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Types of chemical bonds

Bond: Force that holds groups of two or more atomstogether and makes the atoms function as a unit.

Example: H-O-H

Bond Energy: Energy required to break a bond.

Ionic Bond: Attractions between oppositely charged ions.

Example: Na+ Cl-

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Types of chemical bonds

Ionic Compound: A compound resulting from a positive ion

(usually a metal) combining with a negative ion (usually

a non-metal).

Example: M+ + X- MX

Covalent Bond: Electrons are shared by nuclei.

Example: H-H

Polar Covalent Bond: Unequal sharing of electrons bynuclei.

Example: H-F

Hydrogen fluoride is an example of a molecule that has

bond polarity.

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Lewis structures

Lewis Structure: Representation of a molecule that shows

how the valence electrons are arranged among the atoms

in the molecule.

Bonding involves the valence electrons of atoms.

Example: Na● H-H

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Lewis structures of

elements Dots around elemental symbol

– Symbolize valence electrons

Thus, one must know valence electronconfiguration

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Lewis Structures of

moleculesSingle Bond: Two atoms sharing one electron pair.

Example: H2

Double Bond: Two atoms sharing two pairs of electrons.

Example: O2

Triple Bond: Two atoms sharing three pairs of electrons.

Example: N2

Resonance Structures: More than one Lewis Structure can be drawn for a molecule.

Example: O3

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Rules for Lewis structures of

molecules1. Write out valence electrons for each atom2. Connect lone electrons because lone

electrons are destabilizing1. Become two shared electrons

1. Called a “bond”

3. Check to see if octet rule is satisfied1. Recall electron configuration resembling noble

gas1. In other words, there must be 8 electrons (bondedor non-bonded) around atom1. Non-bonded electron-pair

1. Called “lone pair”

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Let’s do some examples

on the board H2

– Duet rule

F2

– Octet rule

O2

N2

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Lewis structures

Example

Write the Lewis Structure for the following

molecules:1) H2O

2) CCl4

1) Where does the carbon go & why?3) PH3

4) H2Se

5) C2

H6

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Lewis structures

continued6) CO2

7) C2H4

8) C2H2

9) SiO2

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Polyatomic ions

If positive charge on ion

Take away electron from central species

If negative charge on ion

Add electron to central species

Example:

H3O+

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Your turn

NH4+

ClO-

OH-

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Resonance structures

When structures can be written inmore than one way

– O3

Actual molecule is “in-between” – Resonance hybrid

Another example – HCO3-

What would its resonance hybrid look like?

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Practice

NO2-

NO3-

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Practice

H2O2

H3O+

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Aberrant compounds

Odd-electron species

– NO

– NO2

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Aberrant compounds

Incomplete octet

– BH3

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Aberrant compounds

Expanded octet

– Some central atoms can exceed an octet

Third period and higher elements cando this

– E.g., Al, Si, P, S, Cl, As, Br, Xe, etc.

– d-orbitals can accommodate extraelectrons

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Examples

AsI5

XeF2

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Practice

SCl6 XeF4

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Practice

SO32-

PO33-

SO2

SO3

H2SeO4

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Electronegativity

Electronegativity: The relative ability of an atom in amolecule to attract shared electrons to itself.

Example: Fluorine has the highest electronegativity.

Similar electronegativities between elements give non- polar covalent bonds (0.0-0.4)

Different electronegativities between elements give

polar covalent bonds (0.5-1.9) If the difference between the electronegativities of two

elements is about 2.0 or greater, the bond is ionic

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Electronegativity

Example

For each of the following pairs of bonds,

choose the bond that will be more polar.

Al-P vs. Al-N

C-O vs. C-S

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Dipole moment

Dipole Moment

A molecule that has a center of positive

charge and a center of negative charge Will line up on electric field

In Debye units

1 D = 3.34 x 10

-30

C

m

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Examples

F2

CO2

H2O

NH3

BF3

CCl4

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Molecular polarity

Net-dipole moment leads to molecularpolarity

Thus the following two that have net-dipole moments are polar:

– H2O

– NH3

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Molecular structure

Molecular Structure: or geometric structure refers to thethree-dimensional arrangement of the atoms in amolecule.

Bond Angle: The angle formed between two bonds in amolecule.

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Molecular structure:

VSEPRThe VSEPR Model: The valence shell electron pair

repulsion model is useful for predicting the molecularstructures of molecules formed from nonmetals.

The structure around a given atom is determined byminimizing repulsions between electron pairs.

The bonding and nonbonding electron pairs (lone pairs)around a given atom are positioned as far apart as possible.

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Molecular Structure:

VSEPRSteps for Predicting Molecular Structure Using the VSEPR

Model

1. Draw the Lewis structure for the molecule.

2. Count the electron pairs and arrange them in the way

that minimizes repulsion (that is, put the lone pairs as

far apart as possible).

3. Determine the positions of the atoms from the way the

electron pairs are shared.

4. Determine the name of the molecular structure from the

positions of the atoms.

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Example

Br 2

CO2

CF4

PF3

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Your turn

NH4+

XeF4

AsI5

SF3+

I3

-

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139_chap10