Molecularity is the number of molecules, atoms, or ions that collide simultaneously in a single elementary reaction step. For an elementary step (and only an elementary step), molecularity matches the stoichiometric coefficients, so it tells you the rate law directly.
Molecularity counts how many particles actually smash into each other in one elementary step of a reaction. One particle rearranging or breaking apart on its own is unimolecular. Two particles colliding is bimolecular. Three particles colliding at the exact same instant is termolecular, and the CED is explicit that these are rare. Getting three particles to hit each other simultaneously, with the right orientation and enough energy, is like trying to get three friends to high-five at the exact same millisecond.
Here's the payoff. For an elementary reaction, the rate law can be read straight off the stoichiometry of the colliding particles. A unimolecular step A → products has rate = k[A]. A bimolecular step A + B → products has rate = k[A][B], and A + A → products has rate = k[A]². This shortcut only works for elementary steps. For an overall reaction made of multiple steps, you must determine the rate law from experimental data, not from the balanced equation.
Molecularity lives in Topic 5.4 (Elementary Reactions) in Unit 5: Kinetics, and it directly supports learning objective 5.4.A, representing an elementary reaction as a rate law expression using stoichiometry. This is the one place in AP Chem where coefficients and rate-law exponents legally match, which is exactly why the exam loves testing it. Topic 5.4 is also the bridge between collision theory (5.2-5.3) and reaction mechanisms (5.7), where you'll use molecularity to write the rate law of the slow step and predict the overall rate law. If you can't read molecularity off an elementary step, mechanisms in 5.7 fall apart.
Keep studying AP Chemistry Unit 5
Elementary Step (Unit 5)
Molecularity only has meaning for an elementary step, a single collision event. An overall reaction doesn't have a molecularity at all, because it's really a sequence of separate collisions stacked together.
Collision Theory (Unit 5)
Collision theory explains why termolecular steps are rare. Every extra particle in a collision multiplies the improbability, since all of them need to arrive at the same point, at the same time, with enough energy and the right orientation.
Rate-Determining Step (Unit 5)
In a multistep mechanism, you apply molecularity to the slow step to write its rate law. If the slow step is bimolecular in A and B, the rate law is k[A][B], and that's what controls the speed of the whole reaction.
Order of the Reaction (Unit 5)
For an elementary step, molecularity and order happen to be the same number. For the overall reaction they can be totally different, because order comes from experiment while molecularity comes from counting particles in one collision.
Molecularity shows up almost entirely in multiple-choice and mechanism-based questions. Classic stems include: given a unimolecular step like the isomerization of cyclopropane, write the rate law (answer: rate = k[C₃H₆]); given several reactions labeled 'assume each is elementary,' pick the one whose rate-law exponents sum to 3 (that's a termolecular step); or evaluate whether a proposed single-step mechanism is consistent with an experimental rate law, like NO + O₃ → NO₂ + O₂ with rate = k[NO][O₃], where the bimolecular step matches the data. The trap version gives you initial-rates data for an overall reaction and tempts you to read the rate law from the balanced equation. Don't. Stoichiometry only gives you the rate law when the problem says the step is elementary. On mechanism FRQs, you'll use molecularity of the rate-determining step to justify a rate law in writing.
Molecularity is a count of particles in one collision; it's always a small whole number (1, 2, rarely 3) and is defined only for elementary steps. Reaction order describes how concentration affects rate for the overall reaction, is determined experimentally, and can be zero or even fractional. They're equal for a single elementary step, which is exactly why people mix them up, but for a multistep reaction the overall order tells you nothing about the molecularity of any individual step.
Molecularity is the number of particles that collide simultaneously in a single elementary step: unimolecular (1), bimolecular (2), or termolecular (3).
For an elementary reaction only, you can write the rate law directly from the stoichiometry, so a bimolecular step A + B → products has rate = k[A][B].
Termolecular steps are rare because three particles almost never collide at the same instant with the right energy and orientation.
You can never assume the overall balanced equation gives the rate law; that experimental rate law must come from data like an initial-rates table.
Molecularity equals reaction order for an elementary step, but the two concepts are not interchangeable for multistep reactions.
In a mechanism, the molecularity of the rate-determining (slow) step sets the rate law for the whole reaction.
Molecularity is the number of molecules, atoms, or ions that collide at the same time in one elementary reaction step. It's tested in Topic 5.4 (Unit 5: Kinetics), where it lets you write a rate law straight from the step's stoichiometry.
Only if the reaction is a single elementary step, and the problem has to tell you that. For any overall reaction, the rate law must be determined experimentally, which is why the exam gives you initial-rates data tables instead of letting you copy the coefficients.
Molecularity counts particles in one collision and applies only to elementary steps, so it's always 1, 2, or rarely 3. Order is found from experiment for the overall reaction and can be 0 or fractional. They match for an elementary step but not necessarily for the overall reaction.
Three particles must collide at exactly the same moment with sufficient energy and the correct orientation, and the probability of that drops sharply compared to a two-particle collision. The CED specifically notes that elementary reactions involving three or more particles are rare.
A unimolecular step involves just one particle decomposing or rearranging, like the isomerization of cyclopropane (C₃H₆). Its rate law is first order in that one species, rate = k[C₃H₆].