Constituents of Atom

John Dalton  first discovered that matter is composed of indivisible particles having an independent existence and called them Atoms (meaning uncut).But towards the end of the 19th century, modern atomic theory, mostly from the works of Thomson, Rutherford,Chadwick, Milliken, Bohr, de Broglie proved that atom is divisible and made up of three fundamental sub-atomic particles namely electron, proton and neutron. Atoms are very reactive and do not exist in the free state,except the atoms of noble gases.

Constituents of Atom

An atom consists of three fundamental particles:

(i)Electron: The presence of electrons was discovered by J.J. Thomson. Electrons are negatively charged particles revolving around the nucleus in orbits of fixed energy.

(ii)Proton: The proton was observed by Goldstein in 1896. Protons are positively charged particles residing in the nucleus.

(iii)Neutron: Neutron was identified by James Chadwick in 1932. These are neutral particles (no charge) residing in the nucleus with protons.

In addition to the above fundamental particles, some uncommon sub-atomic particles have also been postulated:

(i)Positrons: These are the positive counterpart of the electrons. These were discovered by Anderson in1932. These are highly unstable and combine with electrons producing γ-rays (energy radiations).

(ii)Neutrinos and Antineutrinos: These are the particles of small mass and zero charge. These were postulated by Fermi in 1934.

(iii)Pi-mesons (Pions) and µ-mesons (muones) : These particles have a mass intermediate between that of the electron and the proton. The positively and negatively charged mesons were postulated by Yukawa in 1935.Neutral mesons (π) were postulated by Kemmer to account for the binding forces between the nucleons.

Therefore, an atom, on the whole, is neutral because the no. of protons is equal to the no. of electrons. Almost the entire mass of atom is concentrated in a very small part of its total size. This part is called the nucleus and all the protons and neutrons are embedded in it. Radius of the nucleus is approx. 10^-13 cm and that of the  atom is near about 10^-8 cm.

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