BONDINGBONDING

Bonding GeneralitiesBonding Generalities

• Unlike Charges AttractUnlike Charges Attract• Electrons will Be in PairsElectrons will Be in Pairs• Only Valence Electrons are InvolvedOnly Valence Electrons are Involved

Two Main Types of BondsTwo Main Types of Bonds1.1. Ionic BondsIonic Bonds- Electrostatic Attraction - Electrostatic Attraction

Between a + and – Ion That Holds Between a + and – Ion That Holds Together Ionic CompoundsTogether Ionic Compounds

2.2. Covalent BondCovalent Bond – Sharing of an – Sharing of an Valence Electron Pair Between two Valence Electron Pair Between two Nonmetal Atoms to Form a MoleculeNonmetal Atoms to Form a Molecule

In an IONIC bond,electrons are lost or gained,resulting in the formation of IONSin ionic compounds.

FK

FK+ _

FK+ _

The compound potassium fluorideconsists of potassium (K+) ions

and fluoride (F-) ions

In covalent bonding,atoms still want to achievea noble gas configuration(the octet rule).

Octet RuleOctet Rule• When Nonmetal Atoms Share Valence When Nonmetal Atoms Share Valence

Electrons to Form a Covalent Bond-Electrons to Form a Covalent Bond-– It Will Be to Have the Same Number of It Will Be to Have the Same Number of

Valence Electrons as the Closest Noble Valence Electrons as the Closest Noble GasGas

H H 2 electrons 2 electronsEverything Else Everything Else 8 Electrons 8 Electrons

In covalent bonding,atoms still want to achievea noble gas configuration(the octet rule).

But rather than losing or gainingelectrons,atoms now share an electron pair.

In covalent bonding,atoms still want to achievea noble gas configuration(the octet rule).

But rather than losing or gainingelectrons,atoms now share an electron pair.

The shared electron pairis called a bonding pair

Cl2

Chlorineforms

acovalent

bondwithitself

ClClHowwilltwochlorineatomsreact?

ClClEach chlorine atom wants to gain one electron to achieve an octet

ClClNeither atom will give up an electron –chlorine is highly electronegative.What’s the solution – what can theydo to achieve an octet?

Lewis Dot Symbols

A covalent bond is a chemical bond in which two or more electrons are shared by two nonmetal atoms.

Why should two atoms share electrons?

F F+

7e- 7e-

F F

8e- 8e-

F F

F F

Lewis structure of F2

lone pairslone pairs

lone pairslone pairs

single covalent bond

single covalent bond

9.4

1. Draw skeletal structure of compound showing what atoms are bonded to each other.

2. Count total number of valence e-. Add 1 for each negative charge. Subtract 1 for each positive charge.

3. Connect surrounding atoms to central atom with single bonds. Add remaining electrons (2 at a time) such that surrounding atoms follow octet rule (H -2 electrons). Stop adding electrons once the number exceeds value calculated in step 2.

Writing Lewis Structures for Molecular Compounds

9.6

Writing Lewis StructuresWriting Lewis Structures(cont)(cont)

4.4. Make sure that every atom satisfies octet rule Make sure that every atom satisfies octet rule and the total number of valence electrons in and the total number of valence electrons in Lewis Structure is correct.Lewis Structure is correct.

5.5. If a surrounding atom does If a surrounding atom does notnot have an octet; have an octet; move lone pair to bonding position from an move lone pair to bonding position from an adjacent atom to form a double or triple bond.adjacent atom to form a double or triple bond.

Double bond – two atoms share two pairs of electrons

O C O or O C O

8e- 8e-8e-double bonds double bonds

Triple bond – two atoms share three pairs of electrons

N N8e-8e-

N N

triple bondtriple bond

or

Write the Lewis structure of nitrogen trifluoride (NF3).

Step 1 – N is less electronegative than F, put N in center

F N F

F

Step 2 – Count valence electrons N - 5 and F - 7

5 + (3 x 7) = 26 valence electronsStep 3 – Draw single bonds between N and F atoms and complete octets on N and F atoms.Step 4 - Check, are # of e- in structure equal to number of valence e- ?

3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons

9.6

Exception to Generality of Exception to Generality of Bonds and Lone Pairs for AtomBonds and Lone Pairs for Atom1.1. Resonance StructuresResonance Structures2.2. Polyatomic IonsPolyatomic Ions3.3. Exceptions to the Octet RuleExceptions to the Octet Rule

• Incomplete OctetIncomplete Octet• Expanded OctetExpanded Octet• Odd # ElectronsOdd # Electrons

O C O

O

- -O C O

O

-

-

OCO

O

-

- 9.8

What are the resonance structures of the carbonate (CO3

2-) ion?

Coordinate Covalent BondsCoordinate Covalent Bonds• In a coordinate covalent bonds, both In a coordinate covalent bonds, both

electrons in the bond come from one electrons in the bond come from one of the atoms the bond is between.of the atoms the bond is between.

• Usually, coordinate covalent bond is Usually, coordinate covalent bond is betweenbetween– An atom with incomplete octet andAn atom with incomplete octet and– An atom with a lone pairAn atom with a lone pair

Exceptions to the Octet Rule(on Central Atom)

The Expanded Octet (central atom with principal quantum number n≥3

SF6

S – 6e-

6F – 42e-

48e-S

F

F

F

FF

F

6 single bonds (6x2) = 1218 lone pairs (18x2) = 36

Total = 48

9.9

Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond.

Electronegativity - relative, F is highest

9.5

9.5

Lewis Dot Structure for NO Lewis Dot Structure for NO (cont)(cont)• O is more electronegative than NO is more electronegative than N• The atom with a lower The atom with a lower

electronegativity has a lower electronegativity has a lower attraction for the electrons in the attraction for the electrons in the bond and will be the atom with 7 bond and will be the atom with 7 rather than 8 electronsrather than 8 electrons

N OFree radical – unpaired electron

Bond Bond TypeType

Bond Bond LengthLength(pm)(pm)

CC--CC 154154CCCC 133133CCCC 120120CC--NN 143143CCNN 138138CCNN 116116

Lengths of Covalent Bonds

Bond Lengths

Triple bond < Double Bond < Single Bond 9.4

In an IONIC bond,electrons are lost or gained,resulting in the formation of IONSin ionic compounds.

FK

FK+ _

FK+ _

The compound potassium fluorideconsists of potassium (K+) ions

and fluoride (F-) ions

In covalent bonding,atoms still want to achievea noble gas configuration(the octet rule).

Resonance StructuresResonance Structures1.1. RS are Imaginary; the Real RS are Imaginary; the Real

Structure is Intermediate All the RS.Structure is Intermediate All the RS.2.2. Only Electrons Move Between RS; Only Electrons Move Between RS;

Atoms Positions Never ChangeAtoms Positions Never Change3.3. At Least One Atom in RS Will Have a At Least One Atom in RS Will Have a

Non-zero Formal Charge and Won’t Non-zero Formal Charge and Won’t Follow Generality About # Bonds Follow Generality About # Bonds and Lone Pairs.and Lone Pairs.

9.7

An atom’s formal charge is the difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure.

formal charge on an atom in a Lewis structure

=12

total number of bonding electrons( )

total number of valence electrons in the free atom

-total number of nonbonding electrons

-

The sum of the formal charges of the atoms in a molecule or ion must equal the charge on the molecule or ion.

Write the Lewis structure of the carbonate ion (CO32-).

Step 1 – C is less electronegative than O, put C in center

O C O

O

Step 2 – Count valence electrons C - 4 (2s22p2) and O - 6 (2s22p4) -2 charge – 2e-

4 + (3 x 6) + 2 = 24 valence electrons

Step 3 – Draw single bonds between C and O atoms and complete octet on C and O atoms.Step 4 - Check, are # of e- in structure equal to number of valence e- ?

3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons

9.6

Step 5 - Too many electrons, form double bond and re-check # of e-

2 single bonds (2x2) = 41 double bond = 4

8 lone pairs (8x2) = 16Total = 24

Check Lewis Dot StructuresCheck Lewis Dot Structures1.1. Correct Number of Valence Correct Number of Valence

Electrons in StructureElectrons in Structure2.2. Every Atom Follows the Octet RuleEvery Atom Follows the Octet Rule3.3. Every Atom Follows Generality Every Atom Follows Generality

About # Bonds and Lone Pairs About # Bonds and Lone Pairs UNLESS it Follows One of the UNLESS it Follows One of the ExceptionsExceptions

Exceptions to the Octet Rule(on Central Atom)

The Incomplete Octet

H HBeBe – 2e-

2H – 2x1e-

4e-

BeH2

BH3

B – 3e-

3H – 3x1e-

6e-

9.9

Incomplete Octet !4 not 8 e-

B HH

H

Incomplete Octet!6 not 8 e-

Exceptions to the Octet Rule

Odd-Electron Molecules

N – 5e-

O – 6e-

11e-

NO

* When there is an odd # of valence electrons;One atom will have 7 rather than 8 electrons

* Use the electronegativity of atoms to determine which atom has 7 rather than 8 electrons