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Concept of Atom

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Essentially, atoms are the most basic unit of matter, comprising of a proton, neutron and an electron. Protons and neutrons together form a nucleus. The electrons revolve around the nucleus. Protons, as the name suggests, are positively charged, electrons are negatively charged and neutrons are neutral subatomic particles.

Atomic radius is measured in nanometres.

1/109 m = 1 nm

1 m = 109 nm



  • Atom is the smallest particle of an element. It has two parts nucleus and outer nuclear part.
  • Nucleus of the atom consists of protons and neutrons. Protons are positively charged particles. Neutron possess no charge.
  • In outer nuclear part, electrons are present. Electrons are negatively charged particles.
  • Atomic number represents the number of protons in the atom. As an atom is electrically neutral, an atom may contain as many electron as it has protons.
  • The total number of protons and electrons present in one atom of an element is known as its mass number.
  • Atoms of a given atomic number can have different number of neutrons are called isotopes.
  • The atoms of different elements, which have the same mass number but different atomic numbers are called isobars. These have different number of protons but equal sum of number of protons and neutrons.
  • The atoms of different elements, which have the same number of neutrons but different atomic numbers are called isomers.

Charged particles in matter:

  • The discovery of electron and proton is credited to J.J. Thomson and E.Goldstein, respectively.
  • J.J. Thomson proposed that electrons are embedded in a positive sphere
  • Electron was represented as ‘e-‘ and proton as ‘p+’. The mass of a proton is taken as one unit and its charge as plus one where the mass of an electron was considered to be negligible and its charge minus one
  • It seemed that atom consisted of electrons and protons which balanced their charges mutually

Structure of an Atom:

Dalton’s atomic theory suggested that the atom was indivisible and indestructible. But the discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s atomic theory. It was then considered necessary to know how electrons and protons are arranged within an atom. For explaining this, many scientists proposed various atomic models. J.J. Thomson was the first one to propose a model for the structure of an atom.



  • Atoms are the building blocks of all matter.
  • They are very small, Atomic radius is measured in nanometers (1/109 m = 1 nm).
  • Atomic radius is the typical distance from the center of the nucleus to the boundary of the surrounding shells of electrons.


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Daltons atomic theory [00:32:18]
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