How many neutrons are in a nucleus of uranium-238?

A. 92

B. 145

C. 238

D. none of these

146

Explanation:

The mass of uranium - 238 is 238 mass units. The total mass of an element is the sum of its protons and neutrons. There are 92 protons in this isotope, and its mass is 238, so that means that there are, 238 - 92 = 146 neutrons. Thus, as weird as it may seem, the answer must be D.