NIS – CHEMISTRY

Lecture 41Molecular Structures

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Molecular StructuresMolecular formula for compounds do not show how atoms are bonded together in a molecule.In order to show the structure of a molecule we can use different representations:

Space-filling molecular modelBall-and-stick molecular modelStructural formulaLewis structure

Check out Figure 8.13Structural formula and Lewis structure are very useful to show how atoms are bonded.

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Lewis StructuresSteps to draw the Lewis structure of molecules:1- Predict the location of certain atoms.2- Determine the total number of electrons available for bonding.3- Determine the number of electron pairs available for bonding.4- Place the bonding pairs.5- Determine the number of remaining electron pairs.6- Determine whether each atom satisfies the octet rule.

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Lewis StructuresExample: Ammonia is a raw material used in the manufacture of many materials, including fertilizers, cleaning products, and explosives. Draw the Lewis structure for ammonia.

Example: A nitrogen trifluoride molecule contains numerous lone pairs. Draw its Lewis structure.

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Lewis StructuresExample: Carbon dioxide is a product of all cellular respiration. Draw the Lewis structure of carbon dioxide.

Example: Draw the Lewis structure of ethylene, C2H4.

Example: A molecule of carbon disulfide contains both lone pairs and multi-covalent bonds. Draw its Lewis structure.

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Lewis Structures for Polyatomic IonsAlthough the unit acts as an ion, the atoms with polyatomic ion are covalently bonded.Calculate the total number of valence electrons Subtract the charge of the ion in order to find the total number of electrons available for bonding.

Example: PO4-3 5 valence electrons from P4*6 = 24 valence electrons from O43 electrons from the charge Total 32 electrons available for bonding.

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Lewis Structures for Polyatomic IonsExample: Draw the Lewis structure for NH4+.

Example: Draw the Lewis structure for ClO4-.

Example: Draw the Lewis structure for HCO3-.

NIS – CHEMISTRY

Lecture 42Resonance Structures

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Resonance StructuresUsing the same sequence of atoms, it is possible to have more than one correct Lewis structure when a molecule or polyatomic ion has both double bond and a single bond.Example: NO3- Check out Figure 8.14Resosnance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion.

Example: NO2-, SO3-2, CO3-2, O3Each molecule or ion that undergoes resonance behaves as if it has only one structure.Experimentally measured bond lengths show that the bonds are identical to each other.They are shorter than single bonds but longer than double bonds.

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Resonance StructuresExample: Draw the Lewis resonance structure for NO2-.

Example: Draw the Lewis resonance structure for SO2.

Example: Draw the Lewis resonance structure for O3.

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Exceptions to the Octet RuleSome molecules do not obey the octet rule. There are several reasons for these exceptions:

Odd number of valence electronsSuboctet and coordinate covalent bondsExpanded octets

Odd number of valence electrons:Example: NO2, ClO2 and NO

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Exceptions to the Octet RuleSuboctet and coordinate covalent bonds:Suboctet Stable configurations with fewer than 8 electrons present around an atom.Example: BF3A coordinate covalent bond forms when one atom donates both of the electrons to be shared with an atom or ion that needs two electrons.Example: BF3 + NH3Atoms or ions with lone pairs often form coordinate covalent bonds with atoms or ions that need two more electrons.

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Exceptions to the Octet RuleExpanded Octets:These are molecules with central atoms that contain more than 8 valence electrons.This electron arrangement is referred to as an expanded octet.An expanded octet can be explained by considering the d orbital that occurs in the energy levels of elements in period three or higher.Example: PCl5, SF6

Example: Draw the Lewis structure for Xenon tetrafluoride.Example: Draw the Lewis structure for ClF3.