Atomic Structure of an Element

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Atomic Structure of an Element : : Properties of an Elements

Atomic Structure :

  • In 1809, Dalton suggested that atom is the smallest particle of the element and it is indivisible.
  • But in the beginning of 20th century Rutherford, J.J. Thomson etc. suggested that atom is divisible and made up of electrically charged particles.

Electron :

  • The electron is a fundamental particle of an atom which carries a unit negative charge.
  • It was discovered by J.J. Thomson in 1897.

Proton :

  • It is a fundamental particle of an atom carrying a unit positive charge.
  • It was discovered by Rutherford and Goldstein in 1886.

Neutron :

  • It is a fundamental particle of an atom carrying no charge.
  • It was discovered by Chadwick in 1932.

Charge and Mass of Fundamental Sub – Atomic Particles :
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Properties of Cathode Rays :

  • They travel in straight lines
  • They cast shadow of solid objects placed in their path.
  • They cause mechanical motion i.e., they consists of material particles.
  • They carry -ive charge.
  • These rays are deflected when magnetic field is applied on them.
  • They produce heating effect.
  • They cause ionization of gas through which they pass.
  • They produce green fluorescence on the glass walls of the discharge, tube as well as on certain other substances such as zinc suplhide.
  • They effect the photographic plates.
  • They have penetrating power.
  • The ratio of charge / mass (i.e., e / m) = 1.76 x 108 coulombs / g.

Properties of Anode Rays :

  • They travel in straight lines, however, their speed in much less than that of cathode rays.
  • They are made up of material particles.
  • They are positively charged.
  • They deflect in electric and magnetic field.
  • The ratio of charge / mass is found to be different for the particles constituting anode rays when different gases are taken inside the discharge tube.

Properties of Nucleus :

  • Nucleus of an atom consists of protons and neutrons which are collectively known as nucleons.
  • The forces that bind the electrons to the nucleus are electrical or coulombic in nature.
  • Density of nucleus is enormous and is of order of 1014 g/cm.
  • Instability of the nucleus is due to high neutron proton ratio.
  • The radius of the nucleus is around 5 x 10-13 cm (5 Fermi).

Atomic Number ( Z ) :Atomic number of an Element. = Total No. of protons present in the nucleus = Total No. of Electrons present outside the nucleus. ( or )

Z = p = e

Mass Number ( A )Mass Number = Number of Protons + Number of Neutrons

A = p + n (or) A = z + n

Representation

Mass Number  =>A

Atomic Number  =>ZX

e.g., 2311Na, 1735Cl and so on

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