Molecular Geometry.

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Presentation 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