Determine an elementary step in which two identical molecules collide with each other: A + An à B + C. If there are only 2 such molecules in a container, there is only one way they can collide. If we call these molecules A1 and A2, we could represent the collision as A1A2. Suppose now that we have three molecules (A1, A2 and A3); three different collisions are possible (A1A2, A1A3, and A2A3). Determine how many different collisions are possible with four molecules, five, and six? Calculate the general expression for the number of collisions within molecules? In other words, if there are n molecules, determine how many possible collisions can occur?