Class 12 > Unit # 27: Nuclear Physics > Atomic Nucleus & Isotopes

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Atomic Number, Mass Number, Neutron Number, and Isotopes

Core Definitions in Nuclear Physics

1. Atomic Number ($Z$): The number of protons present in an atomic nucleus is called the atomic number (also known as the proton number or charge number). It is denoted by the symbol $Z$.
2. Mass Number ($A$): The total sum of the number of protons and neutrons in a nucleus is called the mass number. It is denoted by $A$ and is also referred to as the nucleon number.
3. Neutron Number ($N$): The difference between the mass number and the atomic number gives the neutron number. It is denoted by $N$ and is calculated as:
   $N = A - Z$
4. Representation of an Element: An element $X$ having mass number $A$ and atomic number $Z$ is standardly represented by the nuclide symbol:
   ${}_{Z}^{A}\text{X}$
   Where $X$ is the chemical symbol for the particular element.

Isotopes

Isotopes are defined as atoms of the same chemical element that have the same atomic number ($Z$) but different mass numbers ($A$) due to a varying number of neutrons ($N$).

Classic Example: Isotopes of Hydrogen

Hydrogen has three well-known isotopes: Hydrogen (Protium), Deuterium, and Tritium.

Isotope	Symbol / Representation	Mass Number ($A$)	Atomic Number ($Z$)	Neutron Number ($N$)
Hydrogen (Protium)	${}_{1}^{1}\text{H}$	1	1	0
Deuterium	${}_{1}^{2}\text{H}$ (or $\text{D}$)	2	1	1
Tritium	${}_{1}^{3}\text{H}$ (or $\text{T}$)	3	1	2

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