Sec 9.7 – Lewis Structures of Molecular Compounds and Polyatomic Ions (con’t) ▫ ▫
•
Writing Lewis Structures for Molecular Compounds Writing Lewis Structures for Polyatomic Ions
Sec 9.8 – Resonance, Formal Charge ▫ ▫
Resonance Formal Charge
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Formal Charge • Formal Charge ▫ The number of valence electrons in an isolated atom minus the number of electrons assigned to that atom in a Lewis structure.
• For any particular atom within the Lewis Structure: FC = # of valence e– – # of lone pair e– – ½ (# of bonding e–)
• A helpful way to remember how to find FC just by looking at the atom’s surroundings: FC = # of valence e– – dots around atom – lines around atom
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Formal Charge • In cases where more than one Lewis structure seems possible, formal charges can be used to determine which structure is most accurate. • Note that formal charges are hypothetical only, and are not real charges ▫ They are a form of ―electron book-keeping‖ that helps in the selection of a probable Lewis structure. ▫ It relates the number of electrons each atom ―owns‖ in the structure, compared to how many valence electrons it typically has
Resonance and Formal Charge
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Chem 1011 – Intersession 2011 Class #6
13-May-11
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Formal Charge • Formal Charge must be considered when determining whether or not a Lewis Structure is plausible • Things to look out for:
▫ The sum of the formal charges must equal: zero for a neutral molecule the charge for a polyatomic ion
▫ Where formal charges are required (ie, in polyatomic ions), they should be as small as possible. ▫ Negative formal charges usually appear on the most electronegative atoms; positive formal charges on the least electronegative atoms. ▫ Structures having formal charges of the same sign on neighbouring atoms are unlikely.
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Problems • Given the two potential structures of the nitronium ion (NO2+) below, identify which is the most plausible. O=N=O
+
O≡N—O
0 +1 0
+
+1 +1 –1
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Manipulating Formal Charges • What if the Lewis Structure we’ve built isn’t a suitable one? How can we manipulate the formal charges? • Example – Perchlorate (ClO4–) ▫ The Lewis Structure, after building the skeletal structure and adding in all lone pairs, looks like:
—
—
O
—
O — Cl — O O
Resonance and Formal Charge
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Chem 1011 – Intersession 2011 Class #6
13-May-11
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Manipulating Formal Charges • Assign all Formal Charges —
–1
—
O
+3
—
O — Cl — O –1
O
Chlorine
e–
–1
–1
Oxygen (Each)
e–
FC = v – lp FC = 7 – 4 – 0 FC = +3
– ½ bp
e–
FC = v e– – lp e– – ½ bp e– FC = 6 – 6 – 1 FC = –1
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Manipulating Formal Charges • Even though all FC add to –1, this is not a desirable structure • Keeping in mind two important points from before: ▫ Where formal charges are required (ie, in polyatomic ions), they should be as small as possible. ▫ Negative formal charges usually appear on the most electronegative atoms; positive formal charges on the least electronegative atoms.
• This means our end goal here is to have a single O atom carrying a –1 formal charge, and all others at zero
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Reducing Formal Charges • We can interpret from Formal Charge that: ▫ Atoms with a positive FC aren’t surrounded by enough electrons ▫ Atoms with a negative FC are surrounded by too many electrons
• If a negative FC resides next to a positive FC, a lone pair from the negative atom can be converted into a bonding pair to be shared with the positive atom, thus neutralizing each others formal charges
Resonance and Formal Charge
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Chem 1011 – Intersession 2011 Class #6
13-May-11
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Reducing Formal Charges —
–1
— —
+3
+2
O
–1
—
–1
—
+1
O— — Cl — O
O — Cl — O
–1
— —
— —
–1
O
— —
O
–1
— —
O
O — Cl — O
–1
—
—
O — Cl — O –1
—
O
—
O
–1
O
–1
O
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Expanded Octet
—
— —
O
— —
O— — Cl — O –1
O
• This is the correct Lewis Structure for ClO4–
▫ How is this possible?! Cl is surrounded by 14 e–!
• Chlorine has an expanded octet ▫ It can disobey the octet rule to satisfy FC because it has access to d-block orbitals to ―store‖ extra electrons beyond the regular 8 e– ▫ This ability is only seen in atoms from Phosphorous (P) onwards. Be mindful of this before forming double bonds!
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Resonance • In this example, we picked 3 oxygen atoms randomly to form double bonds with the central Cl atom; there was no real reasoning behind selecting the atoms we did • Any combination of 3 O atoms double bonded to the Cl, and one O atom singly bonded and carrying a –1 formal charge would be correct • In cases where alternative ways to draw the same correct structure exist, the molecule or ion is said to possess resonance
Resonance and Formal Charge
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Chem 1011 – Intersession 2011 Class #6
13-May-11
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Resonance Structures • We can ―rotate‖ the placement of single / double bonds to create resonance structures for the ClO4– ion • Equivalent Resonance Structures are indicated by a double-headed arrow ▫ Here, 4 resonance structures are possible
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Resonance Structures —
O— — Cl — O
O— — Cl — —O —
–1
O
O –1
—
— — O
—
— —
O— — Cl — —O
O
O — Cl — —O –1
— —
–1
—
O
—
— —
O
— —
— —
O
O
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Problem • Draw the proper Lewis Structure for the Carbonate Ion, CO32– . Assign Formal Charges to all atoms and include all relevant resonance structures. • Complete the same for the Phosphate Ion, PO 43–
Resonance and Formal Charge
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Chem 1011 – Intersession 2011 Class #6
13-May-11
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Resonance Hybrids • The true structure of a molecule or ions is a combination of the different contributing resonance structures. • That resulting structure is known as a resonance hybrid • Example: Ozone (O3) ▫ Has two possible resonance structures ▫ Experiments have shown that both bonds lengths are equal, measuring between typical single and double bond lengths –
: :
:
: :
: :
:
: :
–½
+
–½
: :
+
:
+
:O — O — O ↔ O — O — O:
: :
–
O—O—O
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Resonance Hybrids • Benzene (C6H6) ▫ Is a cyclic structure – the carbons bond together form a ring ▫ Experimental evidence has shown that each C—C bond length is 140pm A C—C double bond is 135pm A C—C single bond is 147pm
▫ The actual benzene structure exists as a resonance hybrid Simplified Representation of Benzene
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Problems •
Draw a plausible Lewis structure for the following. Then, draw all resonance structures as well as the resonance hybrid: (a) SO2 molecule (b) Nitrate (NO3-) ion
Resonance and Formal Charge
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Chem 1011 – Intersession 2011 Class #6
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Next Week! •
Sec 9.9 – Exceptions to the Octet Rule ▫ ▫ ▫
Odd-Electron Species (Suggested Reading Only) Incomplete Octets Expanded Octets
