 In this video you are going to learn what is necessary for a reaction to take place. For a reaction to take place, the reacting particles must collide. But just because they do collide, it doesn't mean they will react. Now consider a reaction between two diatomic molecules, AA and BB, to give two molecules of AB. This can be represented by the equation. These two diatomic molecules can collide in many different orientations, but consider the two orientations here. In orientation 1, the two diatomic molecules A2 and B2 are in the correct orientation to react. In orientation 2, they are not, and so no reaction occurs. Imagine our reaction being that of a ball of yellow plasticine reacting with a ball of red plasticine to give just one ball consisting of the yellow and red plasticine stuck together. If you slowly roll the ball of yellow plasticine towards the ball of red plasticine, they will collide. However, if you pick up the ball of yellow plasticine, the ball of red plasticine will not be stuck to it. No reaction. If you throw the ball of yellow plasticine at the ball of red plasticine, again they will collide. But this time, if you pick up the ball of yellow plasticine, the ball of red plasticine will be stuck to it. There has been a reaction. We call a collision that results in a reaction, a fruitful collision. So, for the collision to be fruitful, the reacting particles must collide with sufficient energy. So to recap, for a reaction to occur, particles must collide with sufficient energy, and particles must be in the correct orientation.