In our
previous blogs, we talked about atomic number and mass number. On the basis of
atomic and mass number, atom is further divided into two species. Today we are going to discuss these species
of the atom. First one is;
Isotopes
In nature, such atoms of some elements have
been identified, which have the same atomic number but different mass numbers.
For example, in the case of hydrogen atom, it has three atomic species, namely
protium (11H), deuterium (21 H or
D) and tritium (31H or T). The atomic number of each one
is 1, but the mass number is 1, 2 and 3, respectively. Other such examples are
(i) carbon 12 and carbon 14, having mass
number 12 and 14 respectively, (ii) chlorine 35 and chlorine 37, having mass 35
and 37 respectively. These species are called Isotopes. Isotopes are defined as
the atoms of the same element, having the same atomic number but different mass
numbers. Therefore, we can say that there are three isotopes of hydrogen atom,
two isotopes of carbon atom and two isotopes of chlorine atom.
Each isotope of an element is a pure
substance. The chemical properties of isotopes are similar but their physical
properties are different. On this account, the question is which one should we
consider as the mass of an atom. For example chlorine occurs in nature in two
isotopic forms, with masses 35 u and 37 u in the ratio of 3:1. and atomic mass
of an element should not be more than one. Hence, the mass of an atom of any
natural element is taken as the average mass of all the naturally occurring
atoms of that element. That is if an element has no isotopes, then the mass of
its atom would be the same as the sum of protons and neutrons in it. But if an
element occurs in more than one isotopic forms, then we have to know the
percentage of each isotopic form and then the average mass is calculated. The
average atomic mass of chlorine atom is 35.5 u. This does not mean that any one
atom of chlorine has a fractional mass. It means that if you take a certain
amount of chlorine, it will contain both isotopes of chlorine and the average
mass is 35.5 u.
Applications
Since the chemical properties of all the
isotopes of an element are the same, normally we are not concerned about taking
a mixture. But some isotopes have special properties which find them useful in
various fields. Such as: (i) An isotope of uranium is used as a fuel in nuclear
reactors. (ii) An isotope of cobalt is used in the treatment of cancer. Etc.
And the second species are Isobars
In case of some elements it was found that they have different atomic number but same mass number. For example calcium and argon. Atomic number of calcium is 20 and atomic number of argon is 18 but the mass number of both elements is 40. That is, the total number of nucleons is the same in the atoms of this pair of element. Hence, the elements having different atomic numbers and same mass numbers are defined as Isobars.
Fore more details refer to Beyond Reaction: Fundamental Chemistry
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