recall from ionic bonding… the partially positive hydrogens of the water molecules attract and...

Recall from Ionic Bonding… The partially positive

hydrogens of the water molecules attract and pull away the negative chloride ions.

The partially negative oxygens of the water molecules attract and pull away the positive sodium ions.

Big Question: For one substance to dissolve another, what is required of both substances?

Intermolecular Forces

inter + molecular forces = forces between molecules


inter + molecular forces = forces between molecules

Intermolecular forces are attractions between molecules.

Intermolecular forces are weaker than covalent or ionic bonds.

Are caused by molecule polarity


• Determine whether a molecule is a gas, liquid, or solid at a given temperature.

Think and Share• How do you think intermolecular forces may

determine the state of matter of a substance?


Why don’t oil and water mix?

Oil is nonpolar.Water is polar.

• Determine the ability of substances to dissolve one another.

– “Like dissolves like”

– Polar molecules dissolve polar molecules and ionic compounds

– Nonpolar molecules dissolve nonpolar molecules only

IMF’s and Solubility

How do intermolecular forces influence the other physical properties of substances?

The diversity of physical properties among covalent compounds is mainly because of widely varying intermolecular attractions.

Properties of SubstancesViscosity

The ability of a substance to resist flowing

Surface TensionHow difficult it is to break the surface of a liquid

Properties of Substances

Evaporation and Vapor PressureA change from liquid to gas


Melting Point

The temperature at which a substance changes from solid to liquid or vice-versa


Boiling Point

The temperature at which a substance changes from liquid to gas or vice-versa

3 Types of Intermolecular ForcesListed in order of increasing strength:•London dispersion forces [low strength[•Dipole-dipole interactions•Hydrogen bonding [highest strength]

van der Waals forces=

London dispersion forces + dipole-dipole interactions

Dipole= separation of + and – charge

a molecule with a + end and a - end

• Occur in ALL molecules (non-polar and polar)

• The weakest of molecular interactions

• Caused by the motion of electronsElectrons are in constant motion. At times, more electrons can temporarily be on one side of a molecule than another.

What do you think happens to the molecule when more electrons are on one side than another?

London Dispersion Forces


Temporary dipoles caused by the movement of electrons in a molecule.

• A single dispersion force is very weak by itself.

• However, when millions and millions of dispersion forces happen in a substance the force dispersion force is a significant intermolecular force.

• The strength of dispersion forces increases as the number of electrons in a molecule increases.


At ordinary room temperature and pressure:

Fluorine and Chlorine = gases

Bromine = liquid

Iodine = solidWhy do you think this is? Explain in terms of dispersion forces.

• Occur in ALL molecules- polar and nonpolar

• The only type of intermolecular force found in nonpolar molecules


Dipole-Dipole Forces

• The + and – ends of polar molecules attract one another.

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

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

Dipole-Dipole Forces

Do you think Dipole-Dipole Forces will occur in nonpolar molecules?

Hydrogen Bonding• Strongest type of

intermolecular force

• Hydrogen bonding always involves hydrogen

• Hydrogen is bonded to a very electronegative atom.

A specialized type of dipole-dipole force

Hydrogen bonding occurs ONLY when a H is

bonded directly to oxygen, nitrogen, or fluorine

A single hydrogen bond has about 5 percent of the strength of the average covalent bond.

Hydrogen bonds are extremely important in determining the properties of water and biological molecules.

Hydrogen Bonding

However, lots of hydrogen bonds add up to be of significant strength.

The “Glue” Between Molecules• Dispersion force

• Dipole-dipole force

• Hydrogen bonding

weakest IM force

strongest IM force

Questions (Think-Pair-Share)

What causes dispersion forces?

Which types of molecules have dispersion forces? (nonpolar or polar)

What types of molecules have dipole-dipole forces?

Determine the strongest type of IM Force in:

Dihydrogen sulfide

Sulfur monoxide

Dinitrogen dihydride

Carbon tetrachloride

How Do you Think IM Forces Affect the Following?

ViscosityThe ability of a substance to resist flowing

Surface TensionHow difficult it is to break the surface of a liquid

EvaporationA change from liquid to gas

Sate of MatterBoiling Point Melting Point

How Do you Think IM Forces Affect the Following?

Viscosityhigh viscosity = STRONG IM Forces

Surface Tensionhigh surface tension = STRONG IM Forces

Evaporationfast evaporation rates = WEAK IM Forces

State of MatterHigh BP = STRONG IM Forces High MP = SRONG IM Forces

recall from ionic bonding… the partially positive hydrogens of the water molecules attract and...

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