Know that the forces and distances between particles (atoms, molecules, ions and electrons) and the motion of the particles affects the properties of solids, liquids and gases

Cambridge IGCSE Physics 0625 – 2.1.2 Particle Model

Objective

Understand how the forces and distances between particles (atoms, molecules, ions and electrons) and the motion of those particles determine the observable properties of solids, liquids and gases.

1. Particles and Forces

The particle model assumes that all matter is made up of tiny particles that are in constant motion. The nature of the forces between these particles governs the behaviour of the material.

• Attractive forces – pull particles toward each other.
• Repulsive forces – prevent particles from occupying the same space.
• Both forces depend on the distance between particles; the magnitude typically follows an inverse‑square or exponential relationship.

2. Inter‑particle Forces in Different States

3. Motion of Particles

Particle motion is characterised by kinetic energy and speed. The average kinetic energy of particles in a substance is directly proportional to its absolute temperature:

\$\$

\langle Ek \rangle = \frac{3}{2}k{\mathrm B}T

\$\$

where \$k_{\mathrm B}\$ is Boltzmann’s constant and \$T\$ is the temperature in kelvin.

• In solids, particles vibrate about fixed positions.
• In liquids, particles vibrate and also translate, allowing flow.
• In gases, particles move freely in random directions, colliding with each other and the container walls.

4. How Forces and Motion Determine Macroscopic Properties

1. Density: Determined by mass of particles per unit volume. Stronger attractive forces pull particles closer, increasing density (as in solids).
2. Shape and \cdot olume: Fixed inter‑particle forces give solids a permanent shape. Weaker forces in liquids allow them to take the shape of their container while retaining volume. In gases, both shape and volume are determined by the container.
3. Pressure: In gases, pressure arises from particle collisions with the container walls:

   \$p = \frac{F}{A} = \frac{1}{3}\rho \overline{v^2}\$

   where \$\rho\$ is the gas density and \$\overline{v^2}\$ is the mean square speed of the molecules.

4. Viscosity: Resistance to flow depends on the strength of intermolecular forces; liquids with strong forces (e.g., honey) are more viscous than those with weak forces (e.g., water).
5. Thermal Conductivity: Energy transfer by particle collisions is more efficient when particles are close together and interact strongly (metals, solids) than when they are far apart (gases).

5. Example: Phase Changes

During melting, heating provides kinetic energy that overcomes part of the attractive forces, increasing inter‑particle distance and allowing particles to move past one another while the temperature remains constant.

During evaporation, particles at the surface acquire enough kinetic energy to completely break free from the attractive forces, entering the gas phase.

6. Summary Checklist

• Identify the type of particles involved (atoms, molecules, ions, electrons).
• Describe how inter‑particle distance changes from solid → liquid → gas.
• Explain how the strength of attractive forces influences density, shape, and volume.
• Relate particle motion (speed, kinetic energy) to temperature and pressure.
• Use the particle model to predict the effect of heating or cooling on a material’s state.