Calcium has at least six stable natural occurring isotopes, and three radioactive ones: Ca-41, 45, 47. The stable ones are (mass)numbered: 40, 42, 44, 46 and 48, plus the only odd one 43.
An isotope has extra/fewer neutrons than the original atom.
So if you know the isotope you can deduce the original atoms' formula.For Ca that would be:20 protons40 mass number