Why is there a measurement of 109.5 degrees between bonds on a tetrahedral?

This is the maximum distance the atoms can be placed away from each other. If you change any of the angles, some of the atoms will be closer together and will experience steric repulsions. By placing the atoms 109.5 degrees apart, you minimize steric repulsions and free energy. ------------------------------------------------------ Edit (AY12345): You know the shape of the molecule. Draw it out. Use a little trigonometry, and the angles can be calculated mathematically.