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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