covalent bonds all atoms want an octet (octet rule) –can share valence e-s e-s occupy molecular...
TRANSCRIPT
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