How do we know that protons and electrons are equal in number?

Protons and electrons are equal in number in a neutral atom because the overall charge of an atom must balance to zero. Each proton carries a positive charge, while each electron carries an equal but opposite negative charge. Since atoms are electrically neutral, the positive charges from protons are offset by the negative charges from electrons, resulting in an equal number of both particles in a neutral atom. This balance is a fundamental principle of atomic structure.