The names ascribed to these materials over the centuries are also used to designate the nuclei themselves. The most common elements, along with one or two others we will run into in this class, are shown in the table below. It is possible to have nuclei with the same atomic number but different atomic mass, i.e. the charge is the same but the number of neutrons is different. Such nuclei are called isotopes of each other. Most nuclei have one isotope which is most common in nature, along with others with lower abundances.
Many isotopes are radioactive, usually decaying away to other elements in a short period of time.
Differing isotopes are distinguished from each other in writing by preceding the element symbol with the atomic mass as a superscript, e.g. 56 Fe. Atoms Because positive nuclei and negative electrons are attracted to each other, they routinely join together to form a stable structure called an atom . The experiments of Ernest Rutherford in 1910 demonstrated that atomic structure was much like the solar system, with light electrons orbiting the massive nucleus. You can explore Rutherford's experiments yourself with this Java applet from the University of Virginia. Such an atom is called an ion , and it is said to be ionized. In astronomy, neutral atoms and positive ions are most common, and a roman numeral is used to indicate the degree of ionization.