You should know why the bond angle is approximately 120 degrees (there are three approximately equivalent hybrid orbitals in the xy plane. The full circle of 360 degrees is thus divided into three approximately equivalent angles. We use the term "approximately" here, because the angles are not exactly equivalent and are not exactly 120 degrees. This is because the atoms to which carbon is bonded are not the same (2H's and 1 C), therrefore the three sigma bonds are not exactly equivalent. The HCH angle is less than 120 and the HCC angle is greater than 120.
