Since  and N have the same number of electrons,  is isoelectronic with ammonia. As shown in the images above,  has a trigonal pyramidal molecular geometry with the oxygen atom at its apex. The  bond angle is approximately 113°, and the center of mass is very close to the oxygen atom. Because the base of the pyramid is made up of three identical hydrogen atoms, the  molecules symmetric top configuration is such that it belongs to the  point group. Because of this symmetry and the fact that it has a dipole moment, the rotational selection rules are ΔJ = ±1 and ΔK = 0. The transition dipole lies along the c'-axis and, because the negative charge is localized near the oxygen atom, the dipole moment points to the apex, perpendicular to the base plane.