Isotopes
Isotopes
Isotopes: These are defined as the atoms of the same element, having the same atomic number but different mass numbers. E.g. there are 3 isotopes of hydrogen atom, namely protium (1H), deuterium (2H) and tritium (3H).
In other words, it can be said that isotopes have same number of protons but differ in the number of neutrons. Each isotope of an element is a pure substance. Since, chemical properties of elements largely depend on their electronic configuration or outermost electrons and as the isotopes of an element have similar electronic configuration.Therefore, isotopes of an element have the same chemical properties. We know that, masses of isotopes of elements are different. Since, physical properties such as density, light scattering etc., depend on mass therefore, these are different for isotopes of an element.
Average Atomic Mass: If an element has no isotopes, the mass of its atom would be the same as the sum of protons and neutrons in it. But if an element occurs in isotopic forms, then from the percentage of each isotopic form, the average mass is calculated as:
Average atomic mass of an element:[(Atomic mass of isotope I x percentage of isotope I) + (Atomic mass of isotope II x percentage of isotope II) +...]
Example: The two isotopic forms of chlorine atom with masses 35u and 37u occur in the ratio of 3:1. Therefore, the average atomic mass of chlorine atom, can be calculated as:
The average atomic mass of chlorine atom:
The 3:1 ratio of isotopes mean 75% of and 25% of
Therefore, average atomic mass of chlorine is
We can calculate the number of neutrons of different isotopes.
Number of Neutrons = Atomic mass - Atomic number
For eg: There are three isotopes of carbon(C) with same numbers of protons and electrons but differ in number of neutrons.
No. of neutrons 12-6 = 6 13-6 =7 14-6 = 8
Here, 35.5 u is not the atomic mass of any one atom of chlorine but it shows that its given amount contains both the isotopes and their average atomic mass is 35.5 u.
NOTE: The fractional atomic masses of elements are due to the fact that all elements have isotopes. The isotopes of an element have different atomic masses. Since the atomic mass of an element is the average atomic mass of all the natural isotopes of that element, most elements have fractional atomic masses.
Applications of Isotopes:
Tritium is the isotope of H whith number of neutrons, electrons and protons respectively: | |||
| Right Option : B | |||
| View Explanation | |||
Which of the following are correct : (a) An isotope of iodine (I-131) is used in the treatment of goiter. (b) An isotope of cobalt (Co-60) is used to remove brain tumours and in the treatment of cancer. | |||
| Right Option : C | |||
| View Explanation | |||
In which one of the following, the number of protons is greater than the number of neutrons but number of protons is less than the number of electrons? | |||
| Right Option : D | |||
| View Explanation | |||
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