Why isn't the mass of the electron included in the mass of an atom on the periodic table?

Because the mass of electrons are extremely smaller than protons and neutrons, it has no effect on atomic mass. Therefore, it's not included.

It takes roughly 1,840 electrons to add up to the mass of ONE proton or neutron.

The highest element on the periodic chart is Uranium.  It's the element with the most complex atoms ... number 92.  

In the nucleus of a Uranium atom are 92 protons, plus 146 neutrons in the most common natural Uranium isotope, for a total atomic weight of 238. A neutral Uranium atom has 92 electrons in a cloud around the nucleus.

It takes roughly 1,840 electrons to add up to the mass of ONE proton or neutron.

So in the most complex atom ... with 92 electrons attached to it ... the combined mass of all of its electrons adds up to about 5% of ONE MORE proton or neutron.

You can see that there's no atom of any element where the electrons add enough mass to be worth considering.

gravity

explanation: