Class 12 Physics Nuclei Measurement of Atomic mass unit

Measurement of Atomic mass unit

  • Mass of atom is very small as compared to the measurable masses which we see around us.
  • Atomic Mass Unit (a.m.u) is used to measure mass of an atom
  • It is denoted by u.
  • Atomic mass unit is defined as (1/12th) of the mass of the carbon.
  • 1 a.m.u =(1/12) x 1.992647 x10-26 kg
    • Where 1.992647 x10-26 = mass of 1 carbon atom.
  • 1 a.m.u =1.67 x10-27
  • To get the exact measurement of the atomic mass, an instrument known as Mass spectrometer is used.

Mass Spectrometer


Problem: -The three stable isotopes of neon: 1020Ne,1021Ne and 1022Ne have respective abundances of90.51%, 0.27% and 9.22%. The atomic masses of the three isotopes are

19.99 u, 20.99u and 21.99 u, respectively. Obtain the average atomic mass of neon.


Atomic mass of1020Ne, m1= 19.99 u

Abundance of 1020Ne, η1 = 90.51%

Atomic mass of1021Ne, m2 = 20.99 u

Abundance of 1021Ne, η2 = 0.27%

Atomic mass of1022Ne, m3 = 21.99 u

Abundance of1022Ne, η3 = 9.22%

The average atomic mass of neon is given as:

m= (m1 η1+ m2 η2+ m3 η3)/ (η1+ η2+ η3)

= (19.99x90.51+20.99x0.27+21.99x9.22)/ (90.51+0.27+9.22)


Nuclei types

  1. Isotopes: - Two nuclei with the same atomic number and different mass number are isotopes of each other.
    1. For example: - There are 3 isotopes of carbon(C) having same atomic number 6 but their mass numbersare differenti.e. 12,13 and 14.(126C), (136C), (146C).
  2. Isobars: -The nuclei which have different atomic number but same mass number are known as isobars.
    1. For example: - Nitrogen (147N) and Carbon (146C)areboth isobars as their mass numbers aresame which is 14 but their atomic numbers are 7 and 6 respectively.
  3. Isotones: -Isotones are those nuclei which have different atomic number but same number of neutrons.
    1. For example: -Boron (125B) and Carbon (136C).
    2. Boron: - Atomic number = 5 and mass number =12.
    3. Carbon: - Atomic number = 6and mass number =13.
    4. But the number of neutronsin Boron = (12-5 =7) and number of neutrons in

carbon = (13-6 = 7) are same.

  1. Nuclide: - Nuclides are collection of nuclei with same atomic number having same number of neutrons.

Charge on a Nucleus

  • Nucleus is positively charged and consists of protons which are (+ively) charged and neutrons are neutral.As a whole nucleus has to be positively charged.
  • Charge on the nucleus is such that the entireatom is electrically neutral as a whole.
  • Atom constitutes of electrons,protons and neutrons.
    • Consider an element ZX
      • Where Z= atomic number, (the number of protons =Z and the number of electrons =Z).
    • Total charge on all the electrons e-= -(Ze).

Total charge on the nucleus has to be equal and opposite of the charge on electron i.e. it should be =+ (Ze)for the atom to be electrically neutral.

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