Lesson 4, Topic 3
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# Molecules

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Most substances cannot exist as individual atoms, instead, they combine their atoms with themselves or other atoms to form molecules.

AÂ moleculeÂ is a group of two or more atoms, held together by attractive forces known as chemical bonds, that form the smallest particle of a substance which can have a separate existence and still retain the composition and chemical properties of that substance.Â

For example, water (H2O) has three atoms, two hydrogen (H) atoms and one oxygen (O) atom.

All molecules are in constant motion. Molecules of a liquid have more freedom of movement than those in a solid. Molecules in a gas have the greatest degree of motion.

### Size of a Molecule:

Matter is made up of molecules which are very small in size. It is in the order of 10-9 to 10-10 m. Thus, one gram of an element contains several millions of molecules. For example, a gram of oxygen has 6.022 x 1023 molecules.

### Determination of Size of a Molecule:

The size of a molecule was first estimated by Lord Rayleigh, in 1890. He put forward that when a drop of oil is placed on top of a water surface, the oil will spread out on top of the water until the thickness of the film of oil was one molecule thick.

This experiment can be easily replicated using the Rayleigh method as follows:

A tray is filled with clean water and lycopodium powder is lightly sprinkled on the water when the surface is still. A drop of Olive oil is then gently released on the surface of the water and it spreads immediately across the surface forming a circular film.

The diameter of the oil film is measured with a ruler, with the radius obtained.

Let the diameter of oil film = d (cm)

Volume of oil = V (cm3)

Area of the oil film  = $$\scriptsize \pi r^2$$

= $$\scriptsize \pi \left (\normalsize\frac {d}{2}\right)^2$$

Thickness of the oil film = $$\frac{Volume}{Area} \\ = \normalsize \frac{V}{\pi \left(\frac{d}{2} \right)^2}\\ = \normalsize \frac{V}{\large \frac{\pi d^2}{4}}$$

= $$\frac{4V}{\pi d^2}$$ cm

From accurate experiments, the thickness of the oil film can be roughly estimated to be $$\scriptsize 2 \: \times \: 10^{-7} \:cm$$

Hence, we can take the size of an oil molecule to be about $$\scriptsize 2 \: \times \: 10^{-7} \:cm$$

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