Why do atoms have no overall electrical charge?
Because atoms (and by extension matter in general) are usually electrically neutral, and have equal number of positively charged and negatively charged particles.
The sign convention between electrons and protons is entirely arbitrary. It would make no difference in any calculations if the proton were suddenly designated a negative charge, and the electron designated a positive charge, (in fact it would make quantum chemical calculations easier!). The point is that electrons, fundamental particles of negligible mass, have an OPPOSITE electric charge to protons, massive nuclear particles.
For the isolated, NEUTRAL atom, there are necessarily equal number of protons and electrons. If an atom loses an electron or so, it becomes a positively charged ion, a cation, i.e.
Why can't an atom,