The Nucleus

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Rutherford and his co-workers, performed series of scattering experiments and concluded that the entire mass and the positive charge is concentrated in a very small region at the centre of the atom, known as nucleus. The radius of the nucleus is about 10^-15m i.e., 10^-5 times that of the atom.

The nucleus consists of positively charged protons and electrically neutral neutrons collectively known as nucleons. The atomic number (Z) of an atom is the number of protons in the nucleus and is the identity of an atom.

The mass number (A) of an atom is the integer nearest to the relative atomic mass and is equal to the number of nucleons in the nucleus. Hence the number of neutrons in the nucleus is A-Z.

A particular nuclear species with a specific atomic number and a mass number is referred to as nuclide.

Atoms with the same number of protons but different number of nucleons are called isotopes.

Examples: ¹H₁, ²H₁ and ³H₁ are isotopes of hydrogen.

Atoms with the same number of neutrons but different number of nucleons are called isotones.

Examples: ³⁰S₁₄, ³¹P₁₅ , ³²S₁₆.

Atoms with the same number of nucleons but different number of protons are called isobars.

Example: ⁴⁰Ca₂₀ and ⁴⁰Ar₁₈

²¹¹Pb₈₂ and ²¹¹Bi₈₃