Molecular Shapes In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry.

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Presentation transcript:

Molecular Shapes In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimized this repulsion. We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory.

Predicting Molecular Geometries To determine the shape of a molecule, we distinguish between lone pairs (or non-bonding pairs, those not in a bond) of electrons and bonding pairs (those found between two atoms). We define the electron pair geometry by the positions in 3D space of ALL electron pairs (bonding or non-bonding). The electrons adopt an arrangement in space to minimize e--e- repulsion.

Predicting Molecular Geometries To determine the electron pair geometry: draw the Lewis structure count the total number of electron pairs around the central atom arrange the electron pairs in one of the above geometries to minimize e--e- repulsion multiple bounds count as one bonding pair

The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles We determine the electron pair geometry only looking at electrons. We name the molecular geometry by the positions of atoms. We ignore lone pairs in the molecular geometry. All the atoms that obey the octet rule have tetrahedral electron pair geometries.

The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles By experiment, the H-X-H bond angle decreases on moving from C to N to O: Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. Therefore, the bond angle decreases as the number of lone pairs increase.

The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles Similarly, electrons in multiple bonds repel more than electrons in single bonds.

Molecules with Expanded Valence Shells To minimize e--e- repulsion, lone pairs are always placed in equatorial positions.

Polar molecules interact with electric fields. If the centers of negative and positive charge do not coincide, then the molecule is polar.

Dipole Moments of Polyatomic Molecules If two charges, equal in magnitude and opposite in sign, are separated by a distance r, then a dipole is established. The dipole moment, m, is given by m = Qr where Q is the magnitude of charge. Dipole Moments of Polyatomic Molecules In a polyatomic molecule, each bond can be a dipole. The orientation of these individual dipole moments determines whether the molecule has an overall dipole moment.