Steric numbers 2-4

The shapes displayed in Fig. 1 are simple and the rules usually easy to apply. instances without nonbonding electrons are:

• linear species (SN=2): BeH₂ (gas phase only;), HgCl₂, CO₂ and ions isoelectronic to it like NO₂⁺, N₃^-and NCO^-;
• the trigonal planar species (SN=3): BF₃,  and
• tetrahedral species (SN=4): CH₄, SiCl₄, POCl₃ and .

AX₂   species with  SN=3  or 4 are bent with nonbonding pairs occupying positions of the trigonal plane or tetrahedron, correspondingly (example water, 1). As predicted by rules (ii) and (iv) the XAX  bond angles are less than the ideal values of 120° (SN =3) or 109.5° (SN=4), and tend to decrease as the electronegativity  variations between  A and X increases. Some instances with their bond angles are:

• SN=3: ClNO (113°);
• SN=4: H₂O (104.5°), H₂S (92°), F₂O (102°).

In AX₃ with SN=4  the nonbonding  pair forces the bonds to be pyramidal. Instances with their bond angles are: