The ties that bind
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The Ties That Bind. Chemical Bonding and Interactions. Chemical Bonding and Interactions. Stable Electron Configurations Electron-Dot (Lewis) Structures Drawing, Rules for Drawing The Octet Rule Some Exceptions to the Rule Ionic Bonding Naming ionic compounds Drawing Covalent Bonding

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The Ties That Bind

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The ties that bind

The Ties That Bind

Chemical Bonding and Interactions


Chemical bonding and interactions

Chemical Bonding and Interactions

  • Stable Electron Configurations

  • Electron-Dot (Lewis) Structures

    • Drawing, Rules for Drawing

    • The Octet Rule

    • Some Exceptions to the Rule

  • Ionic Bonding

    • Naming ionic compounds

    • Drawing

  • Covalent Bonding

    • Naming covalent compounds

    • Drawing

    • Electronegativity and Polar Covalent Compounds

  • Molecular Shapes and the VSEPR Theory

  • Intermolecular Forces of Attraction

    • H-bonds, Dipole-Dipole, Ion-Dipole, London Dispersion Forces


It s all about stability

It’s all about stability…

  • Fact: Noble gases are inert (undergo few, if any, chemical reactions)

  • Theory: The inertness of these gases is a result of their electronic structure

  • If elements could alter their electron structures like those of the noble gases, they would be less reactive.


The ties that bind

Electron Configurations of Cations and Anions

Of Representative Elements

Na [Ne]3s1

Na+ [Ne]

Atoms lose electrons so that cation has a noble-gas outer electron configuration.

Ca [Ar]4s2

Ca2+ [Ar]

Al [Ne]3s23p1

Al3+ [Ne]

H 1s1

H- 1s2 or [He]

Atoms gain electrons so that anion has a noble-gas outer electron configuration.

F 1s22s22p5

F- 1s22s22p6 or [Ne]

O 1s22s22p4

O2- 1s22s22p6 or [Ne]

N 1s22s22p3

N3- 1s22s22p6 or [Ne]


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-1

-2

-3

+1

+2

+3

Cations and Anions Of Representative Elements


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What neutral atom is isoelectronic with H- ?

Na+: [Ne]

Al3+: [Ne]

F-: 1s22s22p6 or [Ne]

O2-: 1s22s22p6 or [Ne]

N3-: 1s22s22p6 or [Ne]

Na+, Al3+, F-, O2-, and N3- are all isoelectronic with Ne

H-: 1s2

same electron configuration as He


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Group

e- configuration

# of valence e-

ns1

1

1A

2A

ns2

2

3A

ns2np1

3

4A

ns2np2

4

5A

ns2np3

5

6A

ns2np4

6

7A

ns2np5

7

Valence electrons are the outer shell electrons of an

atom. The valence electrons are the electrons that

participate in chemical bonding.

9.1


The ties that bind

(Electron Dot Structures)

*We generally place the electrons one four sides of a square around the element symbol.

*Octet rule: we know that s2p6 is a noble gas configuration. We assume that an atom is stable when surrounded by 8 electrons (4 electron pairs).


A violent bond sodium and chlorine

A violent bond: Sodium and Chlorine

  • Sodium is a soft, reactive metal.

  • Chlorine is a greenish-yellow gas which is toxic (WWI poison)

  • Sodium chloride is used for our food.


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Ionic Bonding

Consider the reaction between sodium and chlorine:

Na(s) + ½Cl2(g)  NaCl(s)


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Chemistry In Action:

Sodium Chloride

Mining Salt

Solar Evaporation for Salt


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-

-

-

-

+

Li+

Li

Li

Li+ + e-

e- +

Li+

Li+ +

F

F

F

F

F

F

The Ionic Bond

[He]

[Ne]

1s22s1

1s22s22p5

1s2

1s22s22p6

IONIC BONDING

-bonding due to electrostatic attraction arising from an exchange of electrons.

9.2


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  • ionic compounds consist of a combination of cations and an anions

  • the formula is always the same as the empirical formula

  • the sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero

The ionic compound NaCl

2.6


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Ionic Bonding

*NaCl forms a very regular structure in which each Na+ ion is surrounded by

6 Cl- ions.

Similarly, each Cl- ion is surrounded by six Na+ ions.

There is a regular arrangement of Na+ and Cl- in 3D.

Note that the ions are packed as closely as possible.

Note that it is not easy to find a molecular formula to describe the ionic lattice.


The ties that bind

E = k

Q+Q-

r

lattice energy

cmpd

MgF2

2957

Q= +2,-1

MgO

3938

Q= +2,-2

LiF

1036

LiCl

853

SIDEBAR: Electrostatic (Lattice) Energy

Lattice energy (E) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions.

Q+ is the charge on the cation

Q- is the charge on the anion

r is the distance between the ions

Lattice energy (E) increases as Q increases and/or

as r decreases.

r F < r Cl

9.3


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9.3


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11 protons

11 electrons

11 protons

10 electrons

Na+

Na

17 protons

18 electrons

17 protons

17 electrons

Cl-

Cl

An ion is an atom, or group of atoms, that has a net positive or negative charge.

cation – ion with a positive charge

If a neutral atom loses one or more electrons

it becomes a cation.

anion – ion with a negative charge

If a neutral atom gains one or more electrons

it becomes an anion.

2.5


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A monatomic ion contains only one atom

Na+, Cl-, Ca2+, O2-, Al3+, N3-

A polyatomic ion contains more than one atom

OH-, CN-, NH4+, NO3-

2.5


Test yourself

Test Yourself:

  • MgO

  • MgBr2

  • The combination of the ion of magnesium and the ion of nitrogen


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1 x +2 = +2

1 x +2 = +2

2 x +3 = +6

1 x -2 = -2

3 x -2 = -6

2 x -1 = -2

Formula of Ionic Compounds

Al2O3

Al3+

O2-

CaBr2

Ca2+

Br-

Na2CO3

Na+

CO32-

2.6


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2.6


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2.7


Chemical nomenclature

Chemical Nomenclature

  • Ionic Compounds

    • often a metal + nonmetal

    • anion (nonmetal), add “ide” to element name

barium chloride

BaCl2

potassium oxide

K2O

magnesium hydroxide

Mg(OH)2

potassium nitrate

KNO3

2.7


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  • Transition metal ionic compounds

    • indicate charge on metal with Roman numerals

iron(II) chloride

FeCl2

2 Cl- -2 so Fe is +2

FeCl3

3 Cl- -3 so Fe is +3

iron(III) chloride

Cr2S3

3 S-2 -6 so Cr is +3 (6/2)

chromium(III) sulfide

2.7


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2.7


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Covalent Bonding


The ties that bind

Why should two atoms share electrons?

+

8e-

7e-

7e-

8e-

F

F

F

F

F

F

F

F

lonepairs

lonepairs

single covalent bond

single covalent bond

lonepairs

lonepairs

A covalent bond is a chemical bond in which two or more electrons are shared by two atoms.

Lewis structure of F2

9.4


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single covalent bonds

H

H

H

H

or

H

H

O

2e-

2e-

O

8e-

O

C

O

C

O

O

double bonds

8e-

8e-

8e-

double bonds

O

N

N

triple bond

N

N

triple bond

8e-

8e-

Lewis structure of water

+

+

Double bond – two atoms share two pairs of electrons

or

Triple bond – two atoms share three pairs of electrons

or

9.4


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Lengths of Covalent Bonds

Bond Lengths

Triple bond < Double Bond < Single Bond

9.4


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9.4


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F

H

F

H

Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms

electron rich

region

electron poor

region

e- poor

e- rich

d+

d-

9.5


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X (g) + e- X-(g)

Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond.

Electron Affinity - measurable, Cl is highest

Electronegativity - relative, F is highest

9.5


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9.5


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Bond Polarity and Electronegativity

Electronegativity


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Drawing Lewis Structures

1. Add the valence electrons.

2. Identify the central atom (usually the one with the highest molecular mass, lowest electronegativity, or closest to the center of the periodic table).

3. Place the central atom in the center of the molecule and add all other atoms around it.

4.Place one bond (two electrons) between each pair of atoms.

5.Complete the octet for the central atom.

6.Complete the octets for all other atoms. Use double bonds if necessary.

7. Place remaining electrons on the central atom.


The ties that bind

Write the Lewis structure of nitrogen trifluoride (NF3).

F

N

F

F

Step 1 – N is less electronegative than F, put N in center

Step 2 – Count valence electrons N - 5 (2s22p3) and F - 7 (2s22p5)

5 + (3 x 7) = 26 valence electrons

Step 3 – Draw single bonds between N and F atoms and complete

octets on N and F atoms.

Step 4 - Check, are # of e- in structure equal to number of valence e- ?

3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons

9.6


Trial

Trial

  • Write the Lewis stucture for

    • H2O

    • CO2

    • NH3

    • CH4

    • N2

    • BH3


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Be – 2e-

2H – 2x1e-

H

Be

H

4e-

B – 3e-

3 single bonds (3x2) = 6

3F – 3x7e-

F

B

F

24e-

Total = 24

9 lone pairs (9x2) = 18

F

Exceptions to the Octet Rule

The Incomplete Octet

BeH2

BF3

9.9


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N – 5e-

S – 6e-

N

O

6F – 42e-

O – 6e-

48e-

11e-

F

6 single bonds (6x2) = 12

F

F

Total = 48

S

18 lone pairs (18x2) = 36

F

F

F

Exceptions to the Octet Rule

Odd-Electron Molecules

NO

The Expanded Octet (central atom with principal quantum number n > 2)

SF6

9.9


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Strengths of Covalent Bonds

Bond Enthalpies and Bond Length


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Increasing difference in electronegativity

Covalent

Polar Covalent

Ionic

partial transfer of e-

share e-

transfer e-

Classification of bonds by difference in electronegativity

Difference

Bond Type

0

Covalent

 2

Ionic

0 < and <2

Polar Covalent

9.5


The ties that bind

Classify the following bonds as ionic, polar covalent,

or covalent: The bond in CsCl; the bond in H2S; and

the NN bond in H2NNH2.

Cs – 0.7

Cl – 3.0

3.0 – 0.7 = 2.3

Ionic

H – 2.1

S – 2.5

2.5 – 2.1 = 0.4

Polar Covalent

N – 3.0

N – 3.0

3.0 – 3.0 = 0

Covalent

9.5


Test yourself1

Test Yourself

  • Write the formula and draw the ionic structure

    • Magnesium (II) bromide

    • Iron (III) Oxide

    • Sodium Azide

    • Ammonium Chloride

    • Magnesium Nitrate

    • Ammonium Phosphate

  • Write the name

    • FeO

    • Fe2O3

    • LiBr

    • KCrO4

    • K2Cr2O7


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