What element has atomic mass of 16 and has 8 neutrons?

If you had looked in the periodic table, you would have seen that the element Oxygen has an atomic mass of 16 (or more precisely 15.9994) and if you were to take that atomic mass and subtract the atomic number from it (i.e. 8) you would get a difference of 8 which is the number of neutrons it contains. In short, oxygen is that element.