the number of protons and neutrons
    in a nucleus, determining its stability.

3. An unstable nucleus commonly decays by one of several radioactive modes:
    alpha (a), beta (b), and/or gamma (g) emission.

4. The positively charged particles are held together in the nucleus due to very
    strong binding energy.

5. The energy of nuclear reactions is significantly larger than that associated
    with physical change or chemical reactions.

6. The relative instability of a nucleus is indicated by a characteristic rate of
    decay, measured by its half-life.

7. There are many applications of radioisotopes in medicine, commercial
    processes and research.

1. An understanding of the atomic model (see Atomic Structure module).

2. A qualitative understanding that energy changes are involved in all reactions.

3. Thermochemical calculations.
 
 

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