The central atom has one half-filled orbital in the ground state \(1s^22s^22p_x^12p_y^02p_z^0\) but it has three half-filled orbitals in an excited state. Since one electron is promoted from 2s to 2p orbital during chemical combination. Hence, one 2s and two 2p orbital undergo sp2 hybridization to form three sp2 hybrid orbital of equivalent energy and identical shape and size which takes trigonal planer geometry with the bond angle of 120°.