Since, all isotopes of an element have the same atomic number; hence, all isotopes should contain the same number of protons inside their nuclei. Also, since, different isotopes of an element have different mass numbers, hence, the number of neutrons in the nuclei of isotopes of an element should be different. So, isotopes may also be defined as,
“The atoms of the same element which have the same number of protons but different number of neutrons inside their nuclei are called isotopes of that element.” Isotopes are described by writing the mass number of that isotope as a superscript only the left side (top-left side) of the symbol of the element. The atomic number is written as a subscript only the left side (bottom-left side) of the symbol. For example, isotope of an element But is described as,
Hydrogen (H) has three isotopes having mass numbers 1, 2 and 3, but all having atomic number equal to 1. These three isotopes of hydrogen can be described as follows. Chlorine (Cl) has two isotopes having mass numbers 35 and 37. These are called as chlorine-35, and chlorine-37. Both the isotopes of Cl have the same atomic number equal to 17. So, the two isotopes of chlorine are described as follows.
Carbon (C) has three isotopes having mass number 12, 13 and 14. These are called carbon-12, carbon -13 and carbon14 respectively. These isotopes have the same atomic number equal to 6. The three isotopes of carbon are described below.
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Characteristics of isotopes
(i) Isotopes of an element have the same number of proton inside their nuclei .as a result; all the isotopes of an element contain the same number of electrons.
(ii) Different isotopes of an element have different mass numbers. So isotopes show the following characteristics.
(a) Since, isotopes of an element have the same number of protons and electrons; hence all the isotopes of an element show the same chemical properties, same electronic configurations, and the same number of valence electrons. For example , for the three isotopes of oxygen,
168O 178O 188O
No. of protons 8 8 8
No. of electrons 8 8 8
Electronic configuration 2.6 2.6 2.6
No. of Valence electrons 6 6 6
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Isotopes of an element have different masses. So, the properties which depend upon the atomic mass should be different for different isotopes. Many physical properties e.g., melting point, boiling point, density, etc., depend upon the atomic mass. So different isotopes of an element show different physical properties. For example, physical properties of the two isotopes of hydrogen are different.