It has the same number of protons, otherwise it wouldn't be uranium.
Because of radioactivity, the fraction of rubidium-87 decreases from an initial value of 100% at the time of formation of the mineral, and approaches zero with increasing number of half lives.
At the same time, the fraction of strontium-87 formed increases from zero and approaches 100% with increasing number of half-lives.
Potassium-Argon dating: The element potassium (symbol K) has three nuclides, K39, K40, and K41. K40 can decay in two different ways: it can break down into either calcium or argon.
The ratio of calcium formed to argon formed is fixed and known.
Any argon present in a mineral containing potassium-40 must have been formed as the result of radioactive decay.