Answer: The mass number has changed.
Explanation: The atomic number is also known as the proton number, and if that changes then the name of the element would also change.
Since there is a change in mass it suggests a change in the number of neutrons, meaning that you are now dealing with an 'isotope' of that element. Isotopes behave a little differently and some can be radioactive. The mass number you see on the periodic table tends to be the most common or stable number for that element.
Plus i know this because i took this in science class.
Hope this helps :).
