Atomic Masses and Composition of Nucleus

  • The mass of an atom is so small that it is inconvenient to express it in kilograms.

  • The unit in which atomic and nuclear masses are measured is called atomic mass unit (amu).

  • One amu is defined as 1/12th of the mass of an atom of isotope.

Avogadro’s number = 6.023 × 1023

∴Mass of 6.023 × 1023 atoms of C12 = 12 g

  • Atomic masses can be measured using a mass spectrometer.

  • The different types of atoms of the same element which exhibit the same chemical properties, but differ in mass are called isotopes.


  • The nucleus has the positive charge possessed by the protons. Atomic number is Z. The total charge on an atomic electron is (− Ze) while the charge of the nucleus is (+ Ze).

  • The composition of a nucleus is described using the followings terms and symbols.

Z = Atomic number = Number of protons

N = Neutron number = Number of neutrons

A = Mass number = Z + N = Total number of protons and neutrons

  • Nuclear species can be shown by the notation

    Where, X = Chemical symbol of the species

  • Nuclides with the same mass number (A) are called isobars, and nuclides with the same neutron number (N) are called isotones.

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