2 K + I2 -----} 2 KI
Iodine is diatomic so when it is "by itself" it has to be bonded to another one of its atoms so it has a subscript of 2 on the left and to balance the other side you put a 2 before potassium iodide and then you have to put a 2 in front of the potassium on the left side as well.
