The conventional estimate of the duration of an atomic collision is the time required to cover an atomic diameter or, if there is a force of attraction, the period of one molecular vibration. Whereas this estimate is adequate for the duration of a head-on collision it fails for eccentric collisions. These are described by “effective potential curves,” which were originally introduced to represent dissociation of diatomic molecules by rotation and take into account the centrifugal force. These curves, different from the ordinary potential curves, show broad maxima outside of the well-known minima. The maxima slow down or even repel the approaching atoms. A pair of atoms whose energy carries it just to the top of a maximum may even perform a part of an unstable rotation. In the general case of eccentric collisions the durations cover a wide range between the period of a molecular vibration and the much longer period of a molecular rotation.