ACKNOWLEDGEMENT:
Figure
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Source of Figure
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Figure: The Periodic Table
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sciencenotes.org
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Isotopes:
Isotopes are atoms of the same element with same atomic numbers
but different mass numbers. This is because of the presence of different number
of neutrons in the nucleus.
Example: Cl - 35 and Cl - 37, are 2 isotopes of Chlorine. Both have 17 protons and 17
electrons but Cl - 37 has 20 neutrons, while Cl - 35 has 18 neutrons.
Approximately, Chlorine – 35 is 75% in nature with Chlorine – 37
being 25%.
The Atomic Mass of Chlorine is therefore calculated as:
Ar = 35 x 0.75 + 37 x 0.25 = 35.5
Note: This is how the Ar of various elements shown in
the Periodic Table is calculated. It can be seen that the Ar is
usually a decimal number.
Relative Atomic Mass or
Ar:
The Relative Atomic Mass or Ar of an element is
defines as the mass of an average atom of the element compared with 1/12th the mass of an C - 12 atom which is taken as 12
units.
Figure: The Periodic Table
Isotopes &
Radioactivity:
Some isotopes of the same element are radioactive. For example, H - 3 (Tritium) is radioactive,
while H - 1 (Hydrogen) is not. The 2
neutrons in the Tritium nucleus make it unstable and radioactive in nature.
According to a thumb’s rule; an atom is unstable if the
neutron-proton ratio increases 1.5:1. In Tritium, the neutron-proton ratio is
2:1, thus it is unstable and radioactive.
