VSEPR Theory — Revision Notes

⚡ 30-Second Revision

VSEPR Principle: — Electron pairs repel, arrange to minimize repulsion.
Electron Domains: — Single, double, triple bonds, and lone pairs each count as one domain.
Steric Number (SN): — Sum of bonded atoms + lone pairs on central atom.
Repulsion Order: — LP-LP > LP-BP > BP-BP (Lone Pair > Bond Pair).
Key Geometries (SN, LP):

- SN 2, 0 LP: Linear ($180^circ$) - SN 3, 0 LP: Trigonal Planar ($120^circ$) - SN 3, 1 LP: Bent ($<120^circ$) - SN 4, 0 LP: Tetrahedral ($109.5^circ$) - SN 4, 1 LP: Trigonal Pyramidal ($107^circ$) - SN 4, 2 LP: Bent ($104.5^circ$) - SN 5, 0 LP: Trigonal Bipyramidal ($90^circ, 120^circ$) - SN 5, 1 LP: See-Saw - SN 5, 2 LP: T-shaped - SN 5, 3 LP: Linear - SN 6, 0 LP: Octahedral ($90^circ$) - SN 6, 1 LP: Square Pyramidal - SN 6, 2 LP: Square Planar

Formula for SN (neutral): — $SN = \frac{1}{2} (\text{Valence e}^- \text{ of central atom} + \text{No. of monovalent atoms})$
Formula for SN (ion): — $SN = \frac{1}{2} (\text{Valence e}^- \text{ of central atom} + \text{No. of monovalent atoms} pm \text{Charge})$

2-Minute Revision

VSEPR theory is your go-to model for predicting molecular shapes. Remember, it's all about electron pairs around a central atom trying to get as far apart as possible due to repulsion. Start by drawing the Lewis structure to identify the central atom, its bonding pairs, and lone pairs.

Each bond (single, double, or triple) counts as one 'electron domain,' and each lone pair also counts as one. Summing these gives you the 'steric number' (SN). This SN dictates the 'electron domain geometry' (e.

g., SN=4 means tetrahedral electron geometry). However, the 'molecular geometry' – the actual shape of the atoms – can differ if lone pairs are present. Lone pairs exert stronger repulsive forces than bonding pairs (LP-LP > LP-BP > BP-BP), pushing bond angles to be smaller than ideal.

For instance, $CH_4$ (4 BP, 0 LP) is tetrahedral ($109.5^circ$), $NH_3$ (3 BP, 1 LP) is trigonal pyramidal ($107^circ$), and $H_2O$ (2 BP, 2 LP) is bent ($104.5^circ$), all having a tetrahedral electron domain geometry.

Master the common geometries for SN 2-6 and the effect of lone pairs on bond angles to quickly solve NEET questions.

5-Minute Revision

To ace VSEPR theory, follow a systematic approach. First, always begin by drawing the correct Lewis structure for the molecule or ion. This step is critical for identifying the central atom and accurately counting its valence electrons, bonding pairs, and lone pairs.

Remember that multiple bonds (double or triple) are treated as a single electron domain. Next, calculate the steric number (SN) for the central atom, which is the sum of the number of atoms bonded to the central atom and the number of lone pairs on the central atom.

A useful shortcut for calculating SN is $SN = \frac{1}{2} (\text{Valence electrons of central atom} + \text{Number of monovalent atoms} pm \text{Charge})$.

Once you have the steric number, you can determine the electron domain geometry: SN=2 is linear, SN=3 is trigonal planar, SN=4 is tetrahedral, SN=5 is trigonal bipyramidal, and SN=6 is octahedral. This is the arrangement of *all* electron groups.

Now, to find the molecular geometry, you only consider the positions of the *atoms*. If there are no lone pairs, the molecular geometry is the same as the electron domain geometry. However, if lone pairs are present, they occupy space and exert stronger repulsive forces than bonding pairs (LP-LP > LP-BP > BP-BP).

This stronger repulsion distorts the ideal bond angles and changes the molecular geometry. For example, a molecule with SN=4 and 1 lone pair will have a tetrahedral electron domain geometry but a trigonal pyramidal molecular geometry (like $NH_3$), with bond angles reduced from $109.

5^circ$to about$107^circ$. With 2 lone pairs (like$H_2O$), it becomes bent with angles around$104.5^circ$. For SN=5 (trigonal bipyramidal electron geometry), lone pairs preferentially occupy equatorial positions to minimize$90^circ$ repulsions.

Practice with diverse examples, including noble gas compounds ($XeF_2$, $XeF_4$) and polyatomic ions ($I_3^-$, $ClO_3^-$), to solidify your understanding and speed.

Prelims Revision Notes

VSEPR theory is a qualitative model for predicting molecular geometry based on electron pair repulsion. The key is to determine the steric number (SN) and the number of lone pairs (LP) on the central atom. Each single, double, or triple bond counts as one electron domain. Lone pairs also count as one electron domain. The formula for SN is $SN = \frac{1}{2} (\text{Valence electrons of central atom} + \text{Number of monovalent atoms} pm \text{Charge})$.

Electron Domain Geometries:

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

Molecular Geometries (SN, LP, BP):

SN=2: — (2 BP, 0 LP) $ightarrow$ Linear (e.g., $CO_2$, $BeCl_2$) - $180^circ$
SN=3:

* (3 BP, 0 LP) $ightarrow$ Trigonal Planar (e.g., $BF_3$, $SO_3$) - $120^circ$ * (2 BP, 1 LP) $ightarrow$ Bent (e.g., $SO_2$, $O_3$) - $<120^circ$

SN=4:

* (4 BP, 0 LP) $ightarrow$ Tetrahedral (e.g., $CH_4$, $SiCl_4$) - $109.5^circ$ * (3 BP, 1 LP) $ightarrow$ Trigonal Pyramidal (e.g., $NH_3$, $PCl_3$) - $107^circ$ * (2 BP, 2 LP) $ightarrow$ Bent (e.g., $H_2O$, $H_2S$) - $104.5^circ$

SN=5:

* (5 BP, 0 LP) $ightarrow$ Trigonal Bipyramidal (e.g., $PCl_5$, $AsF_5$) - $90^circ, 120^circ$ * (4 BP, 1 LP) $ightarrow$ See-Saw (e.g., $SF_4$, $TeCl_4$) - $<90^circ, <120^circ$ * (3 BP, 2 LP) $ightarrow$ T-shaped (e.g., $ClF_3$, $BrF_3$) - $<90^circ$ * (2 BP, 3 LP) $ightarrow$ Linear (e.g., $XeF_2$, $I_3^-$) - $180^circ$

SN=6:

* (6 BP, 0 LP) $ightarrow$ Octahedral (e.g., $SF_6$, $TeF_6$) - $90^circ$ * (5 BP, 1 LP) $ightarrow$ Square Pyramidal (e.g., $BrF_5$, $IF_5$) - $<90^circ$ * (4 BP, 2 LP) $ightarrow$ Square Planar (e.g., $XeF_4$, $ICl_4^-$) - $90^circ$

Repulsion Order: Lone pair-lone pair (LP-LP) > Lone pair-bond pair (LP-BP) > Bond pair-bond pair (BP-BP). This order explains the reduction in bond angles from ideal values when lone pairs are present. Lone pairs prefer equatorial positions in trigonal bipyramidal geometry to minimize $90^circ$ repulsions. Molecular polarity is determined by both bond polarity and molecular geometry; symmetrical geometries can lead to nonpolar molecules even with polar bonds.

Vyyuha Quick Recall

To remember the repulsion order: Lone Lone Bond Bond. (LP-LP > LP-BP > BP-BP). Think of 'L' as 'Large' repulsion and 'B' as 'Small' repulsion. So, Large-Large > Large-Small > Small-Small.