What is Atom in chemistry?

Updated on April 8, 2021 06:21 PM IST by Lakshmi Arya

The building blocks of all matter are called atoms .An atom is the smallest characteristic particle of a given element. Atoms are very small and atomic radius is measured in nanometres. Maharishi Kanad and Pakudha Katyayana (Indian philosophers) studied about the particles of matter. The smallest particles of matter were named "Parmanu" by Maharishi Kanad. Democritus (Greek philosopher) called indivisible particles atoms.
The concept of formation of compounds was first given by Antonis L. Lavosier and Joseph L. Proust who gave the laws of chemical combination.

Laws of chemical combination

  1. Law of conservation of mass.
  2. Law of constant proportion.
  1. Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
  2. Law of constant proportion states that in a chemical substance the elements are always present in definite proportions by mass.

Dalton's Atomic Theory

  1. All matter is made of very tiny particles called atoms.
  2. Atoms are indivisible particles which can not be created or destroyed in a chemical reaction.
  3. Atoms of a given element are identical in mass and chemical properties.
  4. Atoms of different elements have different masses and chemical properties.
  5. Atoms combine in the ratio of small whole numbers to form compounds.
  6. The relative number and kinds of atoms are constant in a given compound.


An atom of an element is known with the same symbol and name as of it's element. International Union of Pure and Applied Chemistry (IUPAC) approves names of elements, symbols and units. The first letter of a symbol is always written as a capital letter and the second letter as a small letter.



  1. Symbol of iron, potassium and sodium have been taken from the Latin names Natrium, Kalium and Ferrum respectively.
  2. The name copper was taken from Cyprus, the name of the place where they were found for the first time.

Atomic Mass Unit:--
One atomic Mass Unit is a mass unit equal to exactly one twelfth (1\12th) the mass of one atom of carbon- 12.
The latest short form of atomic Mass Unit is 'u' according to IUPAC.

Relative Atomic Mass Unit:--
Relative Atomic Mass Unit of an element is defined as the average mass of the atom,as compared to 1\12th the mass of one carbon-12 atom.

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Related FAQs

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