Definition of Molecules
A molecule is the smallest particle of a substance which can have a separate existence and still retain the properties of that substance.
Structure, Nature and Size of Molecules
(i) Most substances cannot exist by themselves as individual atoms, rather they combine their atoms with themselves or with other atoms to form molecules. Thus a molecule may be made up of similar atoms of the same element or different atoms of two or more elements. For example a molecule of hydrogen is made up of two atoms of hydrogen but a molecule of water consists of two atoms of hydrogen and one atom of oxygen.
(ii) The molecules of any pure substance are identical they have the same structure, the same mass and the same mechanical properties.
(iii) Molecules are formed by atoms combining in simple proportions
(iv) The simplest model of a molecule is that of a rigid sphere (like a small billiard ball) which can move and collide with other molecules or with a wall and exert attractive or repulsive forces on neighboring molecules. The molecular forces decrease as the distance separating the molecules increase.
(v) Molecules are in constant motion. The motion is random (haphazard or zigzag) in liquids and gases but oscillatory or vibrational in solids.
(vi) The size of a molecule is extremely small – of the order of 10-9– 10-10 m. as a result of the small size, molecules cannot be see with the naked eyes or even with the aid of a microscope: Again because of the small size, one gram of an element contains several millions of molecules. For example one gram of hydrogen contains about 1023 molecules.
Some Ideas about Molecular Size
It is difficult to imagine how tiny the diameter of a molecule actually is. Even the numerical value estimated to be of the order of 10-10 m (10-9 cm) is also not easily and accurately conjectured. The following may enable you to have a better idea of the size of a molecule:
(i) If a fine hair is magnified until its thickness is that of a wide street, a molecule in the hair would then look like a speck of dust in the street
(ii) The tip of a pin contains millions of molecules.
(iii) Two grains of hydrogen contain 6 x 1023 molecules.
If the whole population of the world were to count such a huge number of molecules individually at the rate of five molecules per second, it would take 100 years to count all of them.
Estimating the Size of a Molecule
You can estimate the size or diameter of a molecule by performing the oil film experiment. The principle of the oil film experiment was discovered by Lord Raleigh in 1890. It was known that certain oils when dropped on the surface of water, would spread to form a circular film with the molecules standing up – right. Lord Raleigh argued that if a drop of oil is placed on top of a water surface, the oil will spread out on top of the water surface until the thickness of the oil film is one molecule thick. He, therefore, used this reasoning to obtain the first estimate of the diameter of a molecule. You can repeat Lord Raleigh’s experiment as follows:
(i) Fill a shallow tray with water and allow it to stand until the water is at rest.
(ii) Sprinkle some lycophodium powder lightly on the surface of the water
(iii) Using a graduated pipette with a fine bore, take up a small volume of olive oil and note the reading on the pipette scale
(iv) Drop a very small quantity of the oil on the water surface. Note again the reading on the pipette and obtain the volume of the oil dropped by subtracting the second from the first pipette reading.
(v) Allow the oil to spread, pushing the lycopodium powder outwards and forming a clear thin circular film of oil on thewater surface.
(vi) Measure the diameter of the oil film to the nearest centimeter using a half millimeter scale. Calculate the thickness of the oil film as follows:
Let diameter of oil film = dcm
Let volume of oil drop = Vcm3
Area of oil film = pd 2 = 2 cm2
.’. Thickness of oil film =volumeareaV2=pd2=4Vcm
If you have performed the experiment accurately you should get a value of about 2 × 10-7cm as the thickness of the oil film. Hence the size of an oil molecule is taken as about 2 × 10-7cm