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Atomic Number and Atomic Weight

Atomic number: Number of protons present in the nucleus is called atomic number (Z) of an element.

Atomic weight: The sum of mass number of proton and mass number of neutron is called atomic weight or atomic mass number (A).
  A = Z + n°

Example:
19K39
  Z = 19 and A = 39 as per atomic weight definition.
  A = Z + n°
  n° = A - Z
  n° = 39-19
  n° = 20

Similarly for
53I 127, no of neutron n° = A - Z = 127-53 = 74
 

Isotopes: Atoms of the same element contains different number of neutrons, are called isotopes. They differ by their atomic weight but exhibits same chemical properties. Example – Hydrogen and deuterium.


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