Brownian motion as evidence for existence of atoms.

The idea

Brownian motion is the random, jerky movement of small particles such as smoke particles or pollen grains suspended in a fluid. Under a microscope, the particles move in unpredictable zigzag paths. This motion is explained by collisions with much smaller, fast-moving fluid molecules. Because the molecules move randomly and collide unevenly from different directions, the suspended particle experiences a fluctuating resultant force. The effect is stronger for smaller particles and higher temperatures. Brownian motion provides direct evidence that matter is made of atoms or molecules in continuous random motion, supporting the molecular kinetic theory model. It does not prove the exact size of atoms, but it is strong evidence for their existence and movement.

What earns the marks

• Brownian motion is the random, irregular movement of small visible particles suspended in a fluid, observed as a jerky zigzag path under a microscope.
• The motion is caused by unequal collisions between the suspended particle and fast-moving fluid molecules, producing a fluctuating resultant force.
• The random nature of the motion reflects the random motion of the fluid molecules, so it provides evidence that molecules are in constant random motion.
• Brownian motion supports the existence of atoms and molecules because the visible particle moves only if it is bombarded by smaller unseen particles.
• The effect is more noticeable with smaller suspended particles and at higher temperatures, because collisions are more frequent and energetic.
• Brownian motion is consistent with the molecular kinetic theory model, in which gas or liquid particles move randomly and collide with each other and with surfaces.

Common mistakes and how to put them right

• Saying that Brownian motion is the movement of atoms themselves.

  Put it right: It is the movement of larger suspended particles caused by collisions with atoms or molecules.

• Claiming that Brownian motion proves the size or shape of atoms.

  Put it right: It provides evidence for their existence and random motion, not their exact dimensions.

• Describing the motion as smooth or predictable.

  Put it right: It is random and jerky, with changes in direction caused by uneven molecular collisions.

Exam tips

• Link the observation to the explanation: random jerky motion of a visible particle is caused by uneven collisions with invisible molecules.
• Use the phrase 'evidence for the existence of atoms' rather than 'proof', because scientific evidence supports a model.
• Mention one factor that affects the motion, such as particle size or temperature, and explain its effect in terms of collisions.