Shapes of Molecules: Quick Revision

A brief note on Shapes of Molecules. Read it before a test. Try each question at the end before you open its answer.

VSEPR

• Pairs on the central atom: $\dfrac{V + M \pm q}{2}$, with $V$ its outer electrons, $M$ its singly bonded atoms and $q$ the charge. A double or triple bond is one region.
• 2, 3, 4, 5, 6 pairs: linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral.
• Repulsion: lone with lone $>$ lone with bond $>$ bond with bond.
• The shape names the atoms only. Four pairs with one lone pair: trigonal pyramidal. With two: bent.
• Five pairs: lone pairs go equatorial, giving see-saw and T-shaped. Six pairs: square pyramidal, then square planar.

Valence bond theory

• A bond forms when two half-filled orbitals overlap. More overlap, stronger bond.
• $\text{H}_2$: the energy is lowest at $74\ \text{pm}$, and $435.8\ \text{kJ mol}^{-1}$ is released.
• Head-on overlap: sigma bond. Sideways overlap of parallel p orbitals: pi bond, weaker.
• Single $1\sigma$, double $1\sigma + 1\pi$, triple $1\sigma + 2\pi$.

Hybridisation

• Hybrids can hold lone pairs, as in ammonia and water.
• Unhybridised p orbitals make the pi bonds of ethene and ethyne.

A worked example

Find the hybridisation and shape of the ion $[\text{ICl}_2]^-$.

1. Pairs. Iodine has $7$ outer electrons. Two chlorines add $2$, and the charge adds $1$: $\dfrac{10}{2} = 5$ pairs.
2. Lone pairs. $5 - 2 = 3$ lone pairs. Five pairs mean $sp^3d$.
3. Shape. The three lone pairs fill the equatorial triangle. The two chlorines sit on the axis, so the ion is linear.

Practice with new numbers

A program to try

The program finds the hybridisation, the lone pairs and the shape of six molecules.

Mistakes to avoid

• Shape from the pairs alone. Leave the lone pairs out when you name the shape.
• A double bond as two regions. It counts once.
• Lone pairs axial. With five pairs they go equatorial.
• Pi bonds from s orbitals. Only parallel p orbitals overlap sideways.
• Ethene as $sp^3$. A double-bonded carbon is $sp^2$.

Check yourself

1. What shape is $\text{PH}_3$?

2. How many pi bonds are there in a triple bond?

3. What is the hybridisation of carbon in ethyne?

4. Which is stronger: a sigma bond or a pi bond between the same atoms?

5. Why is $\text{XeF}_4$ square planar and not tetrahedral?