Why is the tetrahedral angle 109.5?

Actually, carbon's bonding angle varies depending on the compound it is in. In methane (CH4), for example, the molecule takes a tetrahedral shape. However, in carbon dioxide (CO2), the molecule takes a linear shape. This is due to VSEPR (Valence Shell Electron Pair Repulsion) theory, which states that lone pair electrons and bonds will spread out as evenly as possible in 3 dimensions The molecular geometry (i.e., shape) of a molecule is determined by the total number of electron pairs and bonds (double and triple bonds count as one bond). The carbon in methane has four bonds and no lone pairs, making its molecular geometry tetrahedral. The angles of a tetrahedron measure 109.6o, so carbon's bonding angle in methane is 109.6o.