Three Electron Groups (m + n = 3)Four Electron Groups (m + n = 4)Five Electron Groups (m + n = 5)Six Electron Groups (m + n = 6)

In the previous area, we experienced how to use VSEPR to predict the geometry about a main atom based on the number of teams attached to a main atom. However, our previous discussion was limited to the straightforward situations where all of the groups were bonded teams (i.e. in the desigcountry AXmEn , n=0). When every one of the groups are bonds, the geometries deserve to be predicted using information in Table 3.2.1 in the previous area. Now we will think about instances wbelow one or more of these teams are lone pairs.

You are watching: Rank the following electron-pair geometries by increasing steric number.

Lone pairs have more powerful repulsive pressure than bonded teams.

When one or even more of the teams is a lone pair of electrons (non-bonded electrons), the experimentally-observed geomeattempt about an atom is slightly various than in the case wright here all groups are bonds. The actual bond angles are similar, however not exactly the same, as those predicted based upon the total variety of groups (the "parent" geometry). When tbelow is a mixture of group types (lone pairs (E) and bonded teams (X)) there are 3 different kinds of angles to consider: bond angles between 2 bonded atoms (X-X angles), angles in between a bonded atom and also a lone pair (X-E angles), and angles between two lone pairs (E-E angles). Empirical evidence shows the complying with trend in the degree of bond angles in around atoms via a mixture of group types:

Trfinish in bond angles:

E-E >X-E >X-X

Using empirical evidence as a guide, we can predict that lone pairs repel various other electron teams more strongly than bonded pairs. The molecular geometry of molecules through lone pairs of electrons are better predicted when we take into consideration that digital repulsion created by lone pairs is more powerful than the repulsion from bonded teams. It is challenging to predict the precise bond angle based on this principle, however we have the right to predict approximate angles, as defined and summarized below in Table (PageIndex1).

Table (PageIndex1): Predictions of molecular geometry and also bond angles around atoms via a mixture of bonded (X) and also unbonded (E) electron groups.

Electron Groups (m + n)


(steric number = 2)


(steric number = 3)


(steric number = 4)


(steric number = 5)


(steric number = 6)

Parent Geometry

(0 lone pairs)


1 lone pair


3 lone pairs



AX2, linear



AX3, trig. plane



AX4, tetrahedron



AX5, trig. bipyramid

90°, 120°


AX6, octahedron






trig. pyramid

AX4E1, see-saw


square pyramid


2 lone pairs



AX2E2, bent

AX3E2, T-shape


square plane




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