it is 120. for those that do not have lone pairs in the center, I used this method.. a cicle is 360 degrees.. trigonal planar has three atoms attached to the center. so 360/3= 120
When they have lone pairs, this does not work because lone pairs makes the angle to decrease more. for example, a molecule with two atoms and lone pairs. the angle doing the math is 360/2= 180, but due to the lone pair, it will be lower than 180..