The atomic number of any element is equal to the number of protons or electrons present in an atom of the element. Since, the atom is electrically neutral, it contains same number of positively charged protons inside the nucleus as it has negatively charged electrons outside the nucleus. In different isotopes of an element, the number of protons and electrons are same, only the number of neutrons inside the nucleus are different. Therefore, the atomic numbers of all isotopes of any element are same. For example, the isotopes of oxygen--oxygen-16, oxygen-17, oxygen-18-- all have the same atomic number, 8.
The atomic mass of an atom is almost equal to the sum of number of protons and neutrons present in the nucleus of the atom. Since, there are different number of neutrons in different isotopes of an element, there is variation in the atomic masses of the isotopes. For example, the isotopes of oxygen-- oxygen-16, oxygen-17, oxygen-18-- have the atomic masses, 15.99, 16.99 and 17.99, respectively. Similarly, the isotopes of boron, carbon and chlorine have different atomic masses but same atomic numbers.
The element carbon, has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. This isotope of carbon, accounts for 98.89 percent of all carbon atoms and it considered as the standard for the atomic mass scale for all elements in the modern periodic table. The second isotope, carbon-13 is the only isotope of carbon having magnetic properties. It is important for structural studies of the compounds that contain carbon in any form. The last isotope of carbon, carbon-14 is radioactive in nature and has a half-life of 5760 years. This isotope of carbon is used to estimate the age of fossils by calculating the amount of carbon-14 in them.
The isotope of carbon, carbon-14, returns to normal form, carbon-12, by going through radioactive decay and emitting neutrons periodically. This is the reason for radioactivity of carbon-14. This decay of the isotopes and their returning to stable forms is shown by other isotopes also, like uranium. The difference is in the time of decay or emission of neutrons. This regular interval may be in thousand years to minutes for different elements' isotopes.