Calculating the Mass of an Electron

[dois]

I'm just looking over an example in my textbook. The example is working with Helium-4.
It lists the mass of He (4.002 603u) and then the mass e^- as 0.000 548u.

How did they get the mass of e^-?

 

[MisterX]

That is not a question about binding energy.

The charge of the electron was found (with some error) with the famous http://en.wikipedia.org/wiki/Oil-drop_experiment"

When you have the electron charge and the mass to charge ratio, it is a simple matter of multiplying them together to get the mass of an electron.

More methods to get the elementary charge:
http://en.wikipedia.org/wiki/Elementary_charge#Experimental_measurements_of_the_elementary_charge