Examples of isotopes

Definition of isotopes

Isotopes may be defined as :

(1) The atoms of an element which have the same number of protons and different number of neutrons are called Isotopes.

(2) The atoms of an element which have the same atomic number but different atomic masses or mass numbers.

Examples of isotopes

** Since isotopes of an element have the same atomic number, each of these contains equal number of protons. They have different atomic masses which is accounted for by the different number of neutrons present in the nucleus. Thus the isotopes of an element are characterised by different number of neutrons in the nucleus.

** The atomic structure of an isotope with atomic number Z and mass number A (atomic mass in amu) can be given as follows :

(1) The number of extranuclear electrons = Z

(2) The number of protons in the nucleus = Z

(3) The mass number A is equal to the total number of protons (Z) and neutrons (N) in the

nucleus. That is,

A = Z + N

∴ N = A – Z

Isotopes of Hydrogen

** There are three isotopes of hydrogen : protium ¹H₁, deuterium ²H₁ or D , and tritium ³H₁ or T .

** Protium is by far the most abundant in natural hydrogen, deuterium about 0.015% and tritium only one out of 10,000,000 hydrogen atoms.

** The atomic number of the three isotopes of hydrogen is 1, while their mass numbers are : protium 1, deuterium 2, and tritium 3.

** Therefore each of the three isotopes has one extranuclear electron and one proton in the nucleus.

** The nucleus of protium is made of one proton only, while the number of neutrons (A – Z) present in deuterium is 2 – 1 = 1, and in tritium 3– 1 = 2.

** The structure of the three isotopes of hydrogen can be pictorially represented as:

Isotopes of Neon

** Neon has been found to consist of three isotopes : ²⁰Ne₁₀, ²¹Ne₁₀, ²²Ne₁₀ . Their percentage abundance is:

²⁰Ne         ²¹Ne        ²²Ne

90.92%    0.257%      8.82%

** The atomic number of the three isotopes of neon is 10, while their mass numbers are 20, 21 and 22 respectively.

** Therefore each of these isotopes has ten extranuclear electrons and ten protons in the nucleus.

** The number of neutrons (A – Z) are : ²⁰Ne, 20 – 10 = 10 ; ²¹Ne, 21 – 10 = 11 ; ²²Ne, 22 – 10 = 12.

** The atomic structure of the isotopes of neon can, therefore, be represented pictorially as:

Isotopes of Oxygen

** Oxygen has three isotopes : ¹⁶O₈ , ¹⁷O₈ , ¹⁸O₈ These are found with the relative abundances of 99.759, 0.037 and 0.204 respectively.

** The atomic number of the above three isotopes of oxygen is 8 while their mass numbers are 16, 17 and 18.

** Therefore each isotope has 8 extranuclear electrons and 8 protons in the nucleus.

** The number of neutrons (A – Z) in the three isotopes is:

¹⁶O     16 – 8 = 8 neutrons
¹⁷O      17 – 8 = 9 neutrons
¹⁸O     18 – 8 = 10 neutrons

** The complete atomic structure of the isotopes of oxygen can be pictorially represented as:

Isotopes of Chlorine

** Chlorine is a mixture of two isotopes : ³⁵Cl_17 ,³⁵Cl₁₇ Their percentage abundance is 75.53 and 24.47 respectively.

** The atomic number of the two isotopes of chlorine is 17 while their mass numbers are 35 and 37.

** Therefore each isotopes has 17 extranuclear electrons and 17 protons in the nucleus.

** The number of neutrons (A – Z) in these isotopes is:

³⁵Cl  35 – 17 = 18 neutrons

³⁷Cl  37 – 17 = 20 neutrons

** The atomic structure of the isotopes of chlorine can be pictorially represented as:

Isotopes of Uranium

** There are three isotopes of uranium : ²³⁴U₉₂ , ²³⁵U_{92 ,} ²³⁸U_{92 ,} 

** Natural uranium consists almost entirely of ²³⁸U, with about 0.72% of ²³⁵U and 0.006% of ²³⁴U.

** These isotopes are particularly important in atomic energy.

** The atomic number of the three isotopes of uranium is 92 and their mass numbers are 238, 235 and 234.

** Thus each isotope has 92 extranuclear electrons and 92 protons.

** The number of neutrons (A – Z) in these isotopes is:

²³⁸U     238 – 92 = 146 neutrons
²³⁵U     235 – 92 = 143 neutrons
²³⁴U    234 – 92 = 142 neutrons

** The atomic structure of the three isotopes of uranium may be represented as:

Isotopes of Carbon

** Carbon has three naturally occurring isotopes.

** The isotopes of carbon are :

Carbon-12: which constitutes 98.89 of all carbon atoms and serves as the standard for the atomic mass scale

Carbon-13: which is the only magnetic isotope, making it very important for structural studies of compounds containing carbon

Carbon-14: which is produced by cosmic rays bombarding the atmosphere.

** Carbon-14 is radioactive, with a half-life of 5760 years.

** The amount of carbon-14 remaining in historical artifacts can be used to estimate their age.

Note:

** Almost every element in nature exists as a mixture of isotopes.

** The isotopes of the elements with atomic numbers 1 to 10, and their structure is listed in the Table blow.

** It may be noted that some elements e.g., fluorine, are monoisotopic. These are found in nature only as a single isotope. About 20 elements are monoisotopic.

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition. .