For carbon, there are a lot of C-12, a couple of C-13, and a few C-14 atoms.
When you average out all of the masses, you get a number that is a little bit higher than 12 (the weight of a C-12 atom).
However, human beings love to see factual precision, and we want to know how old something is.
Please remember that all dating methods, even those termed "absolute," are subject to margins of error. That is a very small amount of possible error range. Modern studies almost always use two or more methods to confirm dating work and to build confidence in the results obtained.
For each dating or chronological method there is a link in the box at right to take you to that section of this page.
There, you will find a brief description of the method, plus links to take you to other webpages with more extensive information.
Hutton, a Scottish geologist, first proposed formally the fundamental principle used to classify rocks according to their relative ages.
He concluded, after studying rocks at many outcrops, that each layer represented a specific interval of geologic time.
A specialized form of cross-dating, using animal and plant fossils, is known as biostratigraphy.There is a time when it loses its extra neutrons and becomes C-12.The loss of those neutrons is called radioactive decay. For carbon, the decay happens in a few thousand years (5,730 years). If you have looked at a periodic table, you may have noticed that the atomic mass of an element is rarely an even number. If you are an atom with an extra electron, it's no big deal. As you learn more about chemistry, you will probably hear about carbon-14. C-14 is considered an isotope of the element carbon.