What is an Atom

In chemistry we deal with chemical components of substances. Their chemical composition helps us in determining the properties of substances. Also, we need to know about what constitutes these substances. John Dalton explained to the world that all substances are made up of small units called atoms.

Just as all the houses are made up of bricks, in the same way, all the matter is made up of atoms. Thus, atoms are the building blocks of all the matter around us. In chemistry, atom is defined as:

An atom is the smallest particle of an element that can take part in a chemical reaction.

Atoms of most of the elements are very reactive and do not exist in the free state (as single atoms). They exist in combination with the atoms of the same element or another element. There are as many kinds of atoms as are elements.

Atoms are very, very small in size. An idea of the extremely small size of atoms can be had from the fact that 35,000,000 copper atoms arranged end to end in a line would cover a distance of about 1 centimetre. The size of an atom is indicated by its radius which is called ‘atomic radius’. Atomic radius is measured in ‘nanometres’. Nanometre is a very small unit of measuring length. The symbol of nanometre isnm.

1 nanometre = 1/109 metre

1 nm = 1/109 metre

1 nm = 10-9 metre




Hydrogen atom is the smallest atom of all. The atomic radius of a hydrogen atom is 0.037 nanometres (or 0.037 nm). If we express the radius of hydrogen atom in metres, it will be 0.037 x 10-9 metre which means 0.000000000037 metre. It is really very, very small.

Atoms are very small that is why we cannot see them even under the most powerful optical microscope. Electron microscope (which uses electrons for its working) can produce extremely magnified images of tiny objects.

Atomic mass of an element –

Actual masses of the atoms of the elements are very, very small. In reality it is not convenient to use such small and complicated figures in our calculations. Hence, it is necessary to define atomic masses in such a way that we get simple figures for them. Therefore, it is defined to use atomic mass unit as a uniform system of describing mass of atoms.

Atomic mass unit = 1/12 the mass of a carbon-12

1 u = 1/12 the mass of a carbon-12

One atomic mass unit (1u) is defined as exactly one-twelfth the mass of an atom of carbon-12.

Existence of atoms –

The atoms of only a few elements called noble gases (such as helium, neon, argon, krypton etc.) are chemically unreactive and exist in the free state (as single atoms). Atoms of most of the elements are chemically very reactive and do not exist in the free state (as single atoms). Atoms usually exist in two ways:

  • In the form of molecules
  • In the form of ions

When atoms form molecules or ions, they become stable because in doing so they acquire the stable electron arrangements of noble gases.




NCERT textbook Questions – Page 35

Question 1:-

Define the atomic mass unit.

Answer 1:-

Atomic mass unit is equal to 1/12  the mass of a carbon-12 atom.

Question 2:-

Why is it not possible to see an atom with naked eyes ?

Answer 2:-

It is not possible to see an atom with naked eyes because an atom is a very, very small particle. The radius of an atom is of the order of 10-10 metre which is extremely small.

Page 39

Question 4:-

How many atoms are present in a:

  • H2S molecule
  • PO43- ion

Answer 4:-

  • Three atoms are present in H2S molecule.
  • Five atoms are present in PO43-

ConclusionAn atom is the smallest unit of an element. An atom cannot be seen from naked eyes. The mass of atom is very small therefore, atomic mass unit is defined as one-twelfth the mass of carbon-12 atom. An atom can exist in the form of a molecule or an ion. Atoms are quite reactive therefore, they cannot exist in free state.

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