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Atoms and Molecules

Modern Atomic Theory

In this lesson you will study about atoms, molecules, elements and compounds.


You must be familiar with the different types of matter around us and the state in which they exist. Do you know what the whole matter around us is made up of?

In the previous learning piece, we studied that matter is made up of minute particles called molecules. But, do you know that molecule is not the only tiny particle of matter? It further consists of a group of very small particles called atoms.

So, we can say that ‘matter consists of very small particles called atoms’.

Let us get very clear knowledge about atoms through this lesson.

The word ‘atom’ is derived from the Greek word ‘atomos’ which means ‘indivisible’. It was the Greek philosopher Democritus who coined the term.

Before coming to the point of describing atoms, let you be introduced to another new term called elements. Elements are the pure substances or matters that consist of only one kind of atoms. For example, Iron is an element that consists of all the atoms of iron. In other words, we can say that one type of atom represents one element or vice versa.

Elements can be solid like gold, liquid like mercury and gaseous like oxygen. They are classified into metals (for example: silver, gold, copper, etc.), non-metals (for example: chlorine, nitrogen, phosphorus, etc.) and inert gases (for example: helium, neon, argon, etc.). Till now more than 109 elements have been discovered, out of which 102 are found in nature.

Now, we can define atom as the smallest particle of an element that posses all the properties of that element.

Dalton's Atomic Theory

In the early nineteenth century, an English chemist named John Dalton (1766- 1844), proposed a theory about atoms known as ‘Dalton’s atomic theory’. It proved to be one of the most important theories of science. The various laws of chemical combination also supported Dalton’s theory. Dalton asserted that ‘atoms are the smallest particles of matter, which cannot be divided further’.

The postulates of  Dalton’s atomic theory are as follows:

• All matter is made up of very tiny particles. These particles are called atoms.

• An atom cannot be divided further, i.e., atoms are indivisible.

• Atoms can neither be created nor destroyed in a chemical reaction.

• All atoms of an element are identical in all respects, e.g. in terms of mass, chemical properties, etc.

• Atoms of different elements differ from each other, i.e., they have different masses and chemical properties.

• Atoms of different elements combine whole-number ratios to form compounds.

• In a given compound, the relative numbers and types of atoms are constant.

Now, let us study how these tiny particles combine to form molecules and how these molecules are represented with symbols of atom through molecular formula.


Most atoms are not stable in their free state. So, they combine with other atoms to form molecules.

The term ‘molecule’ originates from the French word ‘molecule’, which means ‘extremely minute particle’.

A molecule may consist of one or more same or different atom(s). For example, a hydrogen molecule is formed when two hydrogen atoms combine with each other. Similarly, a sulphur molecule consists of 8 sulphur atoms. A water molecule consists of two atoms of hydrogen and one atom of oxygen.



Now, we can easily understand that molecules are of two types:

• Molecules of element: Molecules that consist of similar atoms, for example, hydrogen molecule

• Molecules of compound: Molecules that consist of dissimilar atoms, for example, water molecule

Compounds are pure substances consisting of two or more atoms of different elements that combine chemically in definite ratio.

 For example, water is a compound (molecule of compound) which consists of two hydrogen atoms and one oxygen atom. Atoms that constitute compounds do not show the properties of compound.

Compounds can be solid like potassium permanganate (KMnO4), liquid like water (H2O), or gaseous like carbon dioxide (CO2).

After reading the above discussion about molecules, we can now easily define the molecules.

‘Molecules are defined as small particles of elements or compounds that are capable of independent existence’.

To understand more clearly about molecules, let us take the example of crystals of sugar. If we keep on breaking the crystals of sugar, the smallest particle or crystal of the sugar obtained possess all the properties of the sugar and is called a molecule. If this particle of sugar is further divided, it will reach a point when the sugar molecule will not be divisible further. We will get minute particles of carbon, oxygen and hydrogen atoms of which sugar is made up of. These atoms will not possess the properties of the sugar any more.

Summarising the above discussion

We will study about elements and compounds…

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