Class 9 > Unit # 07:Properties of Matter > Kinetic Molecular Model & Brownian Motion

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Kinetic Molecular Model of Matter & Molecular Motion

Kinetic Molecular Model of Matter

"Matter is made up of tiny particles called atoms, or groups of atoms called molecules, which are always in continuous random motion. Because particles are in continuous motion, an alternative name for this model is 'The particle model of matter'."

Brownian Motion

Direct evidence of molecular motion was first discovered by the botanist Robert Brown in 1827. He observed the irregular, zig-zag motion of pollen grains suspended in water and deduced that the surrounding water molecules were in constant, random motion, colliding with the pollen grains to cause this phenomenon—now known as Brownian motion.

The kinetic molecular theory successfully explains the physical properties of solids, liquids, and gases by analyzing particle positions and motion:

1. Solids (Fig. 7.2a)

Particle Arrangement & Density:

• Molecules are closely packed together and occupy minimum space.
• Molecules are usually arranged in a regular geometric pattern called a lattice.
• There is a high density of particles per unit volume, accounting for the high densities of solids.

Particle Movement & Forces:

• Forces of attraction between particles are very strong.
• Particles are unable to change positions freely.
• Particles vibrate about fixed mean positions and are not entirely stationary.
• Result: Solids maintain fixed shapes and fixed volumes.

2. Liquids (Fig. 7.2b)

Particle Arrangement & Density:

• Molecules are slightly further apart compared to solids.
• Molecules occur in loose clusters with a slightly lower number of particles per unit volume than solids.
• Liquids possess relatively high densities.

Particle Movement & Forces:

• Forces of attraction between particles remain strong.
• Particles are free to move about and slide past one another within the liquid.
• Result: Liquids have fixed volumes but take the shape of their container.

3. Gases (Fig. 7.2c)

Particle Arrangement & Density:

• Molecules are very far apart from one another.
• Molecules are arranged randomly, resulting in a small number of particles per unit volume.

Particle Movement & Forces:

• Forces of attraction between gas particles are negligible.
• Particles are able to move freely in random directions at very high speeds.
• Particles occupy any available space, expanding to fill their container completely.

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