Electrons are the third subatomic particle and is much much smaller than protons and neutrons. They are so small that scientists generally do not account for the mass of the electron when determining the mass of an atom.
Electrons have a negative charge, the opposite charge from the proton. In a neutrally charged atom, the overall charge of the atom will be zero. This means, the number of protons and electrons will be equal. If the number of the protons and electrons are not equal, the atom will have a charge, either positive or negative, depending on which subatomic particle outnumbers the other.