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The word isotopes originates from the Greek language and means equal places. Molecules of different masses of the same element were called isotopes a term first used by Frederick Soddy in 1913.

The name isotope comes from Greek isos meaning equal and topos meaning place and reflects the fact that isotopes are at the same place on the periodic table. Today more than 2200 isotopes of the 92 naturally occurring elements are known.

What is Isotopes?

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Atoms of an element that differ in neutron count are called isotopes. "Isotopes are atoms of an element that have the same  number of electrons but different numbers of neutrons." Different isotopes always have the same number and different mass numbers.

Isotopes occupy an equal place (location) in listings of elements because all isotopes of an element have the same atomic number.

Isotopes Definition

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Definition of isotope

"The isotopes may be defined as atoms possessing the same number of protons but different numbers of neutrons in their respective nuclei." To define isotopes, isotopes occupy the same position in the periodic in the periodic table of the elements.

Isotopes of the same element have different physical properties they may differ in terms of density, relative mass, melting point and so on. Isotopes of an element have different nucleon number.

Radioactive Isotopes

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At the beginning of twentieth century detailed investigations were made of the disintegration of naturally occurring radioactive isotopes. The spontaneous disintegration of atoms is responsible for radioactivity. Those isotopes which decay (break up) in this way are said to be radioactive isotopes.

"The half-life of a radioactive isotopes is the time taken for the amount or concentration of the isotope to fall to half of its original value." The half-life of a radioactive isotope is virtually unaffected by external conditions. It can be used as a measure of the relative stability of an isotope.

Radioactive isotopes uses


Radioactive isotopes are used in the treatment of cancer, studying metabolic pathways in measuring thickness gauges and empty packet detectors and Archaeological and geological uses.

Isotopes of Hydrogen

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It has been found by mass spectrograph that hydrogen has three isotopes namely Protium, Deuterium and Tritium. The three hydrogen isotopes terms of atomic structure have been shown below.

Isotopes of Hydrogen

It may be noted that three isotopes of hydrogen have same properties because of the same electronic configuration. However due to different mass numbers they have different rates of chemical reactions. Difference in properties arising due to the difference in mass number is referred to as isotopic effect.

Carbon Isotopes

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Carbon atom can occur naturally in three different forms namely carbon-12, caron-13 and carbon-14. These carbon atoms have the same number of protons but different numbers of neutrons.

Diagrammatic representation of isotopes of carbon are shown below.

Carbon Isotopes

Oxygen Isotopes

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Oxygen has three stable isotopes. The isotope composition of O in nature varies as a result of isotope fractionation during isotope exchange reactions, phase changes and as a result of kinetic effects.

Diagrammatic representation of isotopes of oxygen are shown below.

Oxygen Isotopes

Table of Isotopes

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The following table summarizes the list of isotopes in a descriptive fashion.

S.No
Isotopes
Symbol
Uranium isotopes 232U, 238U, 235U
2
Nitrogen isotopes 14N, 15N
3
Chlorine isotopes
35Cl, 36Cl, 37Cl
4
Copper isotopes
63Cu, 65Cu 
5
Helium isotopes 3He, 4He
6
Bromine isotopes
79Br, 81Br
7
Sulfur isotopes
32S, 33S, 34S, 36S
8
Lithium isotopes
6Li, 7Li, 8Li
9
Potassium isotopes
39P, 40P, 41P
10
Iron isotopes
54Fe, 56Fe, 57Fe, 58Fe
11
Boron isotopes
10B, 11B
12
Calcium isotopes 40Ca, 42Ca, 43Ca, 44Ca, 46Ca
13
Sodium isotopes
22Na, 24Na
14 Neon isotopes 20Ne, 21Ne, 22 Ne 
15
Iodine isotopes
127I, 129I
16
Phosphorus isotopes
32P
17
Fluorine isotopes
19F
18
Aluminum isotopes
26Al, 27Al
19
Silicon isotopes
28Si, 29Si, 30Si, 32Si
20
Gold isotopes
197Au
21 Isotopes of magnesium
24Mg, 25Mg, 26Mg 

Some of the examples of isotopes are given below.
  • Uranium-238
  • Carbon-14
  • Strontium-90
  • Iodine-131
  • Bismuth-214
  • Polonium-214

Stable Isotopes

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All elements can have a different number of neutrons that is different masses. They are called isotopes of the respective element. Most of the isotopes are unstable, only about 300 are stable.

Stable isotopes do not appear to decay to any other nuclide once they formed. In contrast radioactive isotopes decay spontaneously emitting alpha or beta particles to become stable isotopes.

Nuclear Symbol for Isotopes

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An isotope is represented by a nuclear symbol that has the form

$_{mass number}^{atomic number}\textrm{X}$
or
$_{A}^{Z}\textrm{X}$

X is the chemical symbol of the element. The symbol and the mass number are actually well need to identify an isotope, so the atomic number Z is sometimes omitted.

Uses of Isotopes

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With the ready availability of radioisotopes innumerable uses are known and the typical examples would be included here to give an idea of the vast scope of the technique. They are listed below.
  1. Chemical investigations
  2. Physio-chemical applications
  3. Analytical applications
  4. Age determinations
  5. Medical applications
  6. Agricultural applications
  7. Prospecting of natural resources.
  8. Industrial applications

Atomic Mass of Isotopes

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Naturally occurring isotopes have atomic mass same as the single isotopes. The situation with most elements is more complex, because they occur in nature as a mixture of two or more isotopes. To determine the atomic mass of such an element it is necessary to know not only the masses of the individual isotopes but also their atom percents in nature.

If the atomic mass of an element is known and if it has only two stable isotopes, their abundances can be calculated.

Characteristics of Isotopes

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The isotopes' physical properties are different mainly due to the neutron number variation, present in the nucleus.

Properties such as melting point, boiling point, density etc., which depend upon the atomic mass should be different for different isotopes because the isotopes of an element have different masses.

Isotope Problems

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Example problems based on isotopes are given below.

Solved Examples

Question 1: Boron has two isotopes one with five neutrons and the other with six neutrons. What are the mass numbers and symbols of these isotopes?
Solution:
 
Boron has atomic number of 5, so it has five protons in its nucleus. Therefore the mass numbers of the two isotopes are given by the sum of their numbers of protons and neutrons.

Isotope 1 : B = 5 protons + 5 neutrons = 10 (boron-10)
Isotope 2 : B = 5 protons + 6 neutrons = 11 (boron-11)

So the mass numbers are 10 and 11.
The symbols are $_{10}^{5}\textrm{B}$ and $_{11}^{5}\textrm{B}$
 

Question 2: The half life of the sodium isotope $_{24}^{11}\textrm{Na}$ against beta decay is 15h. How long does it take for seven eights of a sample of this isotope to decay?
Solution:
 
After seven eights has decayed, one eights is left, and $\frac{1}{8} = \frac{1}{2} \times \frac{1}{2} \times \frac{1}{2}$ which is the half lives.
Hence the answer is (3)(15)h = 45h