Molecular Geometry and

Polarity

Review Covalent Bonding

• Covalent bonding entails a sharing of electrons.

• Covalent bonding usually occurs between nonmetals.

• Form individual molecules.

• Shapes of molecules determines properties.

Properties of Covalent Molecules

• Gases, liquids, or solids (made of molecules)

• Low melting and boiling points

• Poor electrical conductors in all phases

• Many soluble in nonpolar liquids but not in water

Covalent Bonds– attain the octet or full valence by sharing pairs of valence electrons.

Covalent Bonds• For every pair of electrons shared between

two atoms, a single covalent bond is formed. 

• Some atoms can share multiple pairs of electrons, forming multiple covalent bonds. 

VSEPR -ChemSaver p 29

• VSEPR stands for Valence Shell Electron Pair Repulsion.  

• Basically, the idea is that covalent bonds and lone pair electrons like to stay as far apart from each other as possible under all conditions. 

• This is because covalent bonds consist of electrons, and electrons don't like to hang around next to each other much because they have the same charge (like charges repel).

VSEPR explains why molecules have their shapes. 

• If carbon has four atoms stuck to it (as in CH4), these four atoms want to get as far away from each other as they can.  This isn't because the atoms necessarily hate each other, it's because the electrons in the bonds “hate” each other.  That's the idea behind VSEPR.

Methane is Tetrahedral

109.50

Sp3 hybridized carbon 4 equivalent C-H bonds (bonds)

All purely single bonds are called bonds

H

CH

HH

Molecular Geometry- ChemSaver p 29

Polarity

• Depending on the percent covalent vs. ionic characteristic of the bond, molecular compounds can have polar covalent or nonpolar covalent bonds

• The higher the percentage of ionic characteristic the more polar the bond will be

Polar vs. Non-polar – ChemSaver p30• Intermolecular forces: are the electrostatic

forces of attraction between molecules (NM-NM).

• Nonpolar: electrons are evenly distributed so no electrical charge on molecule.– London Dispersion ForcesLondon Dispersion Forces

• Polar: electrons are not evenly distributed creating positive and

negative ends on molecule.– Hydrogen BondingHydrogen Bonding – Dipole-dipole ForcesDipole-dipole Forces

POLAR COVALENT • Partial negative charge and partial positive

charge

• Unequal Sharing of Electrons• Moderate boiling points (Most are liquids at

room temperature)

NONPOLAR COVALENT•   No unequal charges anywhere on the

molecule

• Equal Sharing of Electrons• low boiling points (Most are gases at room

temperature)

Hydrogen Bonding

• A very strong type of dipole—dipole (polar ) attraction in which a hydrogen atom is bridged between two highly electronegative atoms, usually N , O, or F.

 

OOHH

HH

OOHH

HH

Covalent bondCovalent bond

Hydrogen bondHydrogen bond

Hydrogen Bonding :Hydrogen Bonding :

Properties due to Hydrogen Bonding

• Higher than expected melting/boiling points

• More viscous substances (liquids are “thicker” to pour)

• Surface tension – an inward pull that minimizes the surface area of a liquid.

• Capillary Action

Hydrogen Bonding :Hydrogen Bonding :

Hydrogen Bonding : Surface TensionHydrogen Bonding : Surface Tension

Only polar covalent molecules have the ability to form dipole-dipole attractions between molecules. Polar covalent molecules act as little magnets, they have positive ends and negative ends which attract each other.

Dipole-Dipole :Dipole-Dipole :

Dipole-Dipole :Dipole-Dipole :

• Because of the constant motion of the electrons, an atom or molecule can develop a temporary (instantaneous) dipole when its electrons are distributed asymmetrically about the nucleus.

• The attractive forces are responsible for Bromine being a liquid and Iodine a solid at room temperature.

London Dispersion Force :London Dispersion Force :

• The electron asymmetry about the nucleus induces a temporary attraction between the non-polar molecules causing the London Dispersion Force.

London Dispersion Force :London Dispersion Force :