Molecular Molecular CompoundsCompounds

Unit 7Unit 7

Covalent BondsCovalent Bonds Sharing pairs of electronsSharing pairs of electrons Covalent bonds are the intra-molecular Covalent bonds are the intra-molecular

attraction resulting from the sharing of attraction resulting from the sharing of electrons between atoms.electrons between atoms.

They result in ‘localized overlaps’ of They result in ‘localized overlaps’ of orbitals of different atoms.orbitals of different atoms.

They also are the result of the attraction of They also are the result of the attraction of electrons for the nucleus of other atoms.electrons for the nucleus of other atoms.

Typical of molecular substances.Typical of molecular substances.

Covalent Bonds Cont.Covalent Bonds Cont. Atoms bond together to form moleculesAtoms bond together to form molecules

– molecules are electrically neutral groups of molecules are electrically neutral groups of atoms joined together by covalent bondsatoms joined together by covalent bonds

– strong attractionstrong attraction Molecules attracted to each other Molecules attracted to each other

weakly form molecular solidsweakly form molecular solids How can we distinguish between an How can we distinguish between an

ionic and a molecular compound?ionic and a molecular compound?– Ionic = contains a metalIonic = contains a metal– Molecular = composed of 2 or more Molecular = composed of 2 or more

nonmetalsnonmetals

Properties of Molecular Properties of Molecular CompoundsCompounds

Strong covalent bonds hold the Strong covalent bonds hold the atoms together within a molecule.atoms together within a molecule.

The intermolecular forces that hold The intermolecular forces that hold one molecule to another are much one molecule to another are much weaker.weaker.

Molecular Compounds can exist in all Molecular Compounds can exist in all 3 states:3 states:– Solids – sugar, ice, aspirinSolids – sugar, ice, aspirin– Liquids – water, alcoholsLiquids – water, alcohols

– Gases – OGases – O22, CO, CO22, and N, and N22O (laughing gas)O (laughing gas)

Properties Cont.Properties Cont. Properties vary depending on the Properties vary depending on the

strength of the intermolecular forces.strength of the intermolecular forces. Usually have a lower MP (than ionic, Usually have a lower MP (than ionic,

metallic)metallic) Generally softGenerally soft Non-conductors (in any state & Non-conductors (in any state &

aqueous)aqueous) Very important to organic chemistry Very important to organic chemistry

and pharmaceutics.and pharmaceutics.

ElectronegativityElectronegativity Measure of the ability of an atom to Measure of the ability of an atom to

attract shared electronsattract shared electrons– Larger electronegativity means atom attracts Larger electronegativity means atom attracts

more stronglymore strongly– Values 0.7 to 4.0Values 0.7 to 4.0

Increases across period (left to right) on Increases across period (left to right) on Periodic TablePeriodic Table

Decreases down group (top to bottom) on Decreases down group (top to bottom) on Periodic TablePeriodic Table

Larger difference in electronegativities Larger difference in electronegativities means more polar bondmeans more polar bond– negative end toward more electronegative atomnegative end toward more electronegative atom

Bond PolarityBond Polarity Covalent bonding between unlike atoms results Covalent bonding between unlike atoms results

in unequal sharing of the electronsin unequal sharing of the electrons– One end of the bond has larger electron One end of the bond has larger electron

density than the otherdensity than the other– Polar covalent – unequal sharingPolar covalent – unequal sharing– Nonpolar covalent – equal sharingNonpolar covalent – equal sharing

The result is The result is bond polaritybond polarity– The end with the larger electron density gets The end with the larger electron density gets

a partial negative chargea partial negative charge– The end that is electron deficient gets a The end that is electron deficient gets a

partial positive chargepartial positive charge

H F••

Dipole MomentDipole Moment Bond polarity results in an unequal electron Bond polarity results in an unequal electron

distribution, resulting in areas of partial positive and distribution, resulting in areas of partial positive and partial negative chargepartial negative charge

Any molecule that has a center of positive charge Any molecule that has a center of positive charge and a center of negative charge in different points is and a center of negative charge in different points is said to have a said to have a dipole momentdipole moment

If a molecule has more than one polar covalent bond, If a molecule has more than one polar covalent bond, the areas of partial negative and positive charge for the areas of partial negative and positive charge for each bond will partially add to or cancel out each each bond will partially add to or cancel out each otherother

The end result will be a molecule with one center of The end result will be a molecule with one center of positive charge and one center of negative chargepositive charge and one center of negative charge

The dipole moment effects the attractive forces The dipole moment effects the attractive forces between molecules and therefore the physical between molecules and therefore the physical properties of the substanceproperties of the substance