Atomic number and Mass number

  • Atom is the smallest, tiny, indivisible particle according to Dalton.
  • Discharge tube experiments revealed that in atom still have many smaller particles are present .
  • The fundamental particles are:
    • Electron
    • Proton
    • Neutron


ELECTRON
  • Discovered by J.J. Thomson. 
  • Charge is -1 or  -1.602x10-19 Coulombs or  -4.8x10-10 e.s.u.
  • Mass is equal to 1/1837th of mass of hydrogen or 9.11x10-31 kg or 0.005486 a.m.u.


PROTON
  • Discovered by Eugen Goldstein.
  • Charge is +1 unit or  +1.602x10-19 coulombs or  +4.8x10-10 e.s.u.
  • Mass is equal to Hydrogen atom or  1.672x10-27 kg or 1.00728 a.m.u.

NEUTRON
  • Discovered by Chadwick
  • Charge is zero (neutral particle)
  • Mass is equal to mass of a proton or 1.675x10-27 kg or 1.008665 a.m.u.

The Important characteristics of the fundamental Particles are shown in the table:






ATOMIC NUMBER (Z)

  • The number of protons or electrons present in the atom is called atomic number.
  • Atomic number was proposed by Moseley.
  • From atomic number, number of electrons and protons present in the atom is known.
  • .˙. Z = No. of electrons or No. of protons

MASS NUMBER (A)

  • The sum of protons and neutrons present in the atom is called mass number (A).
  • A = No. of protons + No. of neutrons.
  • Number of neutrons = A – Z.

  • The number of fundamental particles present in the atom is known from the atomic number and mass number values.
  • Example: Atomic number and mass number of sodium is 11 and 23 respectively. So it contains 11 protons, 11 electrons and 12 neutrons.

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