Molecular structure and covalent bonding

Chapter 8

Key concepts

• Understand the difference between Lewis structures and molecular geometries.

• Know the basic shapes of several molecule types.• Know how to predict molecular shapes using valence-

shell electron repulsion (VSEPR) model.• Understand how overall molecular shape can affect the

dipole moment of the molecule. • Understand the use of valence-bond theory as an

explanation for the VSEPR model.• Know different hybrid orbitals that function in molecular

geometries.

Molecular geometries

• Lewis structures help us understand the type of bonds between atoms in a molecule

• But, they do NOT indicate the geometry in 3-D space.

• VSEPR theory helps describe the actual geometry of molecules based on their covalent bonding.

• two main parameters to know for molecular geometry:– bond length(s) –

– bond angle(s) –

– (there is a third parameter, the dihedral angle, important in molecules with more than one central atom, which we will occasionally encounter)

Molecular geometries

Fundamental geometry

• five fundamental geometries behind all ABn molecular shapes– Linear– Trigonal planar– Tetrahedral– Trigonal bipyramidal– Octahedral

• Table 8.1 (p. 305) illustrates fundamental geometries

valence-shell electron repulsion (VSEPR) model

• used to predict geometries of ABn molecules where A is a p-block element.

• Electron domains (“regions of high electron density”): areas where electrons are most likely found in a molecule.

• two types of electron domains:

• bonding domain -

• non-bonding domain –

• Electron domains repel each other; will force as far away from each other as possible.

the best electronic geometry is the one that minimizes all electron repulsions in the molecule.

Basic principles of VSEPR

The electronic geometry is NOT the molecular geometry.

• Electronic geometry –

• molecular geometry –

Steps for predicting molecular geometries

1. Write Lewis formula and identify a central atom.

• Examples: H2O, NH3, CH4, CO2, SO2, SO32-,

COCl2, SF6, XeF4

2. Count regions of high electron density (electron domains) on that central atom.

• Single bonds, multiple bonds, lone pairs all count as ONE region.

Steps for predicting molecular geometries

3. Determine electronic geometry around central atom.

4. Determine molecular geometry around central atom.

• If there is ≥ 1 non-bonding electron pair in the molecule, the electronic geometry is never the same as the molecular geometry.

Steps for predicting molecular geometries

Non-bonding pairs and multiple bonds affect

the ideal molecular geometry. 5. Adjust molecular geometry for any lone pairs (or

multiple bonds).• Non-bonding domains take up more space than bonding

domains, and have a greater repulsive force that will compress bond angles in the molecule.

• Multiple bonds have higher electron density than single bonds.

multiple bonds compress bond angles between single bonds.

• the amount of compression is Lone pair >> multiple bonds > single bonds

6. Determine hybrid orbitals, describe bonding.

• Hybrid orbitals: How to explain both bonding between atoms and molecular geometries. Atomic orbitals cannot explain this.

• Different atomic orbitals of the central atom mix to form hybrid orbitals that have proper shapes to produce observed molecular geometries.

Steps for predicting molecular geometries

Types of hybrids

• What atomic orbitals combine to form the hybrid orbitals on the central atom?

• sp –• sp2 –• sp3 –• sp3d –• sp3d2 –• The number of hybrid orbitals ________ the

number of atomic orbitals

sp hybrids

sp2 hybrids

sp3 hybrids

sp3d hybrids

suppose I had SF4 instead of PF5. Where would the lone pair be located?

Lone pairs of trig. bipyrimidalelectronic geometries first occupy____________ positions.

sp3d2 hybrids

What if we had XeF4 insteadof SF6? Where would the lonepairs be located?

7. Can another central atom be identified?• Not in current examples, but how about

C2H6?

Steps for predicting molecular geometries

Multiple bonds in the molecule

• consider C2H4, which has a double bond (H2C=CH2).

• What is the electronic geometry on each C? molecular geometry? What hybrid orbitals are used?

• of the 2 bonds between C and C, one is between sp2 orbitals on each C. Where is the other bond?

and bonds

• Bond between sp2 orbitals of carbons called a (sigma) bond. Orbitals overlap head-to-head.

• other bond forms between unhybridized 2p orbitals of carbons. This is called a (pi) bond. Orbitals overlap side-by-side.

bond extends above and below the plane of the bond.

Triple bonds

• Let’s use our molecular geometry process on acetylene (HCCH). What do we get for the molecular geometry?

• What hybrid orbitals are used?

• How are the multiple bonds formed?

8. Determine if molecule is polar or non- polar

• We must examine the dipole moment along each bond in context of the molecular geometry

• Dipoles are vectors, so they add like vectors.

Steps for predicting molecular geometries

To be or not to be…polar…

Two conditions for polarity:1. There must be at least one polar bond

(or lone pair) on the central atom.2. a. the polar bonds (if more than one)

must not cancel each other out, orb. lone pairs (if more than one) must not have a geometry that cancels out.

of our examples, which are polar? which are non-polar?