Particle Model and Density of States

The kinetic particle model explains the properties of solids, liquids and gases in terms of particle arrangement and motion.

Particle arrangements

Property	Solid	Liquid	Gas
Arrangement	Regular pattern	Irregular, close together	Random, far apart
Spacing	Very close	Close (slightly more than solid)	Very far apart
Motion	Vibrate about fixed positions	Move around each other	Move rapidly in all directions
Forces	Strong	Moderate	Very weak (negligible)

Density and the particle model

Density depends on how closely packed the particles are and their individual masses.

• Solids generally have the highest density (particles tightly packed).
• Liquids have slightly lower density than their solid form (particles slightly further apart).
• Gases have very low density (particles far apart with large spaces).

Water is an exception: ice (solid) is less dense than liquid water because the crystal structure of ice creates an open lattice with more space between molecules than in liquid water.

Explaining properties

• Solids have fixed shape because particles vibrate in fixed positions.
• Liquids flow because particles can move past each other.
• Gases fill their container because particles move rapidly in all directions and there are negligible forces between them.
• Gases are easily compressed because of the large spaces between particles. Solids and liquids are virtually incompressible.

Common errors and how to correct them

• Saying particles “expand” when heated. Particles themselves do not change size; they move faster and further apart.
• Saying gases have no particles. Gases have particles, just very far apart.

How to apply this in an exam

Describe the arrangement, spacing and motion of particles for each state. Link these to the macroscopic properties being asked about.