Polarity of Molecules

Electronegativity

The pull an atom has for the electrons it shares with another atom in a bond.

Electronegativity is a periodic trendAs atomic radius increases and number of

electron shells increases, the nucleus of an atom has less of a pull on its outermost electrons

Periodic Table with Electronegativies

increases

decreases

Polar Bond

A polar covalent bond is when there is a partial separation of charge

One atom pulls the electrons closer to itself and has a partial negative charge.

The atom that has the electrons farther away has a partial positive charge

Model Electronegativity of central atom

Electronegativity of outer atom(s)

Subtraction Total

Type of Bond

3.0 3.0 0 Non-polar Covalent

N N

Two atoms sharing equally

N N

Each nitrogen atom has an electronegativity of 3.0

They pull evenly on the shared electrons

The electrons are not closer to one or the other of the atoms

This is a non-polar covalent bond

Atoms sharing almost equally

Electronegativities: H = 2.1 C = 2.5

The carbon pulls on the electrons slightly more, pulling them slightly towards the carbon

Put the difference isn’t enough to create a polar bond

This is a non-polar covalent bond

C HH

H

H

Sharing unevenly

Electronegativities: H = 2.1 C = 2.5 O = 3.5

The carbon-hydrogen difference isn’t great enough to create partial charges

But the oxygen atoms pulls significantly harder on the electrons than the carbon does. This does create a polar covalent bond

This is a polar covalent bond

C OH

H

Showing Partial Charges

There are two ways to show the partial separation of chargesUse of “” for “partial” Use of an arrow pointing towards the partial

negative atom with a “plus” tail at the partial positive atom

C OH

H

+ -C OH

H

Ionic Bonds

Ionic bonds occur when the electronegativies of two atoms are so different that they can’t even share unevenly…one atom just takes them from the other

How to determine bond type

Find the electronegativies of the two atoms in the bond

Find the absolute value of the difference of their valuesIf the difference is 0.4 or less, it’s a non-polar

covalent bondIf the difference is greater than 0.4 but less than

1.4, it’s a polar covalent bondIf the difference is greater than 1.4, it’s an ionic

bond

Let’s Practice

Example:If the bond

is polar, draw the polarity arrow

C – H

O—Cl

F—F

C—Cl

Let’s Practice

Example:If the bond

is polar, draw the polarity arrow

C – H

O—Cl

F—F

C—Cl

2.5 – 2.1 = 0.4 non-polar

3.5 – 3.0 = 0.5 polar

4.0 – 4.0 = 0.0 non-polar

2.5 – 3.0 = - 0.5 polar

Polar Bonds versus Polar Molecules

Not every molecule with a polar bond is polar itselfIf the polar bonds cancel out then the molecule

is overall non-polar.

The polar bonds cancel out.No net dipole

The polar bonds do not cancel out.

Net dipole

The Importance of VSEPR

You must think about a molecule in 3-D (according to VSEPR theory) to determine if it is polar or not!

Water drawn this way shows all the polar bonds canceling out. But water drawn in

the correct VSEPR structure, bent, shows the polar bonds don’t cancel out!

Net dipole

H O H

O H H

Let’s Practice

Example:Is NH3 a

polar molecule?

Let’s Practice

Example:Is NH3 a

polar molecule?

NH H

HElectronegativities:N = 3.0H = 2.1Difference = 0.9 Polar bonds

VSEPR shape = Trigonal pyramidal

Net dipole

Yes, NH3 is polar

Intermolecular Forces

Intra- versus Inter-molecular Forces

So far this chapter has been discussing intramolecular forcesIntramolecular forces = forces within the

molecule (chemical bonds)

Now let’s talk about intermolecular forcesIntermolecular forces = forces between

separate molecules

Breaking Intramolecular forces

Breaking of intramolecular forces (within the molecule) is a chemical change2 H2 + O2 2 H2O

Bonds are broken within the molecules and new bonds are formed to form new molecules

Breaking Intermolecular forces

Breaking of intermolecular forces (between separate molecules) is a physical changeBreaking glass is breaking the intermolecular

connections between the glass molecules to separate it into multiple pieces.

Boiling water is breaking the intermolecular forces in liquid water to allow the molecules to separate and be individual gas molecules.

London Dispersion Forces

All molecules have electrons.

Electrons move around the nuclei. They could momentarily all “gang up” on one side

This lop-sidedness of electrons creates a partial negative charge in one area and a partial positive charge in another.

+ Positively charged nucleus - Negatively charged electron

+-

-

-

-

Electrons are fairly evenly dispersed.

+--

- -As electrons move, they “gang up” on one side.

+

-

London Dispersion Forces

Once the electrons have “ganged up” and created a partial separation of charges, the molecule is now temporarily polar.

The positive area of one temporarily polar molecule can be attracted to the negative area of another molecule.

+ - + -

Strength of London Dispersion Forces

Electrons can gang-up and cause a non-polar molecule to be temporarily polar

The electrons will move again, returning the molecule back to non-polar

The polarity was temporary, therefore the molecule cannot always form LDF.

London Dispersion Forces are the weakest of the intermolecular forces because molecules can’t form it all the time.

Strength of London Dispersion Forces

Larger molecules have more electrons

The more electrons that gang-up, the larger the partial negative charge.

The larger the molecule, the stronger the London Dispersion Forces

Larger molecules have stronger London Dispersion Forces than smaller molecules.

All molecules have electrons…all molecules can have London Dispersion Forces

Dipole Forces

Polar molecules have permanent partial separation of charge.

The positive area of one polar molecule can be attracted to the negative area of another molecule.

+ - + -

Strength of Dipole Forces

Polar molecules always have a partial separation of charge.

Polar molecules always have the ability to form attractions with opposite charges

Dipole forces are stronger than London Dispersion Forces

Hydrogen Bonding

Hydrogen has 1 proton and 1 electron.There are no “inner” electrons. It bonds with the only

one it has.When that electron is shared unevenly (a polar

bond) with another atom, the electron is farther from the hydrogen proton than usual.This happens when Hydrogen bonds with Nitrogen,

Oxygen or FluorineThis creates a very strong dipole (separation of

charges) since there’s no other electrons around the hydrogen proton to counter-act the proton’s positive charge.

Strength of Hydrogen Bond

Hydrogen has no inner electrons to counter-act the proton’s charge

It’s an extreme example of polar bonding with the hydrogen having a large positive charge.

This very positively-charged hydrogen is highly attracted to a lone pair of electrons on another atom.

This is the strongest of all the intermolecular forces.

Hydrogen Bond

N

H H

N

H H

Hydrogen bond

Intermolecular Forces & Properties

IMF’s and Properties

IMF’s are Intermolecular ForcesLondon Dispersion ForcesDipole interactionsHydrogen bonding

The number and strength of the intermolecular forces affect the properties of the substance.

It takes energy to break IMF’sEnergy is released when new IMF’s are

formed

IMF’s and Changes in State

Some IMF’s are broken to go from solid liquid. All the rest are broken to go from liquid gas.

Breaking IMF’s requires energy.

The stronger the IMF’s, the more energy is required to melt, evaporate or boil.

The stronger the IMF’s are, the higher the melting and boiling point

Water

Water is a very small moleculeIn general small molecules have low melting and

boiling pointsBased on it’s size, water should be a gas under

normal conditionsHowever, because water is polar and can form

dipole interactions and hydrogen bonding, it’s melting point is much higher

This is very important because we need liquid water to exist!

IMF’s and Viscosity

Viscosity is the resistance to flowMolasses is much more viscous than

water

Larger molecules and molecules with high IMF’s become inter-twined and “stick” together more

The more the molecules “stick” together, the higher the viscosity

Solubility

In order from something to be dissolved, the solute and solvent must break the IMF’s they form within itself

They must then form new IMF’s with each other

Solubility

- +

- +

- + - +- +

Solvent, water (polar)

+

-- + Solute, sugar (polar)

Water particles break some intermolecular forces with other water molecules (to allow them to spread out) and begin to form new ones with the sugar molecules.

Solubility

Solvent, water (polar)

+

-- + Solute, sugar (polar)

As new IMF’s are formed, the solvent “carries off” the solute—this is “dissolving”

- +

- +

- +- + - +

Solubility

If the energy needed to break old IMF’s is much greater than the energy released when the new ones are formed, the process won’t occurAn exception to this is if more energy is added

somehow (such as heating)

Oil & Water

Water has London Dispersion, Dipole and hydrogen bonding. That takes a lot of energy to break

Water can only form London Dispersion with the oil. That doesn’t release much energy

Much more energy is required to break apart the water than is released when water and oil combine.

Water is polar and can hydrogen bond, Oil is non-polar.

Therefore, oil and water don’t mix!

Surface Tension

Surface tension is the resistance of a liquid to spread out.This is seen with water on a freshly waxed car

The higher the IMF’s in the liquid, the more the molecules “stick” together.

The more the molecules “stick” together, the less they want to spread out.

The higher the IMF’s, the higher the surface tension.

Soap & Water

Soap has a polar head with a non-polar tail

The polar portion can interact with water (polar) and the non-polar portion can interact with the dirt and grease (non-polar).

Polar head

Non-polar tailSoap

Soap & Water

The soap surrounds the “dirt” and the outside of the this Micelle can interact with the water.

The water now doesn’t “see” the non-polar dirt.

Dirt

Soap & Surface Tension

The soap disturbs the water molecules’ ability to form IMF’s and “stick” together.

This means that the surface tension of water is lower when soap is added.

The lower surface tension allows the water to spread over the dirty dishes.

What did you learn about soap?

Soap

Inter-molecular forces

Inter-molecular forces

Works based on

Molecular Geometry

Molecular Geometry

Bonding types &

Structures

Bonding types &

Structures

Determined by

Determined by