Final answer:
Isotopes of selenium have the same number of protons and electrons (34 each) but differ in the number of neutrons, resulting in different atomic masses. This variation does not affect their chemical properties.
Step-by-step explanation:
The element selenium (Se) has an atomic number of 34, indicating that each atom has 34 protons in its nucleus and 34 electrons surrounding the nucleus when it is in an uncharged state. All isotopes of selenium will have these characteristics in common, as the number of protons (and thus electrons) defines the element. However, isotopes of selenium differ in their number of neutrons within the nucleus. The varying numbers of neutrons result in different mass numbers for each isotope, even though the chemical properties remain unchanged because the electronic structure is the same. This means that selenium has multiple isotopes with the same atomic number but different atomic mass numbers.
Natural selenium has six stable isotopes, reflecting the variation in the number of neutrons only. The mass number of an isotope is the sum of the number of protons and neutrons, which can be represented as Se-A, where A represents the mass number for any particular isotope of selenium. Isotopes contribute to the average atomic mass of an element as they occur in nature at different abundances.