Radiometric dating reliability
Uraniums abbreviation is U, so uranium-238 can be more briefly written as U238.
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.
A nuclide of an element, also called an isotope of an element, is an atom of that element that has a specific number of nucleons.
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.
For example, hydrogen-1 and hydrogen-2 are both nuclides of the element hydrogen, but hydrogen-1's nucleus contains only a proton, while hydrogen-2's nucleus contains a proton and a neutron.
Radiometric dating methods are the strongest direct evidence that geologists have for the age of the Earth.
All these methods point to Earth being very, very old -- several billions of years old.