### 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.

Nucleus

• 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.