Chemical bonding i the covalent bond
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Chemical Bonding I: The Covalent Bond. Chapter 9. Group. e - configuration. # of valence e -. ns 1. 1. 1A. 2A. ns 2. 2. 3A. ns 2 np 1. 3. 4A. ns 2 np 2. 4. 5A. ns 2 np 3. 5. 6A. ns 2 np 4. 6. 7A. ns 2 np 5. 7. Valence electrons are the outer shell electrons of an

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Chemical Bonding I: The Covalent Bond

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Chemical bonding i the covalent bond

Chemical Bonding I:The Covalent Bond

Chapter 9


Chemical bonding i the covalent bond

Group

e- configuration

# of valence e-

ns1

1

1A

2A

ns2

2

3A

ns2np1

3

4A

ns2np2

4

5A

ns2np3

5

6A

ns2np4

6

7A

ns2np5

7

Valence electrons are the outer shell electrons of an

atom. The valence electrons are the electrons that

particpate in chemical bonding.


Chemical bonding i the covalent bond

Lewis Dot Symbols for the Representative Elements &

Noble Gases


Chemical bonding i the covalent bond

-

-

-

-

+

Li+

Li

Li

Li+ + e-

e- +

Li+

Li+ +

F

F

F

F

F

F

The Ionic Bond

Ionic bond:the electrostatic force that holds ions together in an ionic compound.

1s22s1

1s22s22p5

1s2

1s22s22p6

[He]

[Ne]

LiF

形成”離子對”


Chemical bonding i the covalent bond

Ionic bond: Other example

  • LiO2

  • Mg3N2


Chemical bonding i the covalent bond

E = k

Q+Q-

r

Compound

Lattice Energy

(kJ/mol)

MgF2

2957

Q: +2,-1

MgO

3938

Q: +2,-2

LiF

1036

LiCl

853

Electrostatic (Lattice) Energy

Lattice energy (U) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions.

E is the potential energy

形成離子固體後, 其穩定性如何?

Q+ is the charge on the cation

Q- is the charge on the anion

r is the distance between the ions

Lattice energy increases as Q increases and/or

as r decreases.

r F- < r Cl-


Chemical bonding i the covalent bond

Born-Haber Cycle for Determining Lattice Energy

Lattice energy: + 1017 kJ

°

°

°

°

°

°

DHoverall = DH1 + DH2 + DH3 + DH4 + DH5


Chemical bonding i the covalent bond

Chemical formula v.s lattice energy

MgCl2(1st ionic energy:738+

2nd ionic energy: 1450

= 2188KJ/mol)

NaCl (2nd inoic energy: 4560KJ/mol)

>> Lattice Energy

so the NaCl2 is not formed!

2+ leading to large attractive force than 1+


Chemical bonding i the covalent bond

+

8e-

8e-

7e-

7e-

F

F

F

F

F

F

F

F

lonepairs

lonepairs

single covalent bond

single covalent bond

lonepairs

lonepairs

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

Covalent compound.

Why should two atoms share electrons?

Lewis structure of F2


Chemical bonding i the covalent bond

Lewis structure of water

single covalent bonds

+

+

H

H

H

or

H

H

H

O

O

O

2e-

8e-

2e-

Octet rule: suitable for 2nd period(2s+ 2p[Ne])

Double bond – two atoms share two pairs of electrons

or

O

O

O

O

C

C

Ex: C2H4

8e-

8e-

8e-

double bonds

double bonds

Triple bond – two atoms share three pairs of electrons

N

or

N

N

N

Ex: C2H2

8e-

8e-

triple bond

triple bond


Chemical bonding i the covalent bond

Lengths of Covalent Bonds

Bond Lengths

Triple bond < Double Bond < Single Bond


Chemical bonding i the covalent bond

Compare the difference between ionic compound

and covalent compound

  • Covalent compound: gas, liquid or Low melting point solid

  • Low solubility in water(can’t conductive)

  • Example: NaCl(solid) and CCl4 (liquid)


Chemical bonding i the covalent bond

F

H

F

H

Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms

electron rich

region

electron poor

region

e- poor

e- rich

d+

d-


Chemical bonding i the covalent bond

X (g) + e- X-(g)

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

Electron Affinity - measurable, Cl is highest

The attraction of single atom to its electrons

Electronegativity - relative, F is highest

The attraction of multi atoms to its co-bonded electrons


Chemical bonding i the covalent bond

The Electronegativities of Common Elements

當形成鍵結的2原子之電負度差異很大時,易形成離子鍵。

反之, 當電負度差異小則易形成極性共價鍵。而電負度相近時

,則形成非極性共價鍵。


Chemical bonding i the covalent bond

Variation of Electronegativity with Atomic Number

過渡金屬有例外~


Chemical bonding i the covalent bond

Increasing difference in electronegativity

Covalent

Polar Covalent

Ionic

partial transfer of e-

share e-

transfer e-

Classification of bonds by difference in electronegativity

Difference

Bond Type

0

Covalent

 2

Ionic

0 < and <2

Polar Covalent


Chemical bonding i the covalent bond

Classify the following bonds as ionic, polar covalent, or covalent: The bond in CsCl; the bond in H2S; and the NN bond in H2NNH2.

Cs – 0.7

Cl – 3.0

3.0 – 0.7 = 2.3

Ionic

H – 2.1

S – 2.5

2.5 – 2.1 = 0.4

Polar Covalent

N – 3.0

N – 3.0

3.0 – 3.0 = 0

Covalent


Chemical bonding i the covalent bond

Oxidation number(氧化數) and Electronegativity

Actually, electronegativity is the way to define the oxidation number of a compound: the numbers of charge been transferred

Ex: NH3 (N 3- / H+ )


Chemical bonding i the covalent bond

Writing Lewis Structures

  • Draw skeletal structure of compound showing what atoms are bonded to each other. Put least electronegative element in the center.

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

  • Complete an octet for all atoms except hydrogen

  • If structure contains too many electrons, form double and triple bonds on central atom as needed.


Chemical bonding i the covalent bond

F

N

F

F

Write the Lewis structure of nitrogen trifluoride (NF3).

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

Step 2 – Count valence electrons N - 5 (2s22p3) and F - 7 (2s22p5)

5 + (3 x 7) = 26 valence electrons

Step 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


Chemical bonding i the covalent bond

O

C

O

2 single bonds (2x2) = 4

1 double bond = 4

8 lone pairs (8x2) = 16

O

Total = 24

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

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

  • 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

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


Chemical bonding i the covalent bond

H

H

C

O

H

C

O

H

formal charge on an atom in a Lewis structure

total number of valence electrons in the free atom

total number of nonbonding electrons

(

total number of bonding electrons

)

1

-

-

=

2

Two possible skeletal structures of formaldehyde (CH2O)

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.

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

對中性分子,所有原子的形式電荷總合為0,對於離子,形式電荷為離子的帶電荷


Chemical bonding i the covalent bond

H

C

O

H

C – 4 e-

O – 6 e-

2H – 2x1 e-

12 e-

formal charge on an atom in a Lewis structure

total number of valence electrons in the free atom

total number of nonbonding electrons

(

total number of bonding electrons

)

1

-

-

=

2 single bonds (2x2) = 4

2

1 double bond = 4

2 lone pairs (2x2) = 4

Total = 12

-1

+1

formal charge on C

= 4 -

2-

½ x 6 = -1

formal charge on O

= 6 -

2-

½ x 6 = +1


Chemical bonding i the covalent bond

C – 4 e-

H

O – 6 e-

C

O

H

2H – 2x1 e-

12 e-

formal charge on an atom in a Lewis structure

total number of valence electrons in the free atom

total number of nonbonding electrons

(

total number of bonding electrons

)

1

-

-

=

2 single bonds (2x2) = 4

2

1 double bond = 4

2 lone pairs (2x2) = 4

Total = 12

0

0

formal charge on C

= 4 -

0-

½ x 8 = 0

formal charge on O

= 6 -

4-

½ x 4 = 0


Chemical bonding i the covalent bond

H

C

O

H

H

C

O

H

-1

+1

0

0

Formal Charge and Lewis Structures

  • For neutral molecules, a Lewis structure in which there are no formal charges is preferable to one in which formal charges are present.

  • Lewis structures with large formal charges are less plausible than those with small formal charges.

  • Among Lewis structures having similar distributions of formal charges, the most plausible structure is the one in which negative formal charges are placed on the more electronegative atoms.

Which is the most likely Lewis structure for CH2O?


Chemical bonding i the covalent bond

-

-

+

+

O

O

O

O

O

O

O

O

O

C

C

C

O

O

O

-

-

-

-

O

O

O

-

-

A resonance structure is one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure.

What are the resonance structures of the

carbonate (CO32-) ion?


Chemical bonding i the covalent bond

The Benzene (C6H6)structure

The measured bonding length is 140 pm, less than C-C(154 nm)

while longer than C=C(133 pm)

分子的表現行為可以看成是共振結構的加成結果,而非

兩種結構的來回切換。


Chemical bonding i the covalent bond

Be – 2e-

2H – 2x1e-

H

Be

H

4e-

B – 3e-

3 single bonds (3x2) = 6

3F – 3x7e-

F

B

F

24e-

Total = 24

9 lone pairs (9x2) = 18

F

Exceptions to the Octet Rule

The Incomplete Octet

BeH2

BF3


Chemical bonding i the covalent bond

Coordinate covalent bond(配位共價鍵):

價電子完全由一原子提供來配對

NH3+BF3H3N-BF3


Chemical bonding i the covalent bond

N – 5e-

S – 6e-

N

O

6F – 42e-

O – 6e-

11e-

48e-

F

6 single bonds (6x2) = 12

F

F

Total = 48

S

18 lone pairs (18x2) = 36

F

F

F

Exceptions to the Octet Rule

(奇墊子分子: 自由基radical)

Odd-Electron Molecules

NO

EX:NO2

The Expanded Octet (central atom with principal quantum number n > 2)

SF6

EX: SCl2 still satisfy the Octet rule


Chemical bonding i the covalent bond

Expanded Octet

XeF4: leave 4 lone pairs to the Xe atom


Margin art 9 14

2nd 週期後的元素因填入3d軌域,價電子會超過八個

Margin Art 9.14

Octet

Expanded Octet


Chemical bonding i the covalent bond

DH° = 436.4 kJ

H2 (g)

H (g)

+

H (g)

DH° = 242.7 kJ

Cl2 (g)

Cl (g)

+

Cl (g)

DH° = 431.9 kJ

HCl (g)

H (g)

+

Cl (g)

DH° = 498.7 kJ

O2 (g)

O (g)

+

O (g)

O

DH° = 941.4 kJ

N2 (g)

N (g)

+

N (g)

O

N

N

The enthalpy change required to break a particular bond in one mole of gaseous molecules is the bond enthalpy.

Bond Enthalpy

Bond Enthalpies

Single bond < Double bond < Triple bond


Chemical bonding i the covalent bond

DH° = 502 kJ

H2O (g)

H (g)

+

OH (g)

DH° = 427 kJ

OH (g)

H (g)

+

O (g)

Average OH bond enthalpy =

502 + 427

2

Average bond enthapy in polyatomic molecules

= 464 kJ


Chemical bonding i the covalent bond

Bond Enthalpies (BE) and Enthalpy changes in reactions

Imagine reaction proceeding by breaking all bonds in the reactants and then using the gaseous atoms to form all the bonds in the products.

DH° = total energy input – total energy released

= SBE(reactants) – SBE(products)

exothermic

endothermic


Chemical bonding i the covalent bond

Type of bonds broken

Number of bonds broken

Bond enthalpy (kJ/mol)

Enthalpy change (kJ/mol)

1

436.4

436.4

1

156.9

156.9

Type of bonds formed

Number of bonds formed

Bond enthalpy (kJ/mol)

Enthalpy change (kJ/mol)

H

H

F

H

F

F

2

568.2

1136.4

Use bond enthalpies to calculate the enthalpy change for:

H2 (g) + F2 (g) 2HF (g)

DH° = SBE(reactants) – SBE(products)

DH° = 436.4 + 156.9 – 2 x 568.2 = -543.1 kJ/mol


Chemical bonding i the covalent bond

Ch9 HW

9.25 Use the Born-Haber cycle outlined in Section 9.3 for LiF to calculate the lattice energy of NaCl. [The heat of sublimation of Na is 108 kJ/mol and (NaCl) = −411 kJ/mol. Energy needed to dissociate mole of Cl2 into Cl atoms = 121.4 kJ].

9.17 Use Lewis dot symbols to show the transfer of electrons between the following atoms to form cations and anions: (a) Na and F, (b) K and S, (c) Ba and O, (d) Al and N.

9.29 How many lone pairs are on the underlined atoms in these compounds? HBr, H2S, CH4.

9.49 Write Lewis structures for these species, including all resonance forms, and show formal charges: (a) (b) Relative positions of the atoms are as follows:


Chemical bonding i the covalent bond

2H2 (g) + O2 (g) 2H2O (g)

Use bond enthalpies to calculate the enthalpy change for:


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