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The Octet RuleThe Octet Rule

Except for hydrogen and helium, atoms aremost energetically stable if they have acompletely filled valence shell.

A completely filled valence shell is called anoctet because eight electrons are involved.

Atoms will form ionic compounds by gainingor losing valence electrons or covalentcompounds by sharing electrons.

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Lewis StructuresLewis Structures

A simple way to show the valence electronspresent in an atom.

Valence electrons are those electrons foundin the highest numbered principal energylevel (PEL).

Valence electrons are found only in the s and

p sublevels and in most cases are theelectrons responsible for bonding.

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Lewis StructuresLewis Structures

The chemical symbol represents the kernel of

the atom.

The kernel of an atom consists of the nucleus

and the core electrons.

Example

X

s px

py

pz

Start with s and proceed cw using Hund’s Rule!

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Lewis Structures – Second PeriodLewis Structures – Second Period

Li Be B C

N O F Ne

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Drawing Lewis StructuresDrawing Lewis Structures

Count the number of valence electrons.

Remember, the number of valence electrons can be determined by the group (family) numbers.

Draw a “skeleton” structure for the covalent

structure assuming single (sigma) bonds.

Terminal atoms will usually be hydrogen,oxygen, and halogens.

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Determine the number of valence electronsthat are left.

Distribute the remaining electrons to theatoms surrounding the central atom to

satisfythe octet rule.

Distribute the remaining electrons as pairsaround the central atom.

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When all of the valence electrons have been

used, ensure that the central atom has anoctet.

If the central atom does not have an octet, form one or more double or

triple (pi) bonds.

To form multiple bonds, move one or more pairs of electrons from a surrounding atom to the bond connecting the central atom.

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Lewis Structure ExamplesLewis Structure Examples

NH3 ammonia

1 N --- 5 val e-

3 H --- 3

8

N

- 62

HH

H

HH

H

N

shared pair

unshared pair

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OF2 oxygen difluoride

1 O --- 6 val e-

2 F --- 14

20 O

- 4

4

F

16

FF

FF O- 12

FF O

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HCN hydrogen cyanide

1 H --- 1 val e-

1 C --- 4 1 N --- 5

10- 4

H C N

H C N

H C N

H C N

A triple bond is needed forboth C and N to have a completeoctet.

6

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NH4+ ammonium

1 N --- 5 val e-

4 H --- 4 -1

8

N HH

H

H

+

A 1 is subtracted because a positively charged polyatomic ion has 1 less electron.

-80

Square brackets areused for anions andcations.

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OO22 oxygenoxygen

2 O --- 12 val e--2

10

O O

O O

O O

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C2H5OH ethyl alcohol (ethanol)

2 C --- 8 val e-

6 H --- 61 O --- 6

20

H

-164

H

H H

HH

H

HH HCC O

H HC C O

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N2H4 hydrazine

2 N --- 10 val e-

4 H --- 4

14 H H

-104

N NH H

H HN NH H

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ResonanceResonance

When there is more than one equivalentLewis structure, all the structures are givento represent the molecule or the polyatomicion.

Resonance can only occur when all thestructures:

satisfy the octet rule

have the same type and number of bonds

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ResonanceResonance

NO2- nitrite

1 N --- 5 val e-

2 O --- 12 + 1 18 - 4

14 -14 0

O ON

NO O

O ON

O ON

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ResonanceResonance

- -O N O

Note that the placement of the atoms in these

structures is the same but the electronarrangement is different.

O N O

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Note that polyatomic ions are placed insidesquare brackets and the charge is placedafter the brackets as a superscript.

It is necessary to include all the structuresseparated by a double-headed arrow.

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Exceptions to the Octet RuleExceptions to the Octet Rule

Three major exceptions to the octet rule:

Molecules or ions with more than eight electrons around the central atom.

Species with fewer than eight electrons around the central atom.

Species with an odd number of valence electrons.

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Expanded OctetsExpanded Octets

Starting with period three, atoms have thecapability to accommodate d electrons (3d).

AX4E molecules such as SF4 are able toaccommodate 4 bonding pairs of electronsand one nonbonding pair of electrons. Thisresults in S being surrounded by 5 electronpairs.

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The favored bonding scenario includes largecentral atoms (starting in the third period)and small terminal atoms such as fluorine,chlorine, and oxygen.

As shown below, S also has the ability to accommodate six pairs of valence electrons as found in SF6.

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Less Than an OctetLess Than an Octet

Molecules having either boron or beryllium as their central atom result in the central atom having only 2 or 3 valence pairs of electrons.

These molecules are very reactive with a molecule having an unshared pair ofelectrons.

BeCl2 BF3

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Odd Number of Valence ElectronsOdd Number of Valence Electrons

Most molecules have an even number ofvalence electrons.

In rare cases, molecules such as NO andNO2, there is one unpaired electron which isvery reactive.

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Odd Number of Odd Number of ValenceValence ElectronsElectrons

nitrogen(II) oxide nitrogen(IV) oxide

N O O N O

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ElectronegativityElectronegativity

Electronegativity is a measure of theattraction of an element for a shared pair ofelectrons.

H Cl H Cl

Comparing the electronegativity values ofhydrogen and chlorine, chlorine has a valueof 3.2 and that of hydrogen is 2.2.

δ-δ+

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Range of Ionic CharacterRange of Ionic Character

% Ionic 100% 50% 5% 0%Character

ΔEN

0.0 – 0.3 Nonpolar Covalent

0.4 – 1.7 Polar Covalent

1.8 – 4.0 Ionic

4.0 0.3

0.01.7

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Properties of Ionic CompoundsProperties of Ionic Compounds

The building blocks of ionic compounds arecations (positive ions) and anions (negativeions).

The force holding an ionic compoundtogether is the ionic bond.

Properties

have only empirical formulas

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high melting and boiling points

solids at room temperature (25°C, 298K)

conductors in the liquid phase (l) or in an

aqueous solution (aq)

some are water soluble, some are not – check solubility chart

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Nonpolar Covalent CompoundsNonpolar Covalent Compounds

The building blocks of nonpolar covalent(molecular) compounds are molecules.

The force holding a nonpolar moleculetogether is the covalent bond.

Properties

have both empirical and molecular formulas

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low melting and boiling points

often a gas or a liquid at room temperature (25°C, 298K)

nonconductors of heat and electricity

insoluble in water

exists as discrete molecular units

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Polar Covalent CompoundsPolar Covalent Compounds

The building blocks of polar covalent(molecular) compounds are molecules.

The force holding a polar molecule togetheris the covalent bond.

Properties

have both empirical and molecular formulas

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usually higher melting and boiling points

often a gas or a liquid at room temperature (25°C, 298K)

some are conductors of heat and electricity

more likely to be water soluble