Question

Why is the relative mass of electron is taken as negligible?

Answer 1

As the mass of an electron is relatively very low (1/1837 times the mass of a proton) therefore,it is considered as negligible in front of other components such as neutrons and protons.

Answer 2

The mass of an electron is 9.1 X 10-31Kg, of a proton is 1.673 X 10-27 Kg, while mass of a neutron is 1.675 X 10-27 Kg. Thus, in comparison to the mass of a proton and a neutron, the mass of an electron is very small. This is why the relative mass of an electron is regarded negligible, and therefore while calculating the mass of an atom, the contribution from mass of electrons is usually ignored. 

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