Radioactive Dating

Because the radioactive half-life of a given radioisotope is not affected by temperature, physical or chemical state, or any other influence of the environment outside the nucleus save direct particle interactions with the nucleus, then radioactive samples continue to decay at a predictable rate. If determinations or reasonable estimates of the original composition of a radioactive sample can be made, then the amounts of the radioisotopes present can provide a measurement of the time elapsed.

One such method is called carbon dating, which is limited to the dating of organic (once living) materials. The longer-lived radioisotopes in minerals provide evidence of long time scales in geological processes. While original compositions cannot be determined with certainty, various combination measurements provide self-consistent values for the the times of formations of certain geologic deposits. These clocks-in-the-rocks methods provide data for modeling the formation of the Earth and solar system.

Isotopes

The different isotopes of a given element have the same atomic number but different mass numbers since they have different numbers of neutrons. The chemical properties of the different isotopes of an element are identical, but they will often have great differences in nuclear stability. For stable isotopes light elements, the number of neutrons will be almost equal to the number of protons, but a growing neutron excess is characteristic of stable heavy elements. The element tin (Sn) has the most stable isotopes with 10, the average being about 2.6 stable isotopes per element.

Nuclear Forces

Within the incredibly small nuclear size, the two strongest forces in nature are pitted against each other. When the balance is broken, the resultant radioactivity yields particles of enormous energy.

Nuclear Size

The size of the nucleus compared to the size of the atom in which it resides is so small that it has invited a number of interesting comparisons. For example, the space inside an atom can be compared to the space in the solar system in a scale model. Scaling the gold nucleus suggests that the atomic radius is some 18,000 times the size of the nucleus. This great disparity in size was first discovered by Rutherford scattering of alpha particles off a thin gold foil. The extremity of this space comparison is highlighted by the fact that an atom with equal numbers of neutrons and protons, the nucleus comprises about 99.97% of the mass of the atom!

Experimental evidence suggests that nuclear matter is almost uniform density, so that the size of a nucleus can be estimated from its mass number.