What are atoms and molecules?

According to Dalton�s atomic theory, all matter whether an element or a compound or a mixture is made up of very small indivisible particles called atoms. An atom is defined as the smallest particle of an element that may or may not be capable of free existence. It is the smallest particle that takes part in a chemical reaction.

Ancient Greek and Indian philosophers have always wondered about the unknown and unseen form of matter. What is a matter ultimately made up of? In around 500 B.C. an Indian philosopher Maharishi Kanad postulated that if we go on dividing matter (padarth), we will get smaller and smaller particles.

Ultimately, a time will come when we will come across the smallest particle beyond which further division will not be possible. These indivisible particles were named �Paramanu� and paramanu was considered to be an indivisible particle.

Almost during the same period ancient Greek philosophers, Democritus and Leucippus also put forward the same idea. They called the smallest indivisible particles �atoms�. During the same era, it was also suggested by Indian philosopher, Pakudha Katyayama, that in some cases, the atoms may not exist in the free state but may exist in the combined state in the form of species, called molecules or compounds

So we may conclude that matter is made up of small particles which may be atoms or molecules. Atoms are considered the building blocks of all matter. Atoms are extremely small particles; they are so small in size that they cannot be seen even under a microscope. The atoms are considered to be spherical. Their sizes are expressed in terms of their radius called the atomic radius.

Atomic radius is usually measured in nanometers (nm). nm is pronounced as nanometers. It is of the order of 10-9 meters. The radius of a hydrogen atom is 0.37 nm.

What are ions?

There are a number of compounds that are formed by the combination of a metal and a non-metal e.g. Sodium chloride. NaCl atom is formed from the metal atom sodium and the non-metal atom chlorine.

Similarly, the Calcium Oxide atom is formed from the metal atom Calcium and the non-metal atom Oxygen. Metal and non-metal ions are present as charged species. In some cases, not only the single atom but the group of atoms may carry a charge. An atom or a group of atoms that carry positive or negative charges is called an �ion�.

The ion-carrying positive charge is called a cation and the ion-carrying negative charge is called an anion. The ions consisting of only a single atom are called mono-atomic ions. Whereas ions consisting of a group of atoms are called polyatomic ions.

What are ionic compounds?

The compounds consisting of the cations and the anions are called ionic compounds. A compound is always made up of the same elements combined together in a fixed proportion by mass.

E.g.:- Water contains 2 Hydrogen and 1 Oxygen atom in the ratio of 1:8 by mass.

Sodium chloride contains Sodium and chlorine in a fixed proportion of 23:35.5 by mass, as NaCl compound has 1 Sodium atom and 1 Chlorine atom. Their atomic masses are Na: 23 AMU, Cl = 35.5 AMU. So in Sodium Chloride, sodium & chlorine are in a fixed proportion by mass i.e. 23:35.5.

Similarly, calcium oxide contains calcium and oxygen in the fixed ratio of 40:16 or 5:2 by mass.

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