Heisenberg's uncertainty principle implies that the exact position and exact velocity of an electron cannot be measured simultaneously. Not only electron, microscopic particles obey this rule ( perhaps that is why, a new physics - quantum physics - was born ) Thus, there are no defined trajectories of electron. You never know where an electron is - within an atom - as Bohr had thought. Two words that are used in quantum mechanical model of atom are 'chance' and 'probability' - In fact, they are the same...