Molecules

Molecules

The smallest particle of a substance which is capable of independent existence is called a molecule. In general, molecule is a group of two or more atoms that are chemically bonded together. It shows all the properties of the substance. Molecules can be divided into two categories.

Molecules of Elements

The molecules of an element are constituted by the same type of atoms. Molecules of many elements are made up of one atom of that element. e.g. noble gases like argon (Ar), helium (He), etc.

The molecules of most of more than one atoms of the non-metals are made up of more than one atoms.

For Example:- A molecule of oxygen consists of two atoms of oxygen. Ozone (O₃) consists of three atoms of oxygen.

Atomicity

It is defined as the number of atoms present in a molecule.

On the basis of atomicity, molecules can be classified as

(i):- Monatomic Molecules They consist of only one atom. e.g. He, Ne, Ar, Xe, Fe, Al, etc.

(ii):- Diatomic Molecules They consist of two atoms. e.g. H₂,O₂, N₂,I₂, Br₂, C₁₂, etc.

(iii):- Triatomic Molecules They consist of three atoms. e.g. O₃.

(iv):- Tetraatomic Molecules They consist of four atoms. e.g. P₄.

(v):- Polyatomic Molecules They consist of more than four atoms. e.g. S₈.

Molecules of Compounds

Atoms of different elements join together in definite proportions forming molecules of compounds.

For Example, H₂O molecule is made up of hydrogen and oxygen elements in the ratio of 1:8 by mass.

Ions

When atoms, groups of atoms or molecules lose or gain electron(s) they become charged. These charged species are known as ions. These can be negatively or positively charged.

Cations

The positively charged ions which are attracted towards cathode in an electric field are known as cations. e.g. Na⁺, K⁺, Ca²⁺, A¹³⁺ , etc.

Anions

The negatively charged ions which are attracted towards anode in an electric field are known as anions. e.g. Cl^–, Br^–, O, N^3-, etc.

Ionic Compounds

The compound which consists of ions as its constituent particles is known as ionic compound. It contains ionic bonds. e.g. sodium chloride, consists of a positively charged) sodium ion (Na⁺ cation) and negatively charged chloride ion (C1^- anion). Calcium oxide or quicklime (CaO) consist of calcium cation (Ca²⁺) and oxide anion (O^2-).

Polyatomic Ion

The compound which consists of ions as its constituent particles is known as ionic compound. It contains ionic bonds. e.g. sodium chloride, consists of a positively charged) sodium ion (Na⁺ cation) and negatively charged chloride ion (C1^- anion). Calcium oxide or quicklime (CaO) consist of calcium cation (Ca²⁺) and oxide anion (O^2-).

Valency

The combining power (or capacity) of an element is called its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.