Subatomic particle mass

Subatomic particle mass, encompassing the masses of protons, neutrons, and electrons, is fundamental to understanding the structure of matter and the behavior of atoms in chemical reactions.

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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