An atom is the smallest particle of an element and shows all the characteristic properties of the element. A molecule, on the other hand, is the smallest particle of an element or a compound, which can exist free in nature and shows all the properties of that clement or compound. To explain clearly the difference between an atom and a molecule, a comparison is shown in the table.



1. A molecule is the smallest particle of an element or a compound.

2. t shows all the properties of the element or the compound.

3. is divisible in a chemical reaction.

4.It .does not retain its identity during .a chemical reaction.

5.It does not exist

6.It exists in free in free state as a state. single atom but the atoms of noble gases are found as single atoms.


“It is a pure substance which cannot be divided into simpler substances’ by chemical methods.”


“An element is a substance in which all the atoms are chemically identical and have the same atomic number.”

Elements exist in all three states, i.e., solids, liquids and gases.


The ancient philosophers believed that only four elements are present on Earth which are: air, water, soil, and fire.

Later investigations proved that this idea is absolutely wrong. None of the four things mentioned above is an element. Air is a mixture of different gases. Water is made up of two gases hydrogen and oxygen. The soil has a lot of things present in it. Fire is not a material thing, it only represents a chemical change. According to present-day knowledge, the number of naturally occurring elements is 92. Everything in this world is made up of these 92 elements.Out of these 92 naturally occurring elements, many are not very common. We shall study only those elements which are well-known and their compounds we use in our daily life. Some of these elements are:

Oxygen, hydrogen, nitrogen, chlorine, carbon, phosphorus, iodine, sulphur, iron, silver, copper, gold, zinc, mercury, aluminum, sodium, potassium, calcium and platinum.


Twenty-five percent of Earth’s crust is composed of compounds of different metals. Metals like gold, silver, platinum which are unreactive, occur mostly in free state while reactive metals like sodium, potassium, tin and aluminum are found in the form of compounds.

Percentage of Some Elements found in Earth’s Crust:

1. Oxygen502. Silicon26
3. Aluminum74. Iron4
5. Calcium36. Potassium2.5
7. Sodium2.58.2
9. Hydrogen0.1410.                 Other




Human efforts have been successful in transforming one element into another. For example, uranium metal has been transformed into plutonium during the project which led to the development of atomic bomb. Plutonium is not found in Earth’s crust.


The elements found in this world are classified into two main groups depending upon their characteristics. One group is called metals and the other is called non-metals.


Comparison of the properties of metals and non-metals.

1.Metals generallyI.                  Non-metals exist
 exist      in    solidin       all      the      three
 state      at     roomphysical                     states                at
 temperature.   Forroom temperature. For
 example,                     gold,

silver                   and         iron,
etc. Mercury and

example,             phosphorus

is a solid, bromine is a
liquid and hydrogen is

 gallium,a        gas        at      room



German silver. Electrodes of most of. the electric cells are made of copper. Electrical instruments, wires for telephones and telegraph, parts of automobiles, and watches are also made of copper. Copper is used in electroplating.

  • Aluminum utensils are often used in kitchens. Electric wires and conductors, parts of cars, trucks, and railway trains, different types of windows and doors, frames, and decoration pieces are made from Aluminum powder mixed with copper oxide is used to prepare scintillating sticks. An important alloy of aluminum is magnalium. It consists of 3 to 10% magnesium mixed with aluminum. It is used to prepare pistons of motor, engines, balances, and airplanes.
  • Gold is used in the ornaments. Many precious objects g., medals, crowns are also made of gold.

Gold and silver are also used in electroplating. Fine wire of gold is used in gold wire embroidery.


(1)’ Carbon is an important non­metal. It has different uses depending upon its different types of allotropes. For example, diamond and graphite have the following uses:

  • Diamond, due to its luster and beauty, is used to preparing ornaments and crowns of kings. Natural diamonds are used in drill machines. are used to make holes in rocks. Fine pieces .of diamond is used in precious watches to protect the wear and tear of various parts.
  • Graphite gives mark when it is rubbed on a >surface: Therefore, it is used, mixed with clay, in pencils. Graphite has high melting point, therefore it is used to prepare crucibles, which are used to melt certain Graphite rods, and plates are used to melt certain metals. Graphite rods and plates are used as electrodes. Carbon is used in gun powder. Carbon has the property to absorb color and odor, so it is used in the sugar industry. It is also used in printing ink, black paint, and boot polishes.

.          (2) Nitrogen is being filled in

electric bulbs. It is used to .prepare ammonia and nitric acid. Nitrogen is food for plants, hence it is. also used to prepare such compounds as urea, ammonium nitrate • and ammonium sulfate which are used as fertilizers.

(3) Sulfur is used to prepare sulphuric acid, which is very important for present-day industries. Matchsticks and gun powder are prepared from sulfur. To increase the strength and elasticity of rubber, sulfur is mixed with it. It is also used for the vulcanization of rubber.