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Geometries predicted for
molecules : 𝐴𝐵𝑛
Molecular shapes: central atom
surrounded by single bond pairs
• The linear geometry for two bond pairs and trigonal
planar geometry for three bond pairs apply to a
central atom that does not have an octet of e’s
• Central atom in a tetrahedral molecule obeys octet
rule with four bond pairs
• Central atoms in trigonal-bipyramidal and octahedral
molecules have five or six bonding pairs & are
expected only when central atom is in period 3 or
higher in P.T
Central atoms with single-bond & lone
pairs
• Electron pair geometry: all the valence e pairs
around central atom
• Molecular geometry: arrangement in space of
central atom and atoms directly attached to it
• Eg 𝑁𝐻3
Effect of lone pairs on Bond angles
• VSEPR model not precise
• Can only predict approximate geometry
• Lone pairs of e’s occupy larger volume than
bonding pairs, increase volume of lone pairs
causes bond pairs to squeeze closer togerher
• Relative strengths of repulsion are as follows:
• Lone pair-lone pair > lone pair-bond pair >
bond pair-bond pair
Central atoms with more than four
valence e pairs
• Central atom has five or six e pairs
• Eg trigonal-bipyramidal has two sets of
positions that are not equivalent
• Positions in trigonal plane lie in the equator of
imaginary sphere around central atom & is
called equatorial positions
• North and South poles represents axial
positions