A century is 100 years. We had no year 0 so the first century was years 1-100 and the second century was years 101-200. So the Century always sounds like it is one ahead of the 100's number of the year. But the 21st century really is from year 2000-2099.

The number of protons remains the same. The mass of a single element can only increase by adding neutrons. The number of protons can not change without changing the element itself.