IMFs, Molecular, Polarity, Ionic compounds

An atom’s ability to attract shared electrons in a covalent bond.

What is electronegativity (EN)?

The difference between two bonded atoms’ electronegativities. Calculate it by subtracting the values and taking the absolute value: ΔEN = |EN₁ − EN₂|.

What is the difference in electronegativity (ΔEN)?

A covalent bond where electrons are shared unequally because the atoms have different electronegativities.

What is a polar bond?

δ+ means partially positive; δ− means partially negative. The more electronegative atom is usually δ−.

What do δ+ and δ− mean?

A separation of partial charges in a polar bond, caused by unequal sharing of electrons. Its arrow points toward the more electronegative atom.

What is a bond dipole?

A molecule with an uneven distribution of charge overall, meaning its bond dipoles do not cancel.

What is a polar molecule?

A molecule with no overall dipole. Its bonds may be nonpolar, or its polar bond dipoles may cancel because of its shape.

What is a nonpolar molecule?

Check whether the bonds are polar with EN calculation, examine the molecule’s 3D shape, and decide whether the bond dipoles cancel.

How do you determine molecular polarity?

CO₂ is linear and symmetrical, so its bond dipoles cancel. H₂O is bent, so its bond dipoles do not cancel.

Why is CO₂ nonpolar but H₂O polar?

BF₃ is trigonal planar and symmetrical, so its dipoles cancel. NH₃ is trigonal pyramidal, so its dipoles do not cancel.

Why is BF₃ nonpolar but NH₃ polar?

Bond polarity describes one bond. Molecular polarity describes the overall charge distribution of the entire molecule.

What is the difference between bond polarity and molecular polarity?

Attractive forces between separate molecules.

What are intermolecular forces (IMFs)?

Attractions caused by temporary changes in electron distribution. They occur in all atoms and molecules.

What are London dispersion forces (LDFs)?

Attractions between the partially positive end of one polar molecule and the partially negative end of another.

What are dipole–dipole forces?

A strong intermolecular attraction that occurs when H bonded directly to N, O, or F is attracted to N, O, or F on a neighbouring molecule.

What is hydrogen bonding?

For similar-sized molecules: London dispersion < dipole–dipole < hydrogen bonding

What is the usual order of IMF strength?

London dispersion forces.

Which IMF occurs in every molecular substance?

All molecules have LDFs. Polar molecules also have dipole–dipole forces. Molecules with H directly bonded to N, O, or F can also hydrogen-bond with each other.

How do you identify the strongest IMF in a substance?

A bond that holds atoms together within a molecule, such as a covalent O—H bond.

What is an intramolecular bond?

An attraction between separate particles or molecules, such as hydrogen bonding between two water molecules.

What is an intermolecular attraction?

Intramolecular bonds hold a molecule together. Intermolecular forces attract separate molecules.

What is the difference between intramolecular and intermolecular forces?

No. Hydrogen must be directly bonded to N, O, or F. H₂O can hydrogen-bond with itself; CH₄ cannot.

Can every molecule containing hydrogen form hydrogen bonds with itself?

I₂ has more electrons and a larger, more polarizable electron cloud, producing stronger London dispersion forces. More energy is needed to separate its molecules.

Why does iodine (I₂) boil at a higher temperature than fluorine (F₂), even though both are nonpolar?

Stronger IMFs generally mean a higher boiling point because more energy is needed to separate the molecules.

How do stronger IMFs affect boiling point?

Stronger IMFs make it harder for molecules to escape into the gas phase, so evaporation is generally slower.

How do IMFs affect evaporation?

Stronger attractions often lead to higher melting points, but molecular shape and how particles pack in a solid also matter.

How do IMFs affect melting point?

Water molecules form hydrogen bonds, which require extra energy to overcome during boiling.

Why does water have a relatively high boiling point?

CH₃OH forms hydrogen bonds, while CH₄ has only London dispersion forces. The stronger attractions in CH₃OH require more energy to overcome

Why does CH₃OH have a higher boiling point than CH₄?

H₂O forms hydrogen bonds; H₂S does not. Water therefore has stronger intermolecular attractions.

Why does H₂O have a higher boiling point than H₂S?

Attraction between particles of the same substance, such as water molecules attracting one another.

What is cohesion?

Attraction between different substances, such as water molecules and a waxy surface.

What is adhesion?

Cohesion between the liquid’s molecules is strong relative to adhesion to the surface, so the liquid tends to minimize its contact area.

Why does a liquid form a rounded, beaded drop?

Adhesion to the surface is strong relative to cohesion within the liquid, encouraging the liquid to spread.

Why does a liquid spread across a surface?

Strong electrostatic attractions between oppositely charged ions in the crystal lattice require substantial energy to overcome.

Why do ionic compounds usually have high melting points?

Water is polar, so it generally dissolves ionic compounds and polar substances more readily than nonpolar substances. Dissolving depends on the attractions between solute and solvent particles.

Why do some molecular substances dissolve in water while others do not?

Identify the particles and their attractions, then explain how the attractions cause the property.

What is the best way to explain a compound’s physical properties?

A nonpolar molecular substance. Its molecules have relatively weak intermolecular attractions, it is insoluble in polar water, and it has no mobile ions.

A solid has a low melting point, does not dissolve in water and does not conduct electricity. What is it most likely?

Boiling point generally increases because more energy is needed to separate molecules.

What happens to boiling point when intermolecular forces become stronger?

Yes. If its shape is symmetrical, the bond dipoles can cancel. Example: CO₂.

Can a molecule have polar bonds but be nonpolar overall?

Boiling overcomes intermolecular attractions. It does not normally break the covalent bonds inside molecules.

What is the difference between boiling and breaking covalent bonds?

Polar substances tend to dissolve in polar solvents, while nonpolar substances tend to dissolve in nonpolar solvents.

What does “like dissolves like” mean?

Surface tension results from cohesive attractions between liquid particles, making the surface resist being stretched or disturbed.

What is surface tension, and how is it related to cohesion?

Cohesion attracts molecules of the same liquid; adhesion attracts the liquid to other surfaces. Their relative strengths affect whether a liquid beads up or spreads.

How do cohesion and adhesion affect a liquid’s behaviour?

Stronger intermolecular forces generally lead to higher boiling points because more energy is needed to separate molecules. They also affect evaporation, surface tension, and other physical properties.

How do intermolecular forces affect the physical properties of a substance?