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Covalent Boning

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Covalent Bonding

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Table of Contents: Covalent Bonding

• Properties of Ionic and Covalent Materials

Click on the topic to go to that section

• Naming Binary Molecular Compounds

• Covalent versus Ionic Bonds

• Resonance Structures

• Lewis Structures

• Polarity

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Covalent Boning

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Covalent versus Ionic Bonds

Covalent Bonding & Molecular Geometry

This form of ibuprofen has virtually no

antiinflammatory effect.

This form of ibuprofen is about 100x more effective at

alleviating pain than the other form.

Even though they consist of the exact same number and kinds of atoms, these two molecules have very different chemical properties.

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Chemical Bonds

Ionic The electrostatic attraction between ions

Covalent The sharing of electrons between atoms

Metallic Each metal atom bonds to other metals atoms within a "sea" of electrons (covered in a later unit)

Chemical bonds hold atoms together to create chemical compounds. There are three basic types of bonds:

Chemical BondsHow ionic or covalent a bond is depends on the difference in electronegativity. The smaller the difference, the more likely electrons are "shared" and the bond is considered covalent, the greater the difference, the more likely electrons have been transferred and the atoms are ionized resulting in an ionic bond.

Li Be B C N O FElectronegativity 1.0 1.6 2.0 2.5 3.0 3.5 4.0

Bond LiF BeF BF CF NF OO FF

Electronegativity 3 2.4 2.0 1.5 1 0.5 0

Increasing Covalent Character

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Chemical Bonds

We can make a few simplifications...

Ionic BondingIonic bonds occur when the difference in electronegativity between

two atoms is more than or equal to 1.7.Na F electronegativity = 3

Covalent BondingIf the difference of electronegativity is less than 1.7, neither atom

takes electrons from the other; they share electrons. This type of bonding typically takes place between two nonmetals or between two metals (metallic bonding).

H Cl electronegativity = 1.1

Ionic v. Covalent Bonding

Ionic bonding results in the formation of a lattice. . not individual molecules.

The chemical formula for an ionic compound is just the ratio of each type of ion in the lattice.

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In contrast, covalent bonding usually results in individual molecules .There are a few exceptions in which a 3D lattice is formed. The bonding and the shapes of these molecules help determine the physical and chemical properties of everything around us!

Ionic v. Covalent Bonding

click here for an animationabout ionic and covalent bonding

1 Which pair of atoms will form a covalent bond?(Hint look at the difference in electronegativities)

A Li and NeB K and BrC C and O

D Na and Cl

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http://njc.tl/bt

http://njc.tl/bt

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2 Which pair of atoms will form a covalent bond?

A Li and IB Na and ClC K and FlD H and O

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3 The atoms that would form the MOST covalent bond would be:

A C HB O HC Cl HD S HE F H

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4 The bond with the LEAST ionic character below would be:A Na FB C FC Si HD Al OE Fe Cl

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Properties of Ionic and Covalent Materials

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Properties of Ionic CompoundsBoiling and Melting Points

Since the attractions between the ions span a short distance, these forces are quite strong resulting in high melting points and boiling points!

Na+ Cl it takes a lot of energy to break an ionic lattice!

Compound Melting Point (C)

NaCl 801

MgO 2852

Properties of Ionic CompoundsConductivity

Since ionic compounds consist of ions, when these ions are free to move, the substance can conduct electricity. To move, they must be in solution or liquid (molten) state.

NaCl (s) Molten NaCl(l)

Lattice is strong, no conductivity Lattice is broken, ions are free to move and conduct

+

+

++

+

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Properties of Metallic SubstancesMelting and Boiling Points

Metallic compounds are held together by nondirectional covalent bonds called metallic bonding. In this type of covalent bond, some electrons are shared but they are loosely held and free to roam. These covalent bonds between the metal atoms are strong! This gives rise to high melting and boiling points!

Metallic Lattice strong metallic covalent bonds

Metal Melting Point

Cu 1085 C

Fe 1585 C

In order to obtain pure metals, the ancients had to melt the metal (metallic substance) out of the rock (an ionic compound).

Copper has a lower melting point so it could be obtained in furnaces at lower temperatures. Furnaces hot enough to extract

iron would come later. Move for answer

REAL WORLD APPLICATION

Why do you think the bronze age (copper mixed with tin) came before the

iron age?

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Since the electrons in metals are free to roam somewhat, metals are good conductors of electricity!

Silver is the most conductive metal and is roughly 510 times more conductive than steel (mostly iron).

Properties of Metallic CompoundsConductivity

Copper is often used in electrical cable rather than silver even though it is roughly 10% less

conductive than silver.

Why?


Copper currently trades for roughly 3 dollars an ounce while silver trades for about 30 dollars a month. It's about the money!!!!

Move for answer

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Like ionic and metallic substances, covalent network solids are giant molecules arranged in 3D crystalline shapes. Here, the atoms involved tend to semimetals like Silicon or Germanium or elemental carbon. Since the bonds are covalent, they are quite strong! This gives rise to high melting and boiling points!

Properties of Covalent Network SubstancesMelting Point and Boiling Point

Glass (75% SiO2) Diamond (pure C)

Melts at 1500 C Melts at 3500 C

Since these substances have higher electronegativities, they keep good tabs on their electrons thereby preventing the electrons from moving. As a result they are largely nonconductive.

Diamond and graphite are both allotropes (different versions) of carbon and vary somewhat in their conductivity.

Properties of Covalent Network SubstancesConductivity

Diamond (C) Graphite (C)

nonconductive a little conductive

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Diamond is notorious for being HARD! This is true for lots of covalent network crystals. Can you think of some applications where hardness is important?

Body Armor

B4C (boron carbide)Drill Bits

polycrystalline diamond


slide for answers

5 Which of the following would NOT conduct electricity in the solid state?

A AlB Al2O3

C NaClD Both A and BE Both B and C

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6 Which of the following would be classified as a covalent network solid?

A NaClB HFC CO2D Ge2O3

E Fe answ

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Molecular Compounds

When atoms are bonded covalently, the atoms are held together by sharing electrons. This occurs between nonmetals such as C,O,S,H,P,N, etc. Unlike in all of the other substances, the atoms form small individual molecules that then interact with each other and their environment. These are called molecular compounds.

P OH H O = C = O

Cl Cl Cl

In covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases. Both atoms use the shared electrons to reach that goal.

http://www.teachersdomain.org/asset/lsps07_int_covalentbond/

http://www.teachersdomain.org/asset/lsps07_int_covalentbond/

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Since these substances contain lots of small molecules, the bonds holding these small molecules together are fundamentally different from the covalent bonds found inside the molecule.

weak intermolecular forces between molecules

Properties of Molecular SubstancesMelting and Boiling Points

They cover a much larger distance and are quite weak giving rise to LOW melting and boiling points!

Molecular compounds contain electronegative nonmetals which do

not lose their electrons easily so they are nonconductive.

As a result they are excellent INSULATORS!

Properties of Molecular SubstancesConductivity

Rubber: (C5H9)250

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Summary of Substances

Ionic Metallic Cov. Network Molecular

metals and nonmetals metals semimetals and

pure carbon nonmetals

Na2O Fe C(diamond) CH4

High MP High MP High MP Low MP

conduct as liquid conduct in all states nonconductive nonconductive

Brittle Malleable Brittle Brittle

7 Which of the following would have the lowest melting point?

A N2

B C(graphite)C C(diamond)D WE LiF

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8 Which of the following will not conduct electricity in any state?

A CuB NaFC FeD CO2E All of these will conduct

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9 Which of the following consists of small individual molecules?

A C(diamond)B SiO2C Cu2OD NaE SO3 an

swer

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10 Which of the following substances has both ionic and covalent bonding within the crystal?

A CuB CuCO3

C LiClD BaE BaF2

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Naming Binary Molecular Compounds

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Use prefixes to indicate the number the atoms.

The second elements name ends in "ide"

Examples

NO2 nitrogen dioxide

P2O5 diphosphorous pentoxide ( pentaoxide>pentoxide)


Look on your reference sheets for the prefixes.

The atom with the lower electronegativity is usually written first.

If there is only one of the first atom, the mono is left off.

Examples

CO carbon monoxide

CO2 carbon dioxide

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11 Chlorine monoxide is

A ClO2

B ClO

C OCl

D O2Cl

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12 Dinitrogen tetroxide is

A NO2 B N2O4

C NO3 D N4O2

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13 H2O is

A Hydrogen monoxide

B Dihydrogen monoxide

C Hydrogen oxide

D Hydrogen dioxide

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14 SO3 is

A sulfate B sulfur oxide C sulfur trioxide

D sulfite

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15 MgO is

A monomagnesium monoxideB magnesium monoxideC monomagnesium oxideD magnesium oxide

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16 P4O10 is

A Phosphorous pentoxide B Tetraphosphorous decoxide

C Phosphorous oxide

D Phosphate

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Lewis Structures

Lewis dot diagrams show valence electrons as dots. To write Lewis dot diagrams , place one dot on each of the four sides of the symbol, before you pair them.

Lewis Dot Structures

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17 How many valence electrons does nitrogen have?

A 2B 3C 4

D 5

E 7

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18The Lewis structure for nitrogen is N

True

False

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Recall that atoms will lose, gain, or share electrons in order to attain a stable, Noble gas configuration. This is usually 8 electrons in an s2p6 configuration.

In covalent bonding, an atom will share electrons in an effort to obtain eight electrons around it (except hydrogen which is stable with 2 valence electrons).

The Octet Rule

Exceptions to the Octet Rule

H needs 2e

Be needs 4e

B needs 6e

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How do electron dot structures represent shared electrons?

An electron dot structure such as H:H represents the shared pair of electrons of the covalent bond by two dots.

H + H H H

Hydrogen atom

Hydrogen atom

Hydrogen molecule

Shared pair of electrons

H

H

1s

1sHydrogen molecule

Structural Formulas

A structural formula represents the covalent bonds by dashes and shows the arrangement of covalently bonded atoms. As in the example below, one shared pair of electrons is represented by one dash.

HH

Hydrogen molecule

Shared pair of electrons

H H

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19 How many electrons are shared by two atoms to create a single covalent bond?

A 2

B 1

answ

erThe halogens form single covalent bonds in their diatomic molecules.

Fluorine is one example.

Single Covalent Bonds

A pair of valence electrons that is not shared between atoms is called an unshared pair, also known as a lone pair or a nonbonding pair.

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In a water molecule, each hydrogen and oxygen atom attains a noblegas configuration by sharing electrons.

Lewis Structure of H2O

The water molecule has two unshared, or lone, pairs of electrons.

2 H + O > O H or O HH

HHydrogen atoms

Oxygen atom

Water molecule

1s 2p2s

1s 1s

O

H H

Water molecule

In the ammonia molecule, NH3, each atom attains a noblegas configuration by sharing electrons.

This molecule has one unshared pair of electrons.

Lewis Structures of NH3

3 H + N > N H or N H

H

H

HHydrogen atom

Nitrogen atom Ammonia

molecule

1s 2p2s

1s 1sH

N

H1s

Ammonia molecule

H

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Drawing Lewis Structures

The P atom has 5 valence electrons.

A Cl atom has 7, and there are three of them.

The total number of valence electrons is:

P

Cl ClCl

1. Find the total number of valence electrons in the polyatomic ion or

molecule.

Drawing Lewis Structures2. The central atom is usually the atom that needs the most electrons to fill its octet. (The atom that needs to make the most bonds).

You can also think of this as the central atoms is the least electronegative element (excluding hydrogen).

Place P in the center, and place the 3 Cl atoms around it. Connect with a single line (a single bond = 2 shared electrons).

3. Connect the other atoms to it by single bonds.

P has 5 valence electrons and needs 3 electrons to fill its octet. Cl only needs 1 electron to fill its octet.

P will be the central atom.

The Cl atoms will surround the P atom.

The single bonds are shown as single lines.

Cl

P

Cl Cl

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4. Count each single bond as a pair (two) of electrons.

5. Add electons to the outer atoms to give each one 8 (a full shell), or just 2 electrons for hydrogen.

6. Do the same for the central atom.

7. Check: Does each atom have a full outer shell (8 except, 2 for hydrogen)?

Have you used up all the valence electrons? Have you used too many electrons?



The N atom has 5 valence electrons

and

each of the three H atoms has 1 so the total number of valence electrons is,

NH3

5 + 3(1) = 8

1. Find the total number of valence electrons in the polyatomic ion or

molecule.

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2. Identify the atom that needs the most electrons to fill its octet it will be the cental atom.


H and all Halogens will usually be terminal atoms, meaning they will always be on the outside because they can only make 1 bond / atom.

H can never be the central atom so N must be

The H atoms will surround the N atom.

The single bonds are shown as single lines.

HN HH

NH3


HN HH

Each H already has two electrons, so that's done. But we have to add electrons to N to make 8.

HN HH

4. Count each single bond as a pair (two) electrons. Now add electrons to the outer atoms to give each one a full shell (2 in the case of H).

5. Place left over electrons around the central atom.

6. Check:Does each atom have a full outer shell ?

7. Have you used up all the valence electrons you started with? Have you used too many electrons?

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20 Which of the following is the correct Lewis Structure for H2O?

A

B

C

D

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H O H

H H O

H H O

H H O

1. Find the total number of valence electrons:

2. Central atom is the least electronegative:


4. Count each single bond as a pair of electrons.

5. Add electrons to the outer atoms to give each one 8 (except H only gets 2).

6. Add electrons to the central atom to give it 8.

7. Check to make sure all valence electrons are used.

21

A

B

C answ

erH H H H H H C C

C C H

H H

H

H H

Which of the following is the correct Lewis Structure for C2H6?

C C H H H H H H

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Lewis Structures for ions

If you are drawing the Lewis Structure for an ION...

A negative ion has extra electrons, add the charge of the ion to your valence electron count.

ClO2 has 1(7) + 2(6) + 1 = 20 electrons

A positive ion is missing electrons, subtract the charge of the ion to your valence electron count.

NH4+ has 1(5) + 4(1) 1 = 8 electrons

22

A

B

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18

How many valence electrons does CO32 have?

C

D

2426

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23

A

B

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8

9

How many valence electrons does H3O+ have?

C

D

1011

Lewis StructuresDraw the Lewis dot structure for the hydronium ion, H3O+

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C

NCl

F

OSB

P

I

H

CO OSi

SeXe

CO2Draw a Lewis Structure For

We ran out of electrons, but carbon does not have an octet

yet!

Now What?

Slide for Answer

Double and Triple Covalent Bonds

Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing two pairs or three pairs of electrons.

A double covalent bond is formed by two atoms sharing 4 electrons.

A triple covalent bond is formed by two atoms sharing 6 electrons.

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Carbon Dioxide, CO2

1. Determine the # of valence electrons.

1 (4) + 2 (6) = 16 e

This leaves 12 electrons, 6 pairs

3. Place lone pairs on oxygen atoms to give each 8.

Double and Triple Covalent Bonds

O C O

O C O2. Form Single Bonds

O C O

Carbon Dioxide, CO2

4. Check: We had 16 electrons to work with; how many have we used?

5. We have used 16 electrons, but Carbon still does not have an octet. We must form DOUBLE BONDS between C and O.

Instead of sharing only 1 pair, a double bond shares 2 pairs. So one pair is taken away from each atom and replaced with another bond.

O C O

O C O

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If you run out of electrons before the central atom has an octet……form multiple bonds until it does.

Writing Lewis Structures

Oxygen molecule

Bonding of O2

1s

2s

2p

1s

2s

2p

O + O > O O or O O

O

O

Oxygenatom

Oxygenatom

Oxygenmolecule

Oxygenmolecule

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A coordinate covalent bond is a covalent bond in which one atom contributes both bonding electrons.

In a structural formula, you can show coordinate covalent bonds as arrows that point from the atom donating the pair of electrons to the atom receiving them.

In a coordinate covalent bond, the shared electron pair comes from one of the bonding atoms.

Carbon has 4 valence electrons, oxygen has 6.

Coordinate Covalent Bonds

C

NCl

F

OSB

P

I

H

CSi

SeXe

CO Draw a Lewis Structure

Carbon has the lower electronegativity, so we will consider it the "central" atom...

O

Slide for Answer

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C

NCl

F

OSB

P

I

H

Si

SeXe

F2 Draw a Lewis Structure

F F

Slide for AnswerF

Draw the Lewis Structure for CO, Carbon monoxide

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Covalent Bond Length

Bond length is affected by the number of atoms being shared between the two nuclei, and the attraction between the negative electrons and the positive nuclei

Single bonds are the longest.

Double bonds are midlength

Triple bonds are the shortest.

Covalent Bond Energy

Triple bonds are the strongest of the three.

Bond Type Bond Energy

C C

C C

C C

348 kJ

614 kJ

839 kJ

Bond strength also varies.

Single bonds are the weakest.

Double bonds are midstrength.

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Covalent Bonds Comparison

Type of Bond

Electrons shared

BondStrength

BondLength

2

4

6

weak

intermediate

strong

long

intermediate

short

24 As the number of bonds between a pair of atoms increases, the distance between the atoms:

A increases

B decreases

C remains unchanged

D varies, depending on the atoms

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25 As the number of bonds between a pair of atoms increases, the strength of the bond between the atoms:

A increases

B decreases

C remains unchanged


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26 As the number of bonds between a pair of atoms increases, the energy of the bond between the atoms:

A increases

B decreases

C remains unchanged


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27 How many electrons are shared by two atoms to create a single bond?

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28 How many electrons are shared by two atoms to create a double bond?

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29 How many electrons are shared by two atoms to create a triple bond?

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30 Using lewis structure drawings, determine which molecule below would have the shortest bond length between atoms?

A O2B F2

C Cl2D COE I2

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31 Which of the following molecules would have the longest CO bond length? Use Lewis structures.

A COB CO2C H2COD CH3OHE The lengths are all the same an

swer

A molecule is a neutral group of atoms joined together by covalent bonds. Air contains oxygen molecules.

A diatomic molecule is a molecule consisting of two atoms. Certain elements do not exist as single atoms; they always appear as pairs.

When atoms turn into ions, this NO LONGER HAPPENS!

HydrogenNitrogenOxygenFluorineChlorineBromineIodine

Remember:HONClBrIF

Diatomic Molecules

H H

N N

O O

H2

N2O2

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32 On the periodic table below, mark which elements exist as diatomic molecules. Note the pattern.


There are three types of ions or molecules that do not follow the octet rule:

#1 Ions or molecules with an odd number of electrons

#2 Ions or molecules with less than an octet

#3 Ions or molecules with more than eight valence electrons (an expanded octet)

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Though relatively rare and usually quite unstable and reactive, there are ions and molecules with an odd number of electrons.

NO is an example:

Exception 1: Odd Number of Electrons

Exception 2: Fewer Than Eight Electrons

Beryllium (Be) this metal is shown to form molecular compounds, rather than ionic compounds as expected; only needs 4 electrons to be stable

Boron (B) only needs 6 electrons to be stable

Memorize these exceptions

B

Be

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The only way PCl5 exists is if phosphorus has 10 electrons around it.

This is called an expanded octet.

Atoms on the third energy level or higher are allowed to expand their octet to 10 or 12 electrons.

These atoms are larger and can accommodate more electrons.

Exception 3: Expanded Octet

How many electrons do these central atoms have around them?

Exception 3: Expanded Octet

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Draw the Lewis dot structure for sulfur hexaflouride, SF6:


Move for answer

Draw the Lewis dot structure for the xenon tetrafluoride, XeF4.


Move for answer

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Draw the Lewis dot structure for boron trifluoride, BF3:

Move for answer

Draw the Lewis dot structure for the iodine tricholoride, ICl3.


Cl I Cl

ClMove for answer

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33

A Boron and Beryllium

B Boron and Helium

C Boron, Beryllium, and Hydrogen

D Boron, Beryllium, Hydrogen and Helium

E Boron, Beryllium, Hydrogen, Helium and Oxygen

[*]

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Which of the following need fewer than 8 valence electrons to be stable?

34 The correct lewis structure for BeCl2 is Cl Be Cl

True False

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35 Elements in the first two rows of the periodic table cannot have expanded octets because their atoms do not have enough space.

True False

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Resonance Structures

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C

NCl

F

OSB

P

I

H

Si

SeXe

O3Draw a Lewis Structure and use that to determine the VSEPR number

For the central oxygen:Electron domains = 3Bonding domains = 2Unpaired electrons = 1

Its VSEPR number is 3 2 1

O

O O

Slide for Answer

Consider the Lewis structure we would draw for ozone, O3:

We would expect the double bond to have a shorter bond length than the single bond.

However, the true, observed structure of ozone shows that both OO bonds are the same length. How can this be?

Resonance

O

OO

OO

O

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One Lewis structure cannot accurately depict a molecule like ozone. Therefore, we use multiple structures, called resonance structures, to describe the molecule.

Ozone has two resonance structures.

Resonance

O

O

OO

O

O

ResonanceThe actual ozone molecule is a synthesis of these two resonance structures.

The bond length for both outer oxygen atoms falls somewhere between the single and double bond length.

O

O

OO

O

O

Resonancestructure

Resonancestructure

Ozone molecule

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Resonance

The nitrate ion, NO31 also requires resonance structures to explain its covalent bonding.

There are three resonance structures for the nitrate ion:

Draw the Lewis dot structure for SO3:

Resonance Structures

move for answer

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36 How many resonance structures can be drawn for the carbonate ion, CO32 ?

A 1B 2C 3D 4E 5

]

answ

erThe benzene molecule is a regular hexagon of carbon atoms with a hydrogen atom bonded to each one. There are two resonance structures for benzene.

Benzene

Benzene, C6H6, is obtained from the distillation of fossil fuels. More than 4 billion pounds of benzene is produced annually in the United States. Because benzene is a carcinogen, its use is closely regulated.

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Localized v. Delocalized electronsIn truth, the shared pairs of electrons do not always remain between adjacent C atoms. They are not localized.

Instead, the electrons are said to be delocalized, meaning that they they can move around the 6carbon ring.

Benzene is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring... we will talk more about this at the end of the year when we study organic chemistry.

<−−> or

Polarity of BondsThough atoms often form compounds by sharing electrons, the electrons are not always shared equally. In a covalent bond, one atom has a greater ability to pull the shared pair toward it.

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Polarity of Bonds

Identical atoms will have an electronegativity difference of ZERO. As a result, the bond is NONPOLAR.

Bonds and Electronegativity

Bond Type

NonPolar Covalent

Polar Covalent

Ionic

Electronegativity Difference

very small or zero

about 0.2 to 1.6

above 1.7 (between metal & nonmetal)

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Therefore, the fluorine end of the molecule has more electron density than the carbon end.

Polarity of Bonds

C F

We use the symbol to designate a dipole (2 poles). The "+" end is on the more positive end of the molecule and the arrow points towards the more negative end.

When two atoms share electrons unequally, a bond dipole results.

Bond Dipoles and Electronegativity

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But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.

Polarity of Molecules

For instance, in the case of CO2:

Each O=C bond is polar, but their directions are 180o from each other, and they cancel out. Polar bonds add

like vectors.


By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.

For a molecule to be polar, it must a) contain one or more dipoles AND b) have these polar bonds arranged asymmetrically

In other words, if all the dipoles are symmetrical, they will cancel each other out and the molecule will be

NONPOLAR.

Many molecules with lone pairs of electrons will be POLAR.

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These are some examples of polar & nonpolar molecules.


330, nonpolar

440, nonpolar

440, polar

431, polar110(?), polarSlide for Answer

Slide for Answer

Slide for Answer

Slide for Answer

Slide for Answer

37 Which of these are polar molecules?

A a, bB a, b, cC a, cD a, c, dE c, e

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38 Sulfur trioxide (SO3) is polar.

True False

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39 Hydrogen sulfide gas (H2S) is nonpolar.

True False

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40 Which of the following contains polar bonds but is a nonpolar molecule?

A CH4

B CS2

C H2SD CF4

E All of these are polar

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