If the atomic number of an element remains the same but the number of neutrons has been increased, which factor has been changed in that element?

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 :).