Class 9 Chemistry Structure of The Atom Atomic Number and Mass Number

Atomic Number and Mass Number

  • Atomic number: -
    • Atomic number constitutes the total number of protons which are present in the nucleus of that atom.
    • It is denoted by ‘Z’.
  • Atomic mass:-
    • Atomic mass is the total number of neutrons and protons which are present inside the nucleus.
    • Mass of electrons is not considered while calculating the mass of the atom and only the mass of neutrons and protons are considered;
    • Since the electrons are the lightest particles their mass is not considered.
    • It is also known as Mass Number.
    • It is denoted by ‘A’.
  • Nucleons à Protons + Neutrons
  • General representation of the element: - (AZX) where A = atomic mass and Z = atomic number.
    • For example:- Hydrogen 11H where atomic number=1 and mass number =1
    • Oxygen 168O where atomic number=8 and mass number =16(8 protons and 8 neutrons).

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