Electron whirl around the atom's nucleus at fantastic speeds. It's convenient to think of them as moving in orbils (paths) as planets travel around the sun. The whirling electrons complete billions of trips around the nucleus in a millionth of a second. The neutral atoms of each element have a definite number of electrons. The number of electrons determines the chemical behavior of the element. Electrons have almost no mass. The mass of one electron in grams is written with a decimal oint followed by 27 zeros and a 9. Electrons belong to the lepton group. See Electron