Covalent Bonding

Covalent Bonding Molecule: neutral group of atoms that are

held together by covalent bonds Covalent Bond: sharing of electrons

between two atoms, “owned” by the two bonded atoms. Occurs most commonly between nonmetal

elements

Covalent bonding: Fluorine

F F

Fluorine has seven valence electrons A second Fluorine atom also has seven

By sharing electrons…Both atoms end up with full orbitals (stable octets)

8 Valence electrons

8 Valence electrons

Covalent Bonding: Oxygen

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n = 1

O1s22s22s22p4

n = 2

+

O2

Atoms share electrons to achieve a stable octet (8 electrons in valence shell).

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O1s22s22s22p4

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Octet Rule

The Octet Rule applies to covalent bonds: atoms bond to achieve 8 valence electrons Exceptions:

Hydrogen is satisfied with 2 valence electrons

Beryllium is content with 4 valence electrons

Boron is satisfied with 6 valence electrons

Bond Strength The strength of a bond is determined by how

many electrons are shared between two atoms.

Single Bond - Atoms share one PAIR of electrons (2 e-) Weakest bond, most flexible

Double Bond – Atoms share TWO pairs (4 e-)of electrons

Triple Bond – Atoms share THREE pairs (6 e-)of electrons Strongest bond, most rigid

(Pg. 182)Relationship between Bond Length and Bond Energies

Bond Energy = Energy required to break a chemical bond Unit: kJ High bond energy = High bond strength

Plot Bond length on the x-axis, Plot bond energy on the y-axis

Result: Bond length decreases as the strength of the bond _____________. Answer: Increases

Covalent Bonds: Lewis Structures Lewis Structure: It shows the molecular arrangement of covalent molecules, using the electron-dot notation. Lone Pair electrons = Unpaired electrons

Q: Do we use lewis structures to illustrate ionic bonds?

Cx

xx

x

x x

F

x x

x

x x

F x

x

xx

x F x

x

xx

x x

F x

x

How To: Lewis Structures (Ex: CH3I)Step 1. Determine the type and number of atoms in

the molecule.Step 2. Determine the total number of valence

electrons available in the molecule.Step 3. Arrange the atoms.

Q: Which atom is central? *(next slide)Step 4. Start by putting two electrons between

atoms for each covalent bond. Then place electrons around each atom to fulfill the Octet Rule

Step 5. Count the electrons to check your work. Ask yourself, does it equal the number of electrons in step 2?

How to: Lewis StructuresQ: Which atom is central? A: If carbon is present, it is the central atom.

Hydrogen is never the central atom. Otherwise, the central atom is the least-electronegative atom.

Q: What does “electronegative” mean?A: Electronegativity: The ability of an atom to

attract electrons in a chemical compound. If an atom is highly electronegative, it is the more “dominant” atom and “hogs the electrons.”

Lewis Structure

o

Co

o o

x

Ho

Co

o oH x

x

H

xH

x H CH

H

H

H

x

xxx

x x

I x

o

No

o o

o o

No

o ox

xxx

x x

I x

x x

xx

x x

I x

xxx

x x I

x

x

o o

Nx

xxx

x x

I

x xx

x x

I x

xx

x x I

x

x

o

o

Co

o o x

xxx

xO x

x

xxx

xO x

x

xxx

xO x

o

Co

o o O = C = O

x x

x x x x

x x

methane (CH4)

carbon dioxide (CO2)

nitrogen triiodide (NI3)

Lewis Structures: Practice

Ammonia, NH3

Hydrogen Sulfide, H2S Silane, SiH4

Phosphorus Trifluoride, PF3

Naming Covalent Molecules

Greek prefixes are used to indicate how many atoms of each element are present in the molecule.

P4O10Prefix needed if first element contributes more than one atom

Root Name of 2nd element + ide

Prefix indicating number of atoms

Name of first element

+ +

tetraphosphorus decoxide

Naming Covalent MoleculesNumb

erPrefix

1 Mono-2 Di-3 Tri-4 Tetra-5 Penta-6 Hexa-7 Hepta-8 Octo-9 Nona-

10 Deca-

The “o” or “a” at the end of a prefix is usually dropped when another vowel follows it. “monooxide”

becomes “monoxide” “pentaoxide”

becomes “pentoxide”

Examples…Name the following - SO3: Sulfur ______?________

Answer: Sulfur trioxide PBr5: _____?_____ _______?________

Answer: Phosphorus pentabromide As2O5: ______?_____ Pentoxide

Answer: Diarsenic Pentoxide

Examples… Write the chemical formulas - Carbon tetriodide

Answer: CI4

Dinitrogen trioxide Answer: N2O3

Silicon dioxide Answer: SiO2