molecular geometry. bonding covalent bonds occur when atoms are at an ideal distance from one...
TRANSCRIPT
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