What are atoms of the same element with different numbers of neutrons called?
The number of protons determine the chemical properties of an atom. The number of protons then determine which element the atom is.
The number of neutrons have no effect on the chemical properties but do affect the nuclear properties. Some ratios of protons to neutrons are more stable than other ratios.
For example U^235 with a ratio of 92 protons to 143 neutrons is an unstable isotope of Uranium and was used to make the first Atomic Bomb. While the U^238 isotope with a ratio of 92 protons to 146 neutrons is used in Nuclear power plants because it much more stable.