Protons and neutrons together are called nucleons, meaning particles that can appear in the atomic nucleus.
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.
Uranium-238 contains 92 protons and 146 neutrons, while uranium-235 contains 92 protons and 143 neutrons.
To keep it short, a nuclide is usually written using the elements abbreviation.
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.
Uraniums abbreviation is U, so uranium-238 can be more briefly written as U238.
They know this because they have been able to accurately date lava flows that happened recently enough for their dates to be known historically, such as the eruption of Mount Vesuvius at Pompeii.By doing so, they can calculate how much time has passed. One system that has been very successful in dating the ages of fossils is potassium-argon dating. Although most potassium isotopes aren’t radioactive, one of them is, and one of its decay products is the gas argon. When rock is melted (think lava), all the argon in the rock escapes, and when the rock solidifies again, only potassium is left.