Section 8.4

Polar Bonds and Molecules

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Bond PolarityCovalent bonds involve electron sharing between atoms. However, they differ in terms of how the bonded atoms share the electrons.

The bonding pairs of electrons are pulled between the nuclei of the atoms sharing the e-

When the atoms in the bond pull Equally (identical atoms are bonded), the bonding electrons are shared equally and the bond is a nonpolar covalent bond.

H2, O2, N2, and Cl2 all have nopolar covalent bonds2

Bond PolarityA polar covalent bond (polar bond), is a covalent bond between atoms in which the electrons are shared unequally.

The more electronegative atoms attracts electrons more strongly and gains a slightly negative charge.

The less electronegative atoms has a slightly positive charge.

Fluorine is the most electronegative and oxygen is the second most electronegative element.

Electronegativity describes the attraction an atom has for electrons when the atom is in a compound.

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Electronegativity Values of Some Elements

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Bond PolarityThe electronegativity difference between two atoms tells you what kind of bond is likely to form.

As the electronegativity difference between two atoms increases, the polarity of the bond increases.

If the electronegativity difference is greater than 2.0, it is very likely that electrons will be pulled away completely by one of the atoms. (an ionic bond will form)

EN Differences Probable Bond Type Example

0.0 – 0.4 Non polar covalent H – H (0.0)

0.5 – 0.99 Moderately polar covalent H – Br (0.8)

1.0 – 2.0 Very polar covalent H – F (1.8)

> 2.0 ionic Na – Cl (2.27)6

Bond PolarityIn hydrogen chloride (HCl) molecule, the H has an electronegativity of 2.2 and the chlorine has an electronegativity of 3.2.

The values are significantly different, so the covalent bond in HCl is polar.

The chlorine atom acquires a slightly negative charge and the hydrogen atoms acquires a slightly positive charge.

Greek letter delta () indicated that the atoms involved in the covalent bond acquire only partial charges, less than 1+ or 1-. + - + - + + -

H – Cl H – O – H H - Br 7

ProblemsPlace the following covalent bonds in order from least to most polar. a. H – Cl b. H – Br c. H – S d. H - C

c & d (tie), b, a

Identify the bonds between atoms of each pair of elements as nonpolar covalent, moderately polar covalent, very polar covalent, or ionic.

a. H & Br b. K & Cl c. C & O d. Cl & F e. Li & O

f. Br & Br

a. moderately polar covalent b. ionic c. nonpolar covalent d. nonpolar covalent e. ionic f. nonpolar 8

Polar MoleculesThe presence of a polar bond in a molecule often makes the entire molecule polar.

In a polar molecule, one end of the molecule is slightly negative and the other end is slightly positive.

In HCl, the partial charges on the hydrogen and chlorine atoms are electrically charged regions or poles.

A molecule that has two poles is called a dipolar molecule.

When polar molecules are placed between oppositely charged plates, they tend to become oriented with respect to the positive and negative plates.

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Polar MoleculesCO2 has two polar bonds and is linear. The C and O lies along the same axis, thus the bond polarities cancel because they are in opposite directions.

CO2 is a nonpolar molecule despite the presence of two polar bonds.

Water also has two polar bonds. The water molecule is bent and the bond polarities do not

cancel and the water molecule is polar.

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Attractions Between Molecules

Molecules can attract each other by a variety of forces.

Intermolecular attractions are weaker than either ionic or covalent bonds.

Intermolecular forces are responsible for determining whether a molecular compound is a gas, a liquid, or a solid at a given temperature.

Van der Waals Forces – are the two weakest attractions between molecules and include both:

• dipole interactions • dispersion forces

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Dipole Interactions

Dipole interactions occur when polar molecules are attracted to one another. The attraction involved occurs between the oppositely charged regions of polar molecules.

The slightly negative region is weakly attracted to the slightly positive regions of another molecule.

Dipole interactions are similar to but much weaker than ionic bonds.

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Dipole Interactions

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Dispersions ForcesDispersion forces are the weakest of all molecular interactions and are caused by the motion of electrons.

They occur between nonpolar molecules.

When the moving electrons happen to be momentarily more on the side of a molecule closest to a neighboring molecule, their electric force influences the neighboring molecule’s electrons to be momentarily more on the opposite side.

This causes an attraction between the two molecules similar but much weaker than the attraction between polar molecules.

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Dispersions ForcesThe strength of dispersion forces generally increases as the number of electrons in a molecule increases.

The halogen diatomic molecules attract each other mainly by means of dispersion forces.

Fluorine and chlorine has relatively few electrons and are gases at room temperature because of their weak dispersion forces.

Bromine has a larger number of electrons and generates larger dispersion forces. Bromine molecules attract each other sufficiently to make it a liquid at room temperature.

Iodine, with a larger number of electrons, is a solid at room temperature.

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Dispersions Forces

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Hydrogen BondsThe dipole interactions in water produce an attraction between water molecules.

Each O – H bond in the water molecule is very polar and the O acquires a slightly negative charge.

The positive region of one water molecules attracts the negative region of another water molecule.

This attraction between the hydrogen of one water molecule and the oxygen of another water molecule is strong compared to other dipole interaction.

This strong attraction, (also found in other H containing molecules) is called hydrogen bonding.

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Hydrogen BondsHydrogen bonds are attractive forces in which a hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom.

The other atom may be in the same molecule or in a nearby molecule.

Hydrogen bonding always involves hydrogen.

The bond between hydrogen and a very electronegative atom is strongly polar.

Hydrogen bonds are the strongest of the intermolecular forces.

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End of Chapter 8

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