Class 10 > Unit # 19:Atomic Structure > Atomic Number , Atomic Mass , Neutron Number & Isotopes

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Atomic Number, Atomic Mass, Neutron Number, and Isotopes

Basic Atomic Parameters & Representation

1. Atomic Number ($Z$): The number of protons in a nucleus is called the atomic number (or proton number / charge number). It is denoted by '$Z$'.
2. Atomic Mass ($A$): The sum of the number of protons and neutrons in a nucleus is called the mass number or atomic mass, denoted by '$A$'. This is also referred to as the nucleon number.
3. Neutron Number ($N$): The difference between the mass number and the atomic number is called the neutron number. It is denoted by '$N$' and is given by the relation: $$N = A - Z$$

Representation of an Element

An element $X$ having mass number $A$ and atomic number $Z$ is conventionally represented by the symbol $_Z^A X$, where $X$ is the chemical symbol for the particular element.

Examples of Element Composition

Name of Element	Atomic Number ($Z$)	Atomic Mass ($A$)	Neutron Number ($N = A - Z$)
Hydrogen	$1$	$1$	$0$
Helium	$2$	$4$	$2$
Lithium	$3$	$7$	$4$
Beryllium	$4$	$9$	$5$
Boron	$5$	$11$	$6$

Isotopes

"Elements having the same atomic number but different mass numbers (or different neutron numbers) are called isotopes."

Alternatively, isotopes can be defined as atoms of the same element having the same number of protons but different numbers of neutrons.

Properties of Isotopes

Isotopes of an element share identical chemical properties because chemical behavior depends on the number of orbital electrons, which remain the same across isotopes. However, they exhibit different physical properties because physical characteristics depend on nuclear mass and neutron count.

Isotopes of Hydrogen

Hydrogen has three natural isotopes:

• Protium ($_1^1\text{H}$): $A = 1$, $Z = 1$, $N = 0$
• Deuterium ($_1^2\text{H}$): $A = 2$, $Z = 1$, $N = 1$
• Tritium ($_1^3\text{H}$): $A = 3$, $Z = 1$, $N = 2$

Fig: Comparative atomic structure of the three isotopes of hydrogen (Protium, Deuterium, and Tritium).

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