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, m_n, however, cannot be measured via mass spectrometry as it lacks a charge.

Particle	Symbol	Relative mass, u	Inertia mass, kg
Proton	p or H+	1.007276466879	1.672621898 x 10-27
Neutron	n	1.00866491588	1.674927471 x 10-27
Electron	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, m_p, from the mass of a deuterium nucleus  (both obtained from mass spectrometry), and adding the mass defect of deuterium, m_defect, which can be measured using X-ray diffraction for the gamma ray released when a neutron captures a proton:

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, m_e, is however calculated from the Rydberg constant, where

 

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