Chemical BondingLewis Structures

Material from karentimberlake.com and H. Stephen Stoker

Forming Chemical Bonds

• According to the Lewis model– an atom may lose or gain enough electrons to

acquire a filled valence shell and become an ion. An ionic bond is the result of the force of attraction between a cation and an anion.

– an atom may share electrons with one or more other atoms to acquire a filled valence shell. A covalent bond is the result of the force of attraction between two atoms that share one or more pairs of electrons.

Ionic Compounds

Vs Covalent

(Molecules) Compounds

• Ionic Compounds–Made up of a metal and a non-metal

–Have a difference in electronegativity of more than 1.7 (Figure 20 pg. 161)

–Formed from ions–Electrons are transferred

• Covalent Molecules–Made up of two non-metals–Have a difference in electronegativity of less than 1.7 (Figure 20 pg. 161)

–Ions are not formed–Electrons are shared

Bond Polarity: NonpolarNonpolar covalent bond Electrons are shared between

atoms with the same electronegativity values.

Difference = 0Examples:

N2 Br2

Bond Polarity: PolarPolar covalent bondElectrons are shared between

different nonmetal atoms Examples:O-Cl O-S N-Cl

Chemical Bonding: The Covalent Bond

Model

←Fig. 5.12(a) In the nonpolar covalent bond present, there is a symmetrical distribution of electron density. (b) In the polar covalent bond present, electron density is displaced because of its electronegativity.

10.7.00 6:16 PM9 Lewis Structure Tutorial

Valence electrons for ElementsRecall that the valence electrons for the elements can be determined based on the elements position on the periodic table.

Lewis Dot Symbol

10.7.00 6:16 PM10 Lewis Structure Tutorial

Lewis Structures

1) Find your element on the periodic table.

2) Determine the number of valence electrons.

3) This is how many electrons you will draw.

10.7.00 6:16 PM11 Lewis Structure Tutorial

Lewis Structures

• Find out which group (column) your element is in.

• This will tell you the number of valence electrons your element has.

• You will only draw the valence electrons.

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10.7.00 6:16 PM12 Lewis Structure Tutorial

Lewis Structures

1) Write the element symbol.

2) Carbon is in the 4th group, so it has 4 valence electrons.

3) Starting at the right, draw 4 electrons, or dots, counter-clockwise around the element symbol.

C

10.7.00 6:16 PM13 Lewis Structure Tutorial

Lewis Structures

C1) Check your

work.2) Using your

periodic table, check that Carbon is in the 4th group.

3) You should have 4 total electrons, or dots, drawn in for Carbon.

10.7.00 6:16 PM14 Lewis Structure Tutorial

Lewis Structures

COn your worksheet, try these elements on your own:

a) Hb) Pc) Cad) Are) Clf) Al

10.7.00 6:16 PM15 Lewis Structure Tutorial

Lewis Structures

HOn your worksheet, try these elements on your own:

a) Hb) Pc) Cad) Are) Clf) Al

10.7.00 6:16 PM16 Lewis Structure Tutorial

Lewis Structures

POn your worksheet, try these elements on your own:

a) Hb) Pc) Cad) Are) Clf) Al

10.7.00 6:16 PM17 Lewis Structure Tutorial

Lewis Structures

Ca

On your worksheet, try these elements on your own:

a) Hb) Pc) Cad) Are) Clf) Al

10.7.00 6:16 PM18 Lewis Structure Tutorial

Lewis Structures

Ar

On your worksheet, try these elements on your own:

a) Hb) Pc) Cad) Are) Clf) Al

10.7.00 6:16 PM19 Lewis Structure Tutorial

Lewis Structures

ClOn your worksheet, try these elements on your own:

a) Hb) Pc) Cad) Are) Clf) Al

10.7.00 6:16 PM20 Lewis Structure Tutorial

Lewis Structures

AlOn your worksheet, try these elements on your own:

a) Hb) Pc) Cad) Are) Clf) Al

10.7.00 6:16 PM21 Lewis Structure Tutorial

Valence electrons and number of bonds

Number of bonds elements prefers depending on the number of valence electrons. In general -

X

Family # Covalent Bonds*

HalogensF, Br, Cl, I

CalcogensO, S

NitrogenN, P

CarbonC, Si

O

N

C

1 bond often

2 bond often

3 bond often

4 bond always

The above chart is a guide on the number of bonds formed by these atoms.

10.7.00 6:16 PM22 Lewis Structure Tutorial

Lewis Structure, Octet Rule Guidelines

When compounds are formed they tend to follow the Octet Rule.Octet Rule: Atoms will share electrons (e-) until it is surrounded by eight valence electrons.

Rules of the (VSEPR) game-i) The Octet Rule works mostly for second period elements.

Many exceptions especially with 3rd period elements (d-orbitals)

ii) H prefers 2 e- (electron deficient)

iii) :C: N: :O: :F:4 unpaired 3unpaired 2unpaired 1unpaired up = unpaired e-4 bonds 3 bonds 2 bonds 1 bond

O=C=O NN O = O F - Fiv) H & F are terminal in the structural formula (Never central)

.. ...

..

.

10.7.00 6:16 PM23 Lewis Structure Tutorial

Atomic ConnectivityThe atomic arrangement for a molecule is usually given. CH2ClF HNO3 CH3COOH H2Se H2SO4 O3

H C F

Cl

HH

N OO

OH O S O H

O

O O O O

H C C

O H H

H O H Se H

In general when there is a single central atom in the molecule, CH2ClF, SeCl2, O3 (CO2, NH3, PO4

3-), the central atom is the first atom in the chemical formula.Except when the first atom in the chemical formula is Hydrogen (H) or fluorine (F). In which case the central atom is the second atom in the chemical formula.

Find the central atom for the following:1) H2O a) H b) O 2) PCl3 a) P b) Cl3) SO3 a) S b) O 4) CO3

2- a) C b) O5) BeH2 a) Be b) H 6) IO3- a) I b) O

10.7.00 6:16 PM24 Lewis Structure Tutorial

Lewis Structures

Carbonic acidFormaldehydeAcetyleneEthylene

Hydrogen chlorideMethaneAmmoniaWater

HH N H C H H ClH

HC C

HC C HH

HC

HHO

H

H2O (8) NH3 (8) CH4 (8) HCl (8)

C2H4 (12) C2H2 (10) CH2O (12) H2CO3 (24)

H

HHO

H

O OC HHO