The smallest unit of substance capable of independent existence, and showing all the properties of that substance is called molecule.
A molecule contains two or more chemically bonded atoms of the same or different type. Therefore, a molecule can be represented in terms of the symbols, and number of atoms of the elements present in it. A shorthand notation of a molecule in terms of the symbols of the various elements present in it is called molecular formula. The molecular formula of a substance may also be defined as follows:
The symbolic representation of a molecule of any substance representing the actual number of various atoms present in it is called its molecular formula. For example, the molecular formula of water is H2O. Thus, one molecule of water contains two atoms of hydrogen and one atom of oxygen.
Molecules are generally classified in terms of number of atoms present in it. For example, the molecules of the gases like hydrogen, oxygen and nitrogen, etc., contain two atoms. Therefore, the molecules of hydrogen (H2), oxygen (O2) and Nitrogen (N2) are termed as diatomic molecules. Molecules of water (H2O) and Carbon Dioxide (CO2) contain three atoms; therefore these are termed as triatomic molecules.
The atoms of noble gases do not combine with other atoms. Therefore, the molecules of noble gases are monoatomic, i.e., noble gases exist in the atomic form.
(i) A molecular formula provides the following information.
(ii) A molecular formula is the shorthand notation of a substance. For example, carbon dioxide is written as CO2. The formula CO2 is much shorter than the name carbon dioxide.
(iii) The molecular formula of a compound represents molecular mass of the substance. For example, the molecular mass of water molecule (H2O) is 18u.
(iv) The molecular formula of any compound gives the name, and actual number of atoms of each kind present in one molecule of the substance. For example, the formula for water is H2O. This means that every molecule of water is made up of 2 atoms of hydrogen and one atom of oxygen.
(v) The molecular formula of a substance also represents one mole of the substance. Therefore, the molecular formula of a substance represents its mass equal to the molar mass of that substance. H2 is the molecular formula of hydrogen. This formula stands for the following:
(vi) H2 is one molecule of Hydrogen. A molecule of hydrogen consists of two atoms of hydrogen.
(vii) There is only one element, i.e., hydrogen, in hydrogen molecule.
(viii) H2 represents mass of hydrogen equal to the molar mass of hydrogen: molar mass of hydrogen is 2.016g mol-1.