However, oxygen atoms can have between 8 and 10 neutrons in the nucleus and therefore the isotopes of oxygen have atomic masses of 16, 17, and 18 a.m.u.(and none are radioactive! Samarium (Sm) has 7 naturally occurring isotopes (3 are radioactive).
Remind them that geologists only use certain radioactive isotopes to date rocks.
Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years.
Samples for dating are selected carefully to avoid those that are altered, contaminated, or disturbed by later heating or chemical events.
In addition to the ages of Earth, Moon, and meteorites, radiometric dating has been used to determine ages of fossils, including early man, timing of glaciations, ages of mineral deposits, recurrence rates of earthquakes and volcanic eruptions, the history of reversals of Earth's magnetic field, and the age and duration of a wide variety of other geological events and processes.
The use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and can be used to date a wide range of natural and man-made materials.
As radioactive Parent atoms decay to stable daughter atoms (as uranium decays to lead) each disintegration results in one more atom of the daughter than was initially present and one less atom of the parent.