Subatomic particle mass

The mass of a proton can be determined via mass spectrometry in the same way as described in a previous article. The mass of a neutron, mn, however, cannot be measured via mass spectrometry as it lacks a charge.


Symbol Relative mass, u

Inertia mass, kg


p or H+ 1.007276466879

1.672621898 x 10-27


n 1.00866491588

1.674927471 x 10-27


e 5.485799090 x 10-4

9.10938356 x 10-31

From eq9 of the article on mass defect, we have:


Hence, the mass of a neutron can be calculated by subtracting the mass of a proton, mp, from the mass of a deuterium nucleus m_{D^+} (both obtained from mass spectrometry), and adding the mass defect of deuterium, mdefect, which can be measured using X-ray diffraction for the gamma ray released when a neutron captures a proton:

n+p\rightarrow D+\gamma

The charge-to-mass ratio of an electron was first estimated by J. J. Thomson, an English physicist, in 1896 using cathode rays. Combining this value with the quantity of charge of a single electron from Robert Millikan’s oil drop experiment, an estimated mass of an electron could be determined. A more precise value, the rest mass of an electron, me, is however calculated from the Rydberg constant, where

m_e=\frac{8\varepsilon _0^{\: 2}h^3cR_{\infty }}{e^4}


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