Explain why the number of protons in an atom must always equal the number of electrons

No. of protons should be equal to the no. of electrons. Each and Every Atom on the Earth is neutral in charge. Every Atom has three particles in it, namely protons neutrons and electrons. Neutrons are neutral and have no charge, Electrons are negatively charged and Protons positively charged. So If we have 10 electrons, then the charge of the atom is - 10. On the other hand, to make it neutrally charged we need to add + 10. That is where you will see no. of protons is also 10. This is why Protons and Electrons are in same no. in an atom.