Electrons are contained in the space surrounding the nucleus of an atom. Electrons are the negatively charged particles of an atom and are necessary in maintaining the balanced charge of the atom.
Electrons are known to be much smaller than protons and neutrons in the atom. They are so much smaller that they don't even contribute to the mass of the atom. The amount of electrons that an atom has is almost always the same as the number of protons, unless it is noted otherwise. However, the number of electrons can change if an electron is gained or lost during interaction with other atoms. Unlike protons, this change in number doesn't effect what element that atom is.