Molecular Geometry — Revision Notes

⚡ 30-Second Revision

VSEPR Theory — Electron pairs repel, arrange to minimize repulsion.
Steric Number (SN) — (Bonded atoms) + (Lone pairs on central atom).
Electron Domain Geometry (EDG)

- SN=2: Linear - SN=3: Trigonal Planar - SN=4: Tetrahedral - SN=5: Trigonal Bipyramidal - SN=6: Octahedral

Molecular Geometry (MG) — Determined by EDG and number of lone pairs.

- SN=4: $ext{AX}_4$ (Tetrahedral), $ext{AX}_3\text{E}$ (Trigonal Pyramidal), $ext{AX}_2\text{E}_2$ (Bent) - SN=5: $ext{AX}_5$ (Trigonal Bipyramidal), $ext{AX}_4\text{E}$ (Seesaw), $ext{AX}_3\text{E}_2$ (T-shaped), $ext{AX}_2\text{E}_3$ (Linear) - SN=6: $ext{AX}_6$ (Octahedral), $ext{AX}_5\text{E}$ (Square Pyramidal), $ext{AX}_4\text{E}_2$ (Square Planar)

Lone Pair Effect — LP-LP > LP-BP > BP-BP repulsion. Lone pairs reduce bond angles.
Polarity — Depends on bond polarity + molecular symmetry. Symmetrical shapes (e.g., linear $ext{CO}_2$, tetrahedral $ext{CCl}_4$) are non-polar even with polar bonds.

2-Minute Revision

Molecular geometry describes the 3D arrangement of atoms in a molecule, crucial for its properties. VSEPR theory is the guiding principle, stating that electron pairs (bonding and lone pairs) around a central atom repel each other, arranging to minimize this repulsion.

The first step is to draw the Lewis structure to identify the central atom, its bonded atoms, and any lone pairs. Calculate the steric number (SN), which is the sum of bonded atoms and lone pairs. This SN determines the electron domain geometry (e.

g., SN=4 means tetrahedral electron domains). The molecular geometry, however, is defined by the positions of the atoms only. Lone pairs exert stronger repulsion than bonding pairs, distorting ideal bond angles and leading to different molecular shapes even for the same electron domain geometry (e.

g., tetrahedral electron domain can yield tetrahedral, trigonal pyramidal, or bent molecular geometries). Remember that multiple bonds count as a single electron domain for VSEPR purposes. For SN=5 (trigonal bipyramidal), lone pairs prefer equatorial positions.

For SN=6 (octahedral), lone pairs prefer trans positions. Polarity is determined by both bond polarity and overall molecular symmetry; symmetrical molecules with polar bonds can be non-polar due to dipole cancellation.

5-Minute Revision

Molecular geometry is the three-dimensional arrangement of atoms in a molecule, a fundamental concept that dictates a molecule's physical and chemical behavior. The Valence Shell Electron Pair Repulsion (VSEPR) theory is the cornerstone for predicting these shapes.

VSEPR posits that electron domains—which can be single, double, or triple bonds, or lone pairs—around a central atom repel each other and will arrange themselves to be as far apart as possible, minimizing repulsion and achieving the most stable configuration.

Each multiple bond to a distinct atom counts as one electron domain.

To apply VSEPR, first, draw the correct Lewis structure for the molecule or ion. Identify the central atom. Count the number of atoms bonded to the central atom and the number of lone pairs on the central atom. The sum of these two is the steric number (SN). The SN determines the electron domain geometry:

SN=2: Linear
SN=3: Trigonal Planar
SN=4: Tetrahedral
SN=5: Trigonal Bipyramidal
SN=6: Octahedral

Next, determine the molecular geometry, which is the arrangement of *atoms only*. Lone pairs significantly influence the molecular geometry by exerting greater repulsive forces than bonding pairs (LP-LP > LP-BP > BP-BP).

This causes distortions in ideal bond angles. For example, a molecule with SN=4 has a tetrahedral electron domain geometry. If it has 4 bonding pairs (e.g., $ext{CH}_4$), its molecular geometry is tetrahedral ($109.

5^circ$). If it has 3 bonding pairs and 1 lone pair (e.g.,$ ext{NH}_3$), its molecular geometry is trigonal pyramidal ($approx 107^circ$). If it has 2 bonding pairs and 2 lone pairs (e.g.,$ ext{H}_2 ext{O}$), its molecular geometry is bent ($approx 104.

5^circ$).

For SN=5 (trigonal bipyramidal electron domain), lone pairs preferentially occupy equatorial positions to minimize $90^circ$ repulsions. This leads to geometries like seesaw ($ext{AX}_4\text{E}$), T-shaped ($ext{AX}_3\text{E}_2$), and linear ($ext{AX}_2\text{E}_3$). For SN=6 (octahedral electron domain), lone pairs occupy positions $180^circ$ apart (trans) to minimize repulsion, leading to square pyramidal ($ext{AX}_5\text{E}$) or square planar ($ext{AX}_4\text{E}_2$) geometries.

Remember that molecular polarity is determined by both bond polarity and the overall molecular geometry. Symmetrical molecules (e.g., $ext{CO}_2$, $ext{CCl}_4$, $ext{BF}_3$) can be non-polar even if they contain polar bonds, because their bond dipoles cancel out. Asymmetrical molecules (e.g., $ext{H}_2\text{O}$, $ext{NH}_3$, $ext{SO}_2$) with polar bonds will be polar overall.

Prelims Revision Notes

1
VSEPR Theory Basics — Electron pairs (bonding and lone) around a central atom repel each other and arrange to maximize separation, minimizing repulsion. This determines the electron domain geometry.
2
Steric Number (SN) — Crucial for VSEPR. $SN = (\text{number of atoms bonded to central atom}) + (\text{number of lone pairs on central atom})$.
3
Electron Domain Geometries (EDG)

* SN=2: Linear * SN=3: Trigonal Planar * SN=4: Tetrahedral * SN=5: Trigonal Bipyramidal * SN=6: Octahedral

1
Molecular Geometry (MG) — Determined by EDG and the number of lone pairs. Only considers atom positions.

* SN=2: $ext{AX}_2$ (0 LP) $ightarrow$ Linear ($180^circ$) e.g., $ext{CO}_2$, $ext{BeCl}_2$. * SN=3: $ext{AX}_3$ (0 LP) $ightarrow$ Trigonal Planar ($120^circ$) e.g., $ext{BF}_3$, $ext{SO}_3$.

$ext{AX}_2\text{E}$ (1 LP) $ightarrow$ Bent ($<120^circ$) e.g., $ext{SO}_2$, $ext{O}_3$. * SN=4: $ext{AX}_4$ (0 LP) $ightarrow$ Tetrahedral ($109.5^circ$) e.g., $ext{CH}_4$, $ext{CCl}_4$.

$ext{AX}_3\text{E}$ (1 LP) $ightarrow$ Trigonal Pyramidal ($approx 107^circ$) e.g., $ext{NH}_3$, $ext{PCl}_3$. $ext{AX}_2\text{E}_2$ (2 LP) $ightarrow$ Bent ($approx 104.5^circ$) e.g., $ext{H}_2\text{O}$, $ext{H}_2\text{S}$.

1
Effect of Lone Pairs — Lone pairs exert greater repulsion than bonding pairs (LP-LP > LP-BP > BP-BP). This causes bond angles to decrease from ideal values.
2
SN=5 (Trigonal Bipyramidal EDG) — Lone pairs prefer equatorial positions.

* $ext{AX}_5$ (0 LP) $ightarrow$ Trigonal Bipyramidal ($90^circ, 120^circ$) e.g., $ext{PCl}_5$. * $ext{AX}_4\text{E}$ (1 LP) $ightarrow$ Seesaw e.g., $ext{SF}_4$. * $ext{AX}_3\text{E}_2$ (2 LP) $ightarrow$ T-shaped e.g., $ext{ClF}_3$. * $ext{AX}_2\text{E}_3$ (3 LP) $ightarrow$ Linear ($180^circ$) e.g., $ext{XeF}_2$, $ext{I}_3^-$.

1
SN=6 (Octahedral EDG) — Lone pairs prefer trans positions.

* $ext{AX}_6$ (0 LP) $ightarrow$ Octahedral ($90^circ$) e.g., $ext{SF}_6$. * $ext{AX}_5\text{E}$ (1 LP) $ightarrow$ Square Pyramidal e.g., $ext{BrF}_5$. * $ext{AX}_4\text{E}_2$ (2 LP) $ightarrow$ Square Planar ($90^circ$) e.g., $ext{XeF}_4$, $ext{ICl}_4^-$.

1
Polarity — A molecule is polar if it has polar bonds AND an asymmetrical geometry where bond dipoles do not cancel. Symmetrical molecules (e.g., $ext{CO}_2$, $ext{CCl}_4$, $ext{BF}_3$) are non-polar even with polar bonds. Asymmetrical molecules (e.g., $ext{H}_2\text{O}$, $ext{NH}_3$, $ext{SO}_2$) are polar.
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Steps for VSEPR — Lewis structure $ightarrow$ Central atom $ightarrow$ Count BP & LP $ightarrow$ Calculate SN $ightarrow$ EDG $ightarrow$ MG.

Vyyuha Quick Recall

To remember the order of repulsion: Lone Pairs Love Big Places. (LP-LP > LP-BP > BP-BP)