AP Chemistry · Exam guide

AP Chemistry Particle-Level Explanations: How to Explain Trends, Boiling Points and Bonding

Many AP Chemistry free-response questions end with a short explanation that asks why. Points are awarded for naming the particles and the force between them, not for describing what happens. This guide gives a pattern you can reuse and two worked examples.

A four-part pattern

Step	What to write
1. Name the particles	Atoms, ions, molecules or electrons, with their identity, such as Na atoms or ethanol molecules.
2. Name the force or interaction	Coulombic attraction, hydrogen bonding, London dispersion forces, dipole-dipole attractions, or ionic bonding.
3. Compare	Say which is stronger or larger and why, using a number such as charge, distance, or number of electrons.
4. Connect to the observation	Say what that difference does to the property, such as the energy required to remove an electron or to boil the liquid.

Example 1: ionization energy

Explain why the first ionization energy of potassium is lower than that of sodium.

Does not earn the pointPotassium is bigger, so it loses an electron more easily.

It gives a fact about size but not the force or the reasoning.

Earns the pointIn both atoms the valence electron feels about the same effective nuclear charge, but in potassium it is in the fourth energy level, farther from the nucleus and shielded by more inner electrons. The Coulombic attraction between the nucleus and that electron is weaker, so less energy is needed to remove it.

It names the electron, the force, the comparison and the effect on the property.

Example 2: boiling point

Ethanol (C2H5OH) boils at about 78 degrees Celsius, and dimethyl ether (CH3OCH3), which has the same formula, boils at about -24 degrees Celsius. Explain the difference.

Earns the pointEthanol molecules have an O-H bond, so they form hydrogen bonds with one another. Dimethyl ether molecules have no hydrogen bonded to oxygen and attract each other only through weaker dipole-dipole forces and dispersion forces. More energy is needed to overcome the hydrogen bonds between ethanol molecules, so ethanol has the higher boiling point.

It compares the strongest force in each substance and links it to the energy needed to separate the molecules.

Common mistakes

Saying that bonds break when a molecular substance boils. The intermolecular attractions are overcome, not the covalent bonds.
Describing a trend without naming the force that causes it.
Using words like "stability" or "wants" for atoms.
Comparing two things without saying which one is larger and why.
Forgetting to mention the species by name.

Frequently asked questions

What does particle-level mean?

An explanation in terms of atoms, ions, molecules and electrons, and the forces between them.

Do I always need to name the force?

Yes. Naming the force, such as hydrogen bonding or Coulombic attraction, is usually the point the question is built around.

How long should an explanation be?

Two to four sentences. One sentence per step of the pattern is a good guide.

What if two things differ in several ways?

Pick the difference that most affects the property, and say why it matters most.

Where can I find the official scoring rules?

Exam formats change a little from year to year, so check the current course and exam description on the College Board site before test day.

Keep practicing

Every lesson in the AP Chemistry course ends with practice items and an Exam focus box, and every unit has a ten-question practice set with worked answers. These free lessons also build the skills the free-response section tests:

See the AP Chemistry course Free lessons and lesson list

Other free-response guides:

AP is a registered trademark of the College Board, which is not affiliated with and does not endorse this site. This guide is our own explanation of the question types and scoring. For the official scoring guidelines, see the College Board.