Molecular Geometry. Bonding Covalent bonds occur when atoms are at an ideal distance from one...

Molecular Geometry

Bonding

• Covalent bonds occur when atoms are at an ideal distance from one another.

• At this distance attractive forces predominate repulsive forces.

• Too close the atoms repel each other. Too far away they do not attract

Bonding

Polarity of Polyatomic Molecules

• Recall polar covalent bonds• For molecules with more

than one covalent bond, polarity depends on individual bonds and shapes of molecules.

• Considering HCl, the molecule is polar because of the shape and electronegativity differences.

Polarity of Polyatomic Molecules

• Consider CO2

• VESPR model signifies linear molecular shape

• C-O bond is polar

• CO2 molecule nonpolar

• Overall molecular polarity is the vector sum of individual dipoles

Polarity of Polyatomic Molecules

• Consider H2O

• We know it is a polar molecule

• H-O bonds are polar• VESPR model predicts

tetrahedral bent molecular shape

• Vector of polarity proves overall polar molecule

Polarity of Polyatomic Molecules

• Predict if CCl4 and CHCl3 are polar.

Covalent Bonding and Orbital Overlap

• When covalent bonds occur we say the orbitals overlap.

Multiple Bonds

• So far we have considered only σ bonds.• In a σ bond the e- density is concentrated

about the nuclear axis; can occur with p or s orbital.

• In a multiple bond overlap within the p orbital occurs perpendicular to the nuclear axis

• The said perpindicular overlap of p orbitals produces a π bond

Multiple Bonds

• Consider ethene 1 σ, 1 π

Delocalized e-

• e- localized when e- are associated w/ π and σ bonds keeping them with 2 atoms.

• Delocalized e- can be associated w/ many atoms. Associated with resonance structures.

• Benzene

Delocalized e-

• In benzene, neither of the two Lewis resonance structures are correct.

• The π e- are spread throughout the entire molecule giving the molecule incredible stability.

Molecular Orbital Theory

• The theory assigns the electrons in a molecule to a series of orbitals that belong to the molecule as a whole.

• relate them to the probability of finding electrons in certain regions of a molecule.

Molecular Orbital Theory

Whenever two atomic orbitals overlap, two molecular orbitals form.

• The lower energy MO concentrates e- density between the nuclei is the bonding molecular orbital.

• The higher energy MO has little e- density between nuclei is the antibonding molecular orbital, signified by a *

Molecular Orbital Theory

• If the e- density is centered about the nucleus it is a σ molecular orbital

• Often represented in energy level diagrams, note σ1s lower energy that σ1s*

Molecular Orbital Theory

• If electrons are not placed in the σ1s orbital they must be placed in σ1s*

• Take theoretical molecule He2

Molecular Orbital Theory

• Bond Order = ½ (#if bonding e- - # of antibonding e-)

• Bond order relates to the stability of covalent bonds

• Bond order of 0 represents no bonds

• 1 represents single, 2 represents double 3 represents triple