An element has mass number 13 and 7 neutrons. What is it an isotope of?
Atoms are made of three types of particles, protons, electrons and neutrons. The mass of an element is determined by the number of protons and neutrons it has as the mass of electrons is negligible. The atomic number is the number of protons in an electron. This is equal to the number of electrons when an atom does not have any net charge.
To determine the element that an atom is of we need to only consider the number of protons. Here the mass of the atom is 13 and the number of neutrons is 7. The number of protons is 13 - 7 = 6.
The element with an atomic number 6 is carbon. It has a stable isotope with mass 13 which forms a small percentage with most of carbon having a mass of 12.