Molecular Shapes and Polarity
Electron Pair Repulsion Theory
The shape of a molecule is determined by the repulsion between electron pairs (both bonding and lone
pairs) around the central atom. These electron pairs arrange themselves in space to minimize repulsion,
leading to specific molecular geometries and bond angles.
Key Principles:
Electron Pair Repulsion: All electron pairs (bonding and lone) repel each other.
Minimizing Repulsion: Molecules adopt shapes that position these electron pairs as far apart as
possible.
Lone Pair Repulsion: Lone pairs of electrons exert a stronger repulsive force than bonding pairs. This
can cause bonding pairs to move closer together, altering bond angles.
Molecular Geometries and Bond Angles
The number of electron domains (bonding pairs and lone pairs) around the central atom dictates the
electron geometry. The molecular geometry describes the arrangement of atoms, which may differ from the
electron geometry if lone pairs are present.
Linear
Description: Two electron domains, arranged 180 degrees apart.
Conditions: Central atom has two bonds (single or double) and no lone pairs.
Example: BeCl2
Bond Angle: 180∘
Trigonal Planar
Description: Three electron domains, arranged in a triangular plane.
Conditions: Central atom has three bonds (single or double) and no lone pairs.
Example: BF3 , Ethene (C2 H4 ) where each carbon has three bonds.
∘
Bond Angle: 120
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, Non-linear (Bent or V-shaped)
Description: Two bonds and one or two lone pairs on the central atom. The shape is not linear due to
the presence of lone pairs.
Conditions:
Two bonds and one lone pair: Example SO2 , Bond Angle ≈
120∘ (due to one lone pair and two
bonds, one of which is a double bond).
Two bonds and two lone pairs: Example H2 O, H2 S, Bond Angle ≈
104.5∘ .
Bond Angle: Varies, but typically less than the ideal angle for the number of electron domains due to
lone pair repulsion.
Pyramidal
Description: Three bonds and one lone pair on the central atom, resembling a pyramid.
Conditions: Central atom has three bonds and one lone pair.
+
Example: NH3 , PH3 , H3 O
Bond Angle: ≈ 107∘ (slightly compressed from ideal tetrahedral due to lone pair repulsion).
Tetrahedral
Description: Four electron domains, arranged in a three-dimensional tetrahedron.
Conditions: Central atom has four bonds and no lone pairs.
+
Example: CH4 , NH4
Bond Angle: 109.5∘
Trigonal Bipyramidal
Description: Five electron domains, consisting of three equatorial positions and two axial positions.
Conditions: Central atom has five bonds and no lone pairs.
Example: PCl5
Bond Angles: 90∘ (between axial and equatorial) and 120∘ (between equatorial).
Octahedral
Description: Six electron domains, arranged in an octahedral shape with four positions in a plane
and one above and one below.
Conditions: Central atom has six bonds and no lone pairs.
Example: SF6
Bond Angles: 90∘
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Electron Pair Repulsion Theory
The shape of a molecule is determined by the repulsion between electron pairs (both bonding and lone
pairs) around the central atom. These electron pairs arrange themselves in space to minimize repulsion,
leading to specific molecular geometries and bond angles.
Key Principles:
Electron Pair Repulsion: All electron pairs (bonding and lone) repel each other.
Minimizing Repulsion: Molecules adopt shapes that position these electron pairs as far apart as
possible.
Lone Pair Repulsion: Lone pairs of electrons exert a stronger repulsive force than bonding pairs. This
can cause bonding pairs to move closer together, altering bond angles.
Molecular Geometries and Bond Angles
The number of electron domains (bonding pairs and lone pairs) around the central atom dictates the
electron geometry. The molecular geometry describes the arrangement of atoms, which may differ from the
electron geometry if lone pairs are present.
Linear
Description: Two electron domains, arranged 180 degrees apart.
Conditions: Central atom has two bonds (single or double) and no lone pairs.
Example: BeCl2
Bond Angle: 180∘
Trigonal Planar
Description: Three electron domains, arranged in a triangular plane.
Conditions: Central atom has three bonds (single or double) and no lone pairs.
Example: BF3 , Ethene (C2 H4 ) where each carbon has three bonds.
∘
Bond Angle: 120
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, Non-linear (Bent or V-shaped)
Description: Two bonds and one or two lone pairs on the central atom. The shape is not linear due to
the presence of lone pairs.
Conditions:
Two bonds and one lone pair: Example SO2 , Bond Angle ≈
120∘ (due to one lone pair and two
bonds, one of which is a double bond).
Two bonds and two lone pairs: Example H2 O, H2 S, Bond Angle ≈
104.5∘ .
Bond Angle: Varies, but typically less than the ideal angle for the number of electron domains due to
lone pair repulsion.
Pyramidal
Description: Three bonds and one lone pair on the central atom, resembling a pyramid.
Conditions: Central atom has three bonds and one lone pair.
+
Example: NH3 , PH3 , H3 O
Bond Angle: ≈ 107∘ (slightly compressed from ideal tetrahedral due to lone pair repulsion).
Tetrahedral
Description: Four electron domains, arranged in a three-dimensional tetrahedron.
Conditions: Central atom has four bonds and no lone pairs.
+
Example: CH4 , NH4
Bond Angle: 109.5∘
Trigonal Bipyramidal
Description: Five electron domains, consisting of three equatorial positions and two axial positions.
Conditions: Central atom has five bonds and no lone pairs.
Example: PCl5
Bond Angles: 90∘ (between axial and equatorial) and 120∘ (between equatorial).
Octahedral
Description: Six electron domains, arranged in an octahedral shape with four positions in a plane
and one above and one below.
Conditions: Central atom has six bonds and no lone pairs.
Example: SF6
Bond Angles: 90∘
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