Chapter 8 “Covalent Bonding”. Covalent Bonds covalent combination of prefix co- (Latin -...

Chapter 8“Covalent Bonding”

Covalent Bonds

•covalent combination of prefix co- (Latin - “together”), valere, - “to be strong”

•2 e-’s shared have strength to hold 2 atoms together

• particle called a particle called a ““moleculemolecule””

MoleculesMoleculesSome elements in nature are molecules:neutral group of atoms covalently

bonded Ex. - air contains O molecules, 2 O atoms Ex. - air contains O molecules, 2 O atoms

joined covalentlyjoined covalently Called “Called “diatomicdiatomic molecule molecule” (O” (O22))

How does H2 form?

•The nuclei repel each other, (both have + charge)

++

(diatomic hydrogen molecule)

+ +

How does H2 form?•nuclei attraction to e-’s stronger than repulsion of nuclei

•e-’s shared•covalent bond

•Only NONMETALS!

++

Covalent bonds•Nonmetals hold valence e-’s• don’t give away e-’s

• still want NGC• share valence e-’s with each other =

covalent bonding

•both atoms count e-’s for NGC

Covalent bonding w/ Fluorine atoms

Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons… …both end with full orbitals

F F8 Valence electrons

Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons… …both end with full orbitals

F F8 Valence electrons

single covalent bond between 2 H atoms

Molecular Compounds• Compounds bonded covalently called

molecular compounds• Molecular compounds have

• lower melting and boiling points • Weaker bond than ionic• gases or liquids at room temperature• a molecular formula:

• Shows how many atoms of each element a molecule contains

Reminder from Ch. 7

• No “molecule” of sodium chloride• Ionic cmpds exist as collection of + & -

charged ions arranged in repeating 3D patterns.

Molecular Compounds• The formula for water is written as H2O

• The subscript “2” behind hydrogen means 2 atoms of hydrogen

• subscript 1 omitted• Molecular formulas do not tell any

information about structure (arrangement of various atoms).

- Page 215

These are some of the different ways to represent ammonia:

1. The molecular formula shows how many atoms of each element are present

2. The structural formula ALSO shows the arrangement of these atoms!

3. The ball and stick model is BEST, because it shows 3D arrangement.

Section 8.2The Nature of Covalent Bonding

A Single Covalent Bond is...• sharing 2 valence e-’s•Only nonmetals and hydrogen.•Different from ionic bond b/c they

actually form molecules.•Two specific atoms joined• In an ionic solid, you can’t tell which

atom e-’s moved from or to

How to show the formation…• It’s like a jigsaw puzzle.• You put the pieces together to end up with the right formula.• Carbon is a special example - can it really share 4 electrons:

1s22s22p2? 2p

1s 2s

C

•Yes, due to electron promotion!

How to show the formation…• It’s like a jigsaw puzzle.• You put the pieces together to end up with the right formula.• Carbon is a special example - can it really share 4 electrons:

1s22s22p2? 2p

1s 2s

Water

Each hydrogen has 1 valence electron- Each hydrogen wants 1 more

The oxygen has 6 valence electrons- The oxygen wants 2 more

They share to make each other complete

H

O

Another example: water is formed with covalent bonds, by using an electron dot diagram

Water• Put the pieces together• The first hydrogen is happy• The oxygen still needs one more

H O

Water• So, a second hydrogen attaches• Every atom has full energy levels

H OH

Note the two “unshared” pairs of electrons

Examples:

1. Conceptual Problem 8.1 on page 220

We’ll do #7 & 8

Double and Triple Covalent Bonds

• Sometimes atoms share more than one pair of valence e-’s

• double bond: atoms share 2 pairs of e-’s (4 total)

• triple bond: atoms share 3 pairs of e-’s (6 total)

• Table 8.1, p.222 - Know these 7 elements as diatomic: Br2 I2 N2 Cl2 H2 O2 F2

Dot diagram for Carbon dioxide• CO2 - Carbon is central

atom ( more metallic )

• Carbon has 4 valence e-’s• Wants 4 more• Oxygen has 6 valence e-’s• Wants 2 moreO

C

The chemistry of CO2 6:44

Carbon dioxide• Attaching 1 oxygen leaves the oxygen

1 short, and the carbon 3 short

OC

Carbon dioxide Attaching the second oxygen

leaves both of the oxygen 1 short, and the carbon 2 short

OCO

Carbon dioxide only solution share more

OCO

Carbon dioxide The only solution is to share more

OCO

Carbon dioxide The only solution is to share more Requires 2 double bonds Each atom can count all the

electrons in the bond

OCO

Carbon dioxide The only solution is to share more Requires two double bonds Each atom can count all the electrons in

the bond

OCO8 valence electrons

Carbon dioxide The only solution is to share more Requires two double bonds Each atom can count all the electrons in

the bond

OCO

8 valence electrons

How covalent bonds form - Mark Rosengarden

How to draw them? Use the handout guidelines:1) Add up all valence e-’s2) Count total e-’s needed to make all atoms

happy (stable)3) Subtract; Divide by 2 (tells you how many

bonds to draw)4) Choose central atom (least electronegative)5) Start w/ most electronegative atom, fill in

remaining valence e-’s to fill atoms up

Examples• NH3, which is ammonia

• N – central atom; has 5 valence electrons, wants 8

• H - has 1 (x3) valence electrons, wants 2 (x3)

• NH3 has 5+3 = 8

• NH3 wants 8+6 = 14

• (14-8)/2= 3 bonds• 4 atoms with 3 bonds

N

H

N HHH

Examples• Draw in the bonds; start with singles• All 8 e- accounted for• Everything full – DONE!

Example: HCN• HCN: C is central atom• N - has 5 valence electrons, wants 8• C - has 4 valence electrons, wants 8• H - has 1 valence electron, wants 2

• HCN has 5+4+1 = 10

• HCN wants 8+8+2 = 18

• (18-10)/2= 4 bonds• 3 atoms with 4 bonds – this will require multiple bonds - not

to H however

HCN• Put single bond between each atom• Need to add 2 more bonds• Must go between C and N (Hydrogen is full)

NH C

HCN Put in single bonds Needs 2 more bonds Must go between C and N, not the H Uses 8 electrons – need 2 more to

equal the 10 it has

NH C

HCN Put in single bonds Need 2 more bonds Must go between C and N Uses 8 electrons - 2 more to add Must go on the N to fill its octet

NH C

Another way of indicating bonds• Often use a line to indicate a bond• Called a structural formula• Each line = 2 valence e-’s

H HO = H HO

Other Structural Examples

H C N

C OH

H

A Coordinate Covalent Bond...• When one atom donates both

electrons in a covalent bond.• Carbon monoxide (CO) is a good

example:

OCBoth the carbon and oxygen give another single electron to share

Coordinate Covalent Bond When one atom donates both

electrons in a covalent bond. Carbon monoxide (CO) is a good

example:

OC

Oxygen gives both of these electrons, since it has no more singles to share.

This carbon electron moves to make a pair with the other single.

Coordinate Covalent Bond When one atom donates both

electrons in a covalent bond. Carbon monoxide (CO)

OCC O

The coordinate covalent bond is shown with an arrow as:

Coordinate covalent bond

•Most polyatomic cations and anions contain covalent & coordinate covalent bonds

•Table 8.2, p.224•Sample Problem 8.2, p.225 •The ammonium ion (NH4

+) can be shown as another example

Bond Dissociation Energies...

•Total energy required to break bond btwn 2 covalently bonded atoms

•High dissociation energy usually means compound relatively

unreactive, b/c it takes hi energy to break bond

Resonance is...• When more than one valid dot diagram

is possible. • Consider the two ways to draw ozone (O3)

• Which one is it? Does it go back and forth?

• It’s hybrid of both, shown by double-headed arrow

• found in double-bond structures!

Resonance in OzoneResonance in Ozone

Neither single structure is correct, actually a hybrid of the two. To show it, draw all possible structures, and join them with a double-headed arrow.

Note the different location of the double bond

ResonanceResonanceOccurs when more than one valid Lewis structure can be written for particular molecule (due to position of double bond)

•resonance structures of carbonate ion (used in production of carbonated beverages).•The actual structure is an avg (or hybrid) of these structures.

Resonance in a carbonate ion (CO3

2-):

Resonance in an acetate ion (C2H3O2

1-):

Polyatomic ions – note the different positions of the double bond.

The 3 Exceptions to Octet rule

• For some molecules, it is impossible to satisfy the octet rule#1. usually when there is an odd

number of valence electrons• NO2 has 17 valence electrons,

because the N has 5, and each O contributes 6. Note “N” page 228

• It is impossible to satisfy octet rule, yet the stable molecule does exist

Exceptions to Octet rule•Another exception: Boron

•Page 228 shows boron trifluoride, and note that one of the fluorides might be able to make a coordinate covalent bond to fulfill the boron

•#2 -But fluorine has a high electronegativity (it is greedy), so this coordinate bond does not form

•#3 -Top page 229 examples exist because they are in period 3 or below

Section 8.3Bonding Theories• OBJECTIVES:

•Describe the relationship between atomic and molecular orbitals.

Section 8.3Bonding Theories• OBJECTIVES:

•Describe how VSEPR theory helps predict the shapes of molecules.

Molecular Orbitals are...• The model for covalent bonding

assumes orbitals are those of individual atoms = atomic orbital

• Orbitals that apply to overall molecule, due to atomic orbital overlap are molecular orbitals• bonding orbital is molecular orbital

occupied by 2 e-’s of covalent bond

Molecular Orbitals - definitions• Sigma bond- 2 atomic orbitals combine

to form molecular orbital symmetrical along axis connecting nuclei

• Pi bond- bonding e-’s likely above and below bond axis (weaker than sigma)

• Note pictures - next slide

Hybridization video – 1:36

- Pages 230 and 231

Sigma bond is symmetrical along axis between 2 nuclei.

Pi bond is above and below bond axis - weaker than sigma

Attractive & repulsive forces in H2 bond• + nuclei repel• e-’s repel• Nuclei and e-’s attract• Attractive forces stronger than repulsion• As nuclei distances decrease, PE decreases• If distance decreases more, PE increases

b/c increased repulsion• Bond forms w/ bond length = interatomic

distance (PE minimum)

VSEPR: stands for...•Valence Shell Electron Pair Repulsion• Predicts 3D shape of molecules• The name tells you the theory:

• Valence shell = outside e-’s• Electron Pair repulsion = e- pairs try to get

as far away as possible from each other.• determines angles of bonds.

VSEPR• Based on # of pairs of ve-’s, bonded &

unbonded.• Unbonded pair called lone pair.• CH4 - draw structural formula

• Has 4 + 4(1) = 8• wants 8 + 4(2) = 16• (16-8)/2 = 4 bonds

VSEPR for methane (a gas):

• Single bonds fill all atoms.

• 4 pairs of e-’s pushing away

• The furthest they can get away is 109.5°

4 atoms bonded• Basic shape -

tetrahedral• pyramid w/

triangular base• Same shape for

everything with 4 pairs CH H

H

H 109.5º

Other angles, pages 232 - 233

•Ammonia (NH3) = 107o

•Water (H2O) = 105o

•Carbon dioxide (CO2) = 180o

- Page 232

Methane has an angle of 109.5o, called tetrahedral

Ammonia has an angle of 107o, called pyramidal

Note the unshared pair that is repulsion for other electrons.

VSEPR models VSEPR theory video 4:52

VSEPR song – 4:33

Hybrid Orbitals

• Provides info for molecular bonding & shape

Hybridization w/ single bonds

• Orbitals combine • C outer e- configuration 2s2 2p2 but one 2s e-

promoted to 2p• One 2s e- and 3 2p e-’s• Allows bond in methane (CH4)

• All bonds same due to orbital hybridization• Mix to form 4 sp3 hybrid orbitals

Hybridization w/ double bonds

• Ethene C2H4 – 1 C-C double bond and 4 C-H single bonds

• sp2 hybrid orbitals form from one 2s and two 2p atomic orbitals of C

Section 8.4Polar Bonds and Molecules• OBJECTIVES:

•Describe how electronegativity values determine the distribution of charge in a polar molecule.


•Describe what happens to polar molecules when they are placed between oppositely charged metal plates.


•Evaluate the strength of intermolecular attractions compared with the strength of ionic and covalent bonds.


•Identify the reason why network solids have high melting points.

Bond Polarity•do covalent bonds always share

equally?•e-’s pulled - tug-of-war, btwn nuclei

• In equal sharing (such as diatomic molecules), the bond is called nonpolar covalent bond

Bond Polarity• When 2 different atoms bond covalently, unequal sharing• more electronegative atom stronger

attraction• slightly negative charge

•polar bond

Lower case delta

Electronegativity?Electronegativity?

The ability of an atom in a The ability of an atom in a molecule to attract shared molecule to attract shared electrons to itself.electrons to itself.

Linus Pauling1901 - 1994

Bond Polarity• Refer to periodic table w/ EN• Consider HCl

H = electronegativity of 2.1Cl = electronegativity of 3.0• Polar bond

• Cl: slight - charge• H: slight + charge

Bond Polarity• Partial charges, much less than 1+ or

1- in ionic bond

H ClPartial charges

and

Bond Polarity• Can also be:

• arrow points to more EN atomKNOW Table 8.3, p.238 shows how

electronegativity indicates bond type

H Cl

Polar molecules• Sample Problem 8.3, p.239• polar bond tends to make entire molecule “polar”• areas of “difference”

• HCl has polar bonds, thus polar molecule• molecule w/ 2 poles called dipole, like

HCl

Polar molecules• effect of polar bonds on polarity of entire

molecule depends on molecule shape• CO2 has 2 polar bonds

• linear • nonpolar molecule

Polar molecules• effect of polar bonds on molecule polarity

depends on shape• water has 2 polar bonds - bent shape• highly electronegative O pulls e- away from H • very polar!

polar bond of water molecule

Chemistry of water 4:46

bonding animations

Polar and non-polar covalent bonds - Mark Rosengarten

Attractions between molecules p. 240• makes solid & liquid molecular cmpds possible• weakest called van der Waal’s forces - there

are two kinds:#1. Dispersion forces: weakest of all, caused by

motion of e- - increases as # e- increases (momentarily more on side of molecule closest to neighboring molecule, neighboring molecule’s e-’s move to opp side)halogens start as gases; bromine (l); iodine (s) – all in Group 7A

#2. Dipole interactions• Occurs when polar molecules

attracted to each other• 2. Dipole interaction happens

in water Figure 8.25, page 240 • +region of one molecule

attracts -region of another molecule

#2. Dipole interactions

• Occur when polar molecules attracted to each other

• Slightly stronger than dispersion forces• Opposites attract, but not completely hooked

like ionic solids

H F

H F

#2. Dipole Interactions

#3. Hydrogen bonding• …is the attractive force caused by

hydrogen bonded to N, O, F, or Cl• N, O, F, and Cl very electronegative, so

this is very strong dipole• And, the hydrogen shares with lone pair

in molecule next to it• This is strongest of the intermolecular

forces

Order of Intermolecular attraction strengths

1) Dispersion forces are weakest2) Little stronger are dipole

interactions3) Strongest is H bonding4) All are weaker than ionic bonds

#3. Hydrogen bonding defined:• When H atom is: a) covalently bonded to a

highly EN atom, AND b) is also weakly bonded to unshared e- pair of nearby highly EN atom• The H is left very e- deficient (only had 1 to

start with!) - it shares with something nearby

• H is ONLY element with no shielding for its nucleus when involved in covalent bond!

Hydrogen Bonding(Shown in water)

HH

O+ -

+

H HO+-

+

This H is bonded covalently to: 1) the highly negative O, and 2) a nearby unshared pair.

H bonding allows H2O to be a liquid at room temp

HH

O H HO

HH

O

H

H

OH

HO

H

HO HH

O

Attractions and properties•Why are some chemicals gases,

some liquids, some solids?•Depends on type of bonding!•Table 8.4, page 244

•Network solids – solids where all atoms covalently bonded to each other

Attractions and properties

• Figure 8.28, page 243• Network solids melt at very high

temps, or not at all (decomposes)

• Diamond does not really melt, but vaporizes to a gas at 3500 oC

• SiC, used in grinding, has melting pt of 2700 oC

Covalent Network CompoundsCovalent Network CompoundsSome covalently bonded substances DO NOT form discrete molecules.

Diamond, a network of covalently bonded carbon atoms

Graphite, a network of covalently bonded carbon atoms

Ionic/Covalent Bond Song

Chapter 8 “Covalent Bonding”. Covalent Bonds covalent combination of prefix co- (Latin -...

Documents

Transcript of Chapter 8 “Covalent Bonding”. Covalent Bonds covalent combination of prefix co- (Latin -...