1 Molecular Geometry and Polarity Part B: Lewis Structure Dr. Chin Chu River Dell Regional High...

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Molecular Geometry and Polarity

Part B: Lewis Structure

Dr. Chin Chu

River Dell Regional High School

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Writing Lewis Structures for Ionic Compounds

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Writing Lewis Structures for Molecules and Polyatomic

Ions

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Depiction of Covalent Compounds and Polyatomic Ions

Covalent compounds and polyatomic ions can be described by•Molecular formula (molecular compounds)

•Structural formula (depicts the arrangement of the atoms in space)

•Lewis structure: depicts the arrangement of the electrons around the atoms in a molecule

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Writing Lewis Structures

• The Lewis Structures will be written for molecules that obey the Octet Rule

• AND

• Exceptions to the Octet Rule– Electron deficiency– Expanded Octet Rule– Odd electron molecules

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Writing Lewis Structure1. Count the number of valence electrons of each

atom. Add the totals for all atoms and make adjustments for charge (-1 charge gives one more electron, +2 charge two less electrons, etc.)

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1. Count the number of valence electrons of each atom. Add the totals for all atoms and make adjustments for charge (-1 charge gives one more electron, +2 charge two less electrons, etc.)

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Writing Lewis Structure2. Arrange atoms in the diagram as follows:

a) Choose the least electronegative atom as the central atom. This is usually the atom closest to the left side of the periodic table (the largest atom)

b) Generally arrange other atoms around the central atom (CA).

i. Hydrogen can only form bond and hence never be the central atom.

ii. Hydrogen in oxyanions and oxyacids is usually bonded to an oxygen.

iii. Carbon has four single bonds (except in CO and CN-1 where it has three bonds and an unshared valence electron pair.

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2. Arrange atoms in the diagram as follows:

a) Choose the least electronegative atom as the central atom. This is usually the atom closest to the left side of the periodic table (the largest atom)

b) Generally arrange other atoms around the central atom (CA).

i. Hydrogen can only form bond and hence never be the central atom.

ii. Hydrogen in oxyanions and oxyacids is usually bonded to an oxygen.

iii. Carbon has four single bonds (except in CO and CN-1 where it has three bonds and an unshared valence electron pair.

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Writing Lewis Structure3. Place the valence electrons around the atom as

follows:1st: Connect outer atoms to the central atom with a

single covalent bond. Each single bond takes two valence electrons.

2nd: Calculate the number of remaining valence electrons.

3rd: Fill the octets of outer atoms.

4th: place any remaining electrons on the central atom.

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3. Place the valence electrons around the atom as follows:

1st: Connect outer atoms to the central atom with a single covalent bond. Each single bond takes two valence electrons.




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3. Place the valence electrons around the atom as follows:

1st: Connect outer atoms to the central atom with a single covalent bond. Each single bond takes two valence electrons.




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Writing Lewis Structure4. Check the central atom to see whether the octet rule

has been satisfied. If not, make multiple bonds (double or triple) to the central atom until the octet rule is satisfied. This is done by moving an unshared electron pair on an outer atom to the central atom. Remember, only selected few elements are capable of forming multiple bonds, C, N, O, P, S and rarely Cl.

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4. Check the central atom to see whether the octet rule has been satisfied. If not, make multiple bonds (double or triple) to the central atom until the octet rule is satisfied. This is done by moving an unshared electron pair on an outer atom to the central atom. Remember, only selected few elements are capable of forming multiple bonds, C, N, O, P, S and rarely Cl.

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Writing Lewis StructureBut there are exceptions to the octet rule: Less than an octet (“incomplete octet”, e.g.,

BeCl2, BF3) If there are not enough electrons to follow the octet

rule, then the least electron negative atom is left short of electrons.

http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Lewis_Theory_of_Bonding/Violations_of_the_Octet_Rule



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Writing Lewis StructureBut there are exceptions to the octet rule: More than an octet (“expanded octet” e.g.,

PCl5, SF6, XeOF4) If there are too many electrons to follow the octet

rule, then the extra electrons are placed on the central atom.

SF6How come the octet rule

being violated? 3rd period elements have

d- orbitals available to accommodate electrons beyond the octet.

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Writing Lewis StructureBut there are exceptions to the octet rule: Odd numbers of valence electrons (e.g., NO,

NO2)

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Writing Lewis Structure5. For ions, include the charge. This is done by

placing brackets around the structure and writing the charge outside the brackets as a right-hand superscript.

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Writing Lewis StructureThe correct Lewis structure has been obtained

when: Atoms are correctly linked. All valence electrons (including charges) have

been used. Atoms have an octet (except hydrogen). There

are some exceptions to the octet rule. Formal charges on all atoms are minimized. A charge other than zero is shown.

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Resonance• When more than one dot diagram with the same

connections are possible.• Use double arrows to indicate it is the “average” of

the structures.

• NO2-

• Which one is it?• Does it go back and forth.• It is a mixture of both, like a mule.• NO3

- CO3-2 SO3

-2

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• When more than one dot diagram with the same connections are possible.

• Use double arrows to indicate it is the “average” of the structures.

1 Molecular Geometry and Polarity Part B: Lewis Structure Dr. Chin Chu River Dell Regional High...

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