Molecules

Molecules are formed when two or more atoms chemically join together. The atoms, which make up a molecule, can be from a single element or from two or more different elements. For example, two atoms of hydrogen form a hydrogen molecule (H₂), and on the other hand three atoms from hydrogen and oxygen, form water (H₂O) molecule.

WATER MOLECULE

Compounds

A compound is a substance formed when atoms of two or more different elements chemically join together. The smallest portion of a compound is called molecule. The molecules formed by two different elements are called compounds. When different elements are joined, the atoms lose their original elements’ property and gain different property from the elements they are composed of. Compounds always contain the same ratio of its component atoms and can be separated only by chemical means and not by physical means. Thus, all compounds are molecules, but not all molecules are compounds. The splitting of a chemical compound is called analysis and the formation of a compound from simpler compounds or elements is called synthesis.

    The list of compounds is given below.

• Water (H₂O)
• Carbon dioxide (CO₂)
• Nitrogen dioxide (NO₂)

Mixtures

The mixtures are made when different elements or compounds are physically intermingled. Air is a mixture of gases that is about 78% nitrogen and 21% oxygen. Mixtures can be separated into its original components by physical means, and often retain many of the properties of its original components. For example, when you physically mix sulphur (S) powder and iron (Fe) filings, it forms a mixture of iron and sulphur. This new mixture will retain the original properties of iron and sulphur. Using a magnet, which attracts the iron filings and leaves the sulphur behind, can separate iron and sulphur mixture. When the mixture of iron and sulphur is heated on a test tube, they react together and form a new compound called iron sulphide (FeS). This new compound will neither have the properties of iron nor sulphur and cannot be separated by physical means.

Molecular Forces

Cohesive force: Intermolecular attraction between molecules of the same substance is called as cohesive force.

Adhesive force: The attraction between the molecules of different substances is called an adhesive force.

Chemical Equation

It is a symbolic representation of a chemical reaction in the form of symbols and formulae, wherein the reactant entities are on the left hand side and the product entities on the right hand side.

Example

    Addition of two hydrogen atoms and an oxygen atom forms a water molecule. The chemical equation for this reaction is below.

H₂ + O → H₂O

    Subscript number denotes a molecule formed by two atoms of a same element. In the above equation, the number 2 as in H₂ indicates that two hydrogen (H) atoms form a hydrogen molecule (H₂). The atomic mass is the mass of an atom whereas molecular mass is the mass of all the atoms that form a molecule. Thus the atomic mass of an oxygen (O₂) molecule is 16 and its molecular mass is 32 (16 x 2).

Balancing chemical equations: The equation has to be balanced so that during the chemical reaction there are no atoms lost or gained. It means that the number of atoms of each element on the left side should be equal to that on the right side. For example, when we add two molecules of hydrogen (2H₂) and one molecule of oxygen (O₂) we get two molecules of water (2H₂O).

2H₂ + O → H₂O