The attraction between the electons and the nucleus, called the electromagnetic force , holds the atom together. The electromagnetic force, familiar to us as the force that produces sparks when we take off a sweater, the force that produces lightning, powers motors, etc. is one of the four fundamental forces of nature, along with gravity, and the strong and weak nuclear forces. The nucleus contains postitively charged protons and neutral neutrons . Here's periodic table of the elements. Nuclei with the same number of protons but different numbers of neutrons are called isotopes.
Many elements have several common isotopes.
The charge of the proton is equal and opposite to the charge of the electron. Neutral atoms have equal numbers of electrons and protons, and thus have no net electrical charge. This is the form of matter most familar to us. But when heated or exposed to radiation, atoms can lose electrons and become ions, which are atoms with an imbalance between electrons and protons, i.e. they are electrically charged. The process by which an atom loses electrons is called ionization .