In a system with 4 atoms and 1 lone pair, predict whether the lone pair will be in a B site or an A site? Explain.

B

Explanation:

The compound corresponds to a molecule AX4E where A is the Central atom, X are the other atoms present and E is the lone pair of electrons.

The shape of this molecule is based on a trigonal bipyramid since the molecule contains five electron domains.

Since there are four bond pairs and one lone pair, the molecular geometry is see-saw with equatorial bond angle <120° and axial bond angle <90°.

For AX4E (five electron pairs), the lone pair should occupy an equatorial position instead of an axial position since the equatorial position has two neighboring electron pairs that are positioned at about 90° apart from each other and two other neighboring electron pairs that are positioned at 120° from each other. The axial position has 3 neighboring electron pairs positioned at 90° from each other and another neighboring electron pair positioned at 120°. Hence the equatorial position experiences lesser repulsion compared to the axial position.

Hence in the structure pictured in the question, position B is a preferred position for the lone pair instead of position A.

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