1. Molecular Compounds Unit 7

2. Covalent Bonds Sharing pairs of electrons Sharing pairs of electrons Covalent bonds are the intra-molecular attraction resulting from the sharing of electrons between atoms. Covalent bonds are the intra-molecular attraction resulting from the sharing of electrons between atoms. They result in ‘localized overlaps’ of orbitals of different atoms. They result in ‘localized overlaps’ of orbitals of different atoms. They also are the result of the attraction of electrons for the nucleus of other atoms. They also are the result of the attraction of electrons for the nucleus of other atoms. Typical of molecular substances. Typical of molecular substances.

3. Covalent Bonds Cont. Atoms bond together to form molecules Atoms bond together to form molecules –molecules are electrically neutral groups of atoms joined together by covalent bonds –strong attraction Molecules attracted to each other weakly form molecular solids Molecules attracted to each other weakly form molecular solids How can we distinguish between an ionic and a molecular compound? How can we distinguish between an ionic and a molecular compound? –Ionic = contains a metal –Molecular = composed of 2 or more nonmetals

4. Properties of Molecular Compounds Strong covalent bonds hold the atoms together within a molecule. Strong covalent bonds hold the atoms together within a molecule. The intermolecular forces that hold one molecule to another are much weaker. The intermolecular forces that hold one molecule to another are much weaker. Molecular Compounds can exist in all 3 states: Molecular Compounds can exist in all 3 states: –Solids – sugar, ice, aspirin –Liquids – water, alcohols –Gases – O 2, CO 2, and N 2 O (laughing gas)

5. Properties Cont. Properties vary depending on the strength of the intermolecular forces. Properties vary depending on the strength of the intermolecular forces. Usually have a lower MP (than ionic, metallic) Usually have a lower MP (than ionic, metallic) Generally soft Generally soft Non-conductors (in any state & aqueous) Non-conductors (in any state & aqueous) Very important to organic chemistry and pharmaceutics. Very important to organic chemistry and pharmaceutics.

6. Electronegativity Measure of the ability of an atom to attract shared electrons Measure of the ability of an atom to attract shared electrons –Larger electronegativity means atom attracts more strongly –Values 0.7 to 4.0 Increases across period (left to right) on Periodic Table Increases across period (left to right) on Periodic Table Decreases down group (top to bottom) on Periodic Table Decreases down group (top to bottom) on Periodic Table Larger difference in electronegativities means more polar bond Larger difference in electronegativities means more polar bond –negative end toward more electronegative atom

7. Bond Polarity Covalent bonding between unlike atoms results in unequal sharing of the electrons Covalent bonding between unlike atoms results in unequal sharing of the electrons –One end of the bond has larger electron density than the other –Polar covalent – unequal sharing –Nonpolar covalent – equal sharing The result is bond polarity The result is bond polarity –The end with the larger electron density gets a partial negative charge –The end that is electron deficient gets a partial positive charge H F 

8. Dipole Moment Bond polarity results in an unequal electron distribution, resulting in areas of partial positive and partial negative charge Bond polarity results in an unequal electron distribution, resulting in areas of partial positive and partial negative charge Any molecule that has a center of positive charge and a center of negative charge in different points is said to have a dipole moment Any molecule that has a center of positive charge and a center of negative charge in different points is said to have a dipole moment If a molecule has more than one polar covalent bond, the areas of partial negative and positive charge for each bond will partially add to or cancel out each other If a molecule has more than one polar covalent bond, the areas of partial negative and positive charge for each bond will partially add to or cancel out each other The end result will be a molecule with one center of positive charge and one center of negative charge The end result will be a molecule with one center of positive charge and one center of negative charge The dipole moment effects the attractive forces between molecules and therefore the physical properties of the substance The dipole moment effects the attractive forces between molecules and therefore the physical properties of the substance