Triangular pyramid molecular

This time, however, only three hydrogen atoms are to be attached. Although the electrons are located toward the corners of a tetrahedron, the molecular shape is called trigonal pyramidal, not tetrahedral, because the atoms lie at the corners of a triangular pyramid ... 

It is very important to recognize that the name of the molecular structure is always based on the positions of the atoms. The placement of the electron pairs determines the structure, but the name is based on the positions of the atoms. Thus it is incorrect to say that the NH3 molecule is tetrahedral. It has a tetrahedral arrangement of electron pairs but not a tetrahedral arrangement of atoms. The molecular structure of ammonia is a trigonal pyramid (one triangular side is different from the other three), rather than a tetrahedron. 

Then we describe the molecular geometry as the arrangement of the atoms. In each of these molecules, the N atom is at the apex of a (shallow) trigonal pyramidal arrangement and the other three atoms are at the corners of the triangular base of the pyramid. Thus, the molecular geometry of each molecule is described as trigonal pyramidal. 

Four electron pairs, all bonding, yield the same electron-pair and molecular geometries tetrahedral (Line 4). The tetrahedral methane molecule, CH4, looks like a tall pyramid with a triangular base (Fig. 13.5[a]). Each bond angle is 109.5°—the tetrahedral angle. 

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