Why are isotopes electrically neutral?
Isotopes are electrically neutral because they possess an equal number of protons (+) and electrons (-).
Let's look at the example of carbon.
The most common version is C-12 (6p, 6n, 6e) whose mass of 12amu comes from the 6 protons and 6 neutrons. The balance of 6 protons and 6 electrons makes the atom electrically neutral (no charge).
Another version of carbon is C-14. This version has 6 protons, 8 neutrons, and 6 electrons. The mass comes from adding the protons and neutrons. Since the number of protons and electrons is equal, the isotope has no charge.
Here is a video which talks about the subatomic particles found in an isotope.
Hope this helps!