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
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 : 35Cl17 ,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 , 235U92
, 238U92 ,
** 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
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. .
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