The mass of an electron is approximately what fraction of a proton's mass?

• A. 1
• B. 1/1840
• C. 1840
• D. 1/2

Answer: (B)

The mass of an electron is vastly smaller than the mass of a proton, by about a factor of a couple thousand. In numbers, the electron mass is about 9.11 × 10^-31 kg and the proton mass about 1.67 × 10^-27 kg. Dividing gives roughly 5.5 × 10^-4, which is about 1/1837. Rounding gives commonly used approximately 1/1840. So the electron’s mass is about one-thousandth of the proton’s mass, i.e., 1/1840 of it.

That’s why the correct choice is the fraction 1/1840. The other options would imply equal mass, the electron being far heavier than the proton, or only half as heavy, which don’t match the actual masses.