Know that microscopic particles may be moved by collisions with light fast-moving molecules and correctly use the terms atoms or molecules as distinct from microscopic particles
Cambridge IGCSE Physics 0625 – 2.1.2 Particle Model
2.1.2 Particle Model
Learning Objective
Students will be able to:
Explain how microscopic particles are set into motion by collisions with fast‑moving molecules.
Distinguish clearly between the terms atoms, molecules and the more general term microscopic particles.
Key Concepts
All matter is composed of particles that are too small to be seen with the naked eye.
These particles are in constant random motion; the speed of this motion increases with temperature.
Collisions between particles transfer kinetic energy and can cause motion of otherwise stationary particles.
The terminology:
Atom – the smallest unit of a chemical element that retains its chemical properties.
Molecule – two or more atoms chemically bonded together.
Microscopic particle – a generic term that includes atoms, molecules, ions, electrons, neutrons, etc.
How Collisions Cause Motion
When a fast‑moving molecule (or atom) collides with a stationary microscopic particle, part of its kinetic energy is transferred. The result is that the previously stationary particle begins to move. This process can be described qualitatively and, where appropriate, quantitatively.
For a perfectly elastic collision between two particles of masses \$m1\$ and \$m2\$ with initial velocities \$u1\$ and \$u2\$, the final velocities \$v1\$ and \$v2\$ are given by:
\$\$
v1 = \frac{(m1 - m2)u1 + 2m2 u2}{m1 + m2}, \qquad
v2 = \frac{(m2 - m1)u2 + 2m1 u1}{m1 + m2}
\$\$
In most IGCSE contexts we consider the target particle initially at rest (\$u_2 = 0\$), simplifying the expression for the target’s speed after impact to:
\$\$
v2 = \frac{2 m1}{m1 + m2}\,u_1
\$\$
Distinguishing Atoms, Molecules and Microscopic Particles
Term
Definition
Example
Typical Size (≈)
Atom
Smallest unit of a chemical element that retains its properties.
Hydrogen atom (H), Carbon atom (C)
0.1 nm
Molecule
Two or more atoms chemically bonded together.
O₂, H₂O, CO₂
0.2–0.5 nm
Microscopic particle
Any particle too small to be seen directly; includes atoms, molecules, ions, electrons, neutrons, etc.
Electron, ion (Na⁺), dust particle in air
10⁻⁹ m to 10⁻⁶ m (varies widely)
Practical Example: Brownian Motion
Brownian motion is the observable jittery movement of tiny particles (e.g., pollen grains) suspended in a fluid. The motion results from countless collisions with the much smaller, rapidly moving molecules of the fluid.
Suggested diagram: A series of arrows representing fast‑moving molecules striking a larger suspended particle, causing it to jiggle.
Summary Checklist
All matter consists of microscopic particles that are always moving.
Higher temperature → faster particle motion → more energetic collisions.
Collisions can transfer kinetic energy, setting previously stationary particles into motion.
Remember:
Atoms = single element units.
Molecules = groups of atoms bonded together.
Microscopic particles = the broad category that includes both.
Sample Examination Questions
Explain why a dust particle suspended in air moves randomly even though it is much larger than an air molecule.
State the difference between an atom and a molecule. Give one example of each.
In a sealed container, the temperature is increased. Describe qualitatively how this affects the frequency and energy of collisions between gas molecules and a suspended microscopic particle.