Since all atoms of the same element have the same number of protons, different nuclides of an element differ in the number of neutrons they contain.
The rules are the same in all cases; the assumptions are different for each method.
To explain those rules, I'll need to talk about some basic atomic physics. Hydrogen-1's nucleus consists of only a single proton.
I found several good sources, but none that seemed both complete enough to stand alone and simple enough for a What is radiometric dating?
Simply stated, radiometric dating is a way of determining the age of a sample of material using the decay rates of radio-active nuclides to provide a 'clock.' It relies on three basic rules, plus a couple of critical assumptions.
Young-Earth creationists -- that is, creationists who believe that Earth is no more than 10,000 years old -- are fond of attacking radiometric dating methods as being full of inaccuracies and riddled with sources of error.
When I first became interested in the creation-evolution debate, in late 1994, I looked around for sources that clearly and simply explained what radiometric dating is and why young-Earth creationists are driven to discredit it.