Atoms And Molecules

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A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence.

The number of atoms constituting a molecule is referred to as atomicity.

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

Compound	Combining Elements	Ration by mass
Water	Hydrogen (H), Oxygen (O)	1:8
Ammonia	Nitrogen (N), Hydrogen (H)	14:3
Carbon dioxide	Carbon (C) Oxygen (O)	3:8

Calculation of ratio by a number of atoms for a water.

Thus, the ratio by number of atoms for water is H:O = 2:1

The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element.

Based upon atomicity we can classify molecules as follows.

Monoatomic molecules: Noble gases like helium, neon, argon, etc., exist in atomic forms, i.e., they are monoatomic. For example, helium, neon and argon exist as He Ne and Ar respectively.

Diatomic molecules: These molecules consist of two atoms. For example, hydrogen (H₂ ) oxygen (O₂), nitrogen (N₂₎, chlorine (Cl₂), carbon monoxide (CO), hydrogen chloride (HCl) and sodium chloride (NaCl).

Triatomic molecules: These molecules consist of three atoms. For example. ozone (O₃) is a triatomic molecule. Similarly, carbon dioxide (CO₂), nitrogen dioxide (NO₂) and hydrogen sulphide (H₂S) are examples of triatomic molecules.

Tetraatomic molecules: These molecules consist of four atoms. For example, P₄ is a tetra atomic molecule of phosphorus.

Polyatomic molecules: Any molecule containing more than four atoms is called a polyatomic molecule. For example, sulphur (S₈), methane (CH₄), nitric acid (HNO₃), ethanol (C₂H₅OH).

1. An ion is a charged particle and can be negatively or positively charged.
2. A negatively charged ion is called an ‘anion’.
3. A positively charged ion is called ‘cation’.
4. Ions may consist of a single charged atom or a group of atoms that have a net charge on them.

Take, for example, sodium chloride (NaCl).

Its constituent particles are positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl^–).

A group of atoms carrying a charge is known as a polyatomic ion.