An isotope, X, of a particular element has an atomic number of 8 and a mass number of 18. Therefore:

A. X is an isotope of oxygen.

B. X has 10 neutrons per atom.

C. X has an atomic mass of 15.9994.

D. A and B.

E. A, B, and C.

D

Explanation:

Because the atomic number is 8, we can see that it is indeed an isotope of oxygen. If the mass number is 18, this means that the protons and neutrons added together = 18, so there are 10 neutrons. It does not have an atomic mass of 15.9994 like the element on the periodic table, however, as this is a confirmed isotope and not an average.