Final answer:
The isotope notation is a way to represent different isotopes of an element. For ions, the notation is not used, but we can determine the number of neutrons by considering the atomic number and the charge of the ion.
Step-by-step explanation:
The isotope notation is a way to represent different isotopes of an element. It consists of the element symbol, the atomic number (number of protons), and the mass number (sum of protons and neutrons). For example, the isotope notation for carbon-12 is ¹²C.
In the case of Se2-, the isotope notation is not used because it represents an ion, not an isotope. However, we can still determine the number of neutrons. The atomic number of selenium (Se) is 34, so it has 34 protons. The charge of Se2- ion tells us that it has gained 2 electrons. Since the number of protons remains the same, the number of neutrons must also be 34.