Covalent Bonds• All atoms want an octet (Octet Rule)

– Can share valence e-s• e-s occupy molecular orbitals• Where are e-s located?

– Attraction and repulsion of e-s and p+s

• Molecule is more stable than atoms– Minimum energy (Ep)

• Can share 1, 2, or 3 pair e-s– Single, double, triple bond

Bond Properties

• Bond distance• Bond energy• Polarity

Bond Distance

Distance between nuclei of bonded atoms– Distance at lowest Ep– single > double > triple– Smaller atoms bond closer together

• H – F H – Cl H – Br H – I (9.2) (12.7) (14.1) (16.1) relative

bond dist

Bond Energy

• Energy needed to break mol of cov bonds– At minimum energy

• bal of attr and repulsion of e- and p+ of both atoms

• e- free to move about both nuclei = more stable

– C C < C C < C C (347) (612) (820) units = kJ/mol– Smaller atom held together stronger

• H – F H – Cl H – Br H – I (565) (431) (360) (297) kJ/molInversely related

Polarity

• Unequal sharing of e-s– Due to diff in electronegativities

• F = 4.0 Fr = 0.8

– Dipole moment• Measure of the strength of polarization• Polar molecules have charged ends

Lewis Structures

• Show atoms and their arrangement in a molecule

• Octet rule– All atoms gain an octet (except H and

He)

• Know rules pg 166• See powerpoint on Lewis Structures

Lewis Structures

• Bonding and nonbonding e- pairs• Resonance

– Formal charge

• Exceptions to the octet rule– Octet deficiencies– Expanded octets

• Molecular geometry– VSEPR theory

• Hybridization

Resonance

• 2 or more Lewis Struct for same molec– Equal, major, minor contributors

• Formal Charge (for each central atom)– Used to determine major Lewis Structure– Calc FC for the central atom(s)

FC = eval – (enb + ½eb)

H .. .. .. H C O H H C O H

H H H

Determination of Formal Charge FC = eval – (enb + ½eb)

H .. .. .. H C O H H C O H H H H

• FCC = 4 – [0 + (½•8)] FCC = 4 – [2 + (½•6)]

= 0 = -1

• FCO = 6 – [2 + (½•4)] FCO = 6 – [2 + (½•6)]

= 0 = +1 the better structure has: 1- all atom’s FC as close to 0 as possible.

2- if not 0 then negative value is on the more electroneg element

Exceptions to Octet Rule

• Octet deficiencies– Odd electron species

• B, Be, and sometimes N form compounds with less than an octet

Exceptions to Octet Rule

Octet deficienciesOdd electron species

B, Be, and sometimes N form compounds with less than an octet

Draw Lewis Structure for BeF2

Expanded Octet• Some species (3rd period and

beyond) exceed the octet rule• Empty 3d sublevel (or higher) can

accommodate extra e-s– Terminal atoms usually halogen or O

• Never occurs with 2nd period elements– No d sublevel to house extra e-s

Molecular Geometry

• VSEPR Theory– Valence Shell Electron Pair Repulsion

Theory• All e- pairs (bonding and nonbonding) occupy

orbitals (shells)• All orbitals, being (-) charged repel each

other, and push one another as far away as possible

– All orbitals orient themselves as far apart as possible

Molecular Geometry• Based on number of electron clouds around

the central atom– Single, double, triple bonds, and nonbonding e-

pairs all count as 1 cloud

• 2 to 6 pairs of e-s on central atom– Therefore 2 to 6 clouds around central atom

• Unshared (nonbonding) e- pairs affect the geometry of the molecule but not the orientation of the e- clouds about the central atom– Know descriptions and bond angles from tables

7.3 p 188 and 7.7 p 190

Nonbonding Clouds Molecular Geometry

• The nonbonding cloud is not seen as part of the molecule

• The geometry of the molecule and the geometry of the e- clouds are the same if there are no nonbonding clouds present

• The geometry of the molecule and the geometry of the e- clouds are different if there are nonbonding clouds present

Geometry of Molec with all bonding e- clouds

Effects of Nonbonding e- Clouds on Geometry

Effects of Nonbonding e- Clouds on Geometry

Effects of Nonbonding e- Clouds on Geometry

Effects of Nonbonding e- Clouds on Geometry

Structural Nonpolarity

• Bonds indicate molecule should be polar but it is not– Symmetrical molecule

• Linear, trig planar, tetrahed, triag bipyr, octa

– Central atom bonded to 2 or more of the same atoms

– Opposite pulls on e-s cancel each other

Structural Nonpolarity

Water is polar because of the unequal pull on the electrons – the molecule in not symmetrical

Structural Nonpolarity

Because of the equal attraction for the shared pair of e-s, and the symmetry the molecule, carbon dioxide is nonpolar

Hybridization

• Atomic orbital blend together and form new molecular orbitals – Called hybrid orbitals

• The same # of MO are formed as the # AO that blend together– 1 s and 1 p AO form 2 sp MO– 1 s and 2 p AO form 3 sp2 – 1 s and 3 p AO form 4 sp3 – 1 s and 3 p and 1 d AO form 5 sp3d – 1 s and 3 p and 2 d AO form 6 sp3d2

2s

These new orbitals are called hybrid orbitals

The process is called hybridization

This means is that the s and one p orbital are mixed together and form 2 sp hybrid molecular orbital.

Formation of sp hybrid orbitalsFormation of sp hybrid orbitals

The combination of an s orbital and a p orbital produces 2 new orbitals called sp orbitals.

Formation of spFormation of sp22 hybrid orbitalshybrid orbitals

Formation of spFormation of sp33 hybrid orbitalshybrid orbitals

sp3d and sp3d2 Orbitals

sp3d sp3d2

Sigma and Pi Bonds

• Sigma () bond s are created by an end to end overlap of molecular orbitals– All single bonds are bonds

• Pi bonds are formed by a side by side overlap of MO. The e-s are delocalized, moving more freely about both nuclei that e-s in a sigma bond.– Double bond = 1 bond, and 1 bond– Triple bond = 1 bond and 2 bonds.

and Bonds