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Science Class 10 Notes for Periodic Classification of Elements
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” But
in some triads all the threc elements possessed nearly the same atomic masses, therefore the
law was rejected
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g
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In the triad ofFe, Co, Ni, all the three elements have a nearly
equal atomic mass and thus does not follow the above law
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eight note of musical scale
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But later on several new element were discovered
whose properties did not fit into law of octaves
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Thus, Newland‟s classification was not accepted
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According
to Mendeleev “the properties of the elements are a periodic function of their atomic masses
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Merits of Mendeleev’s Periodic Table:

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(i) At some places the order of atomic weight was changed in order to justify the chemical and
physical nature
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(iii) One of the strengths of Mendeleev‟s periodic table was that, when inert gases were
discovered they could be placed in a new group without disturbing the existing
order
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(ii) There are eight groups indicated by Roman Numerals I, II, III, IV, V, VI, VII, VIII
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The elements that are present on the left hand side in each
group constitute sub-group A while those on the right hand side form sub-group B
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(iii) There are six periods (numbered 1, 2, 3, 4, 5 and 6)
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The first half ofthe elements are
placed in the upper left corners and the second half occupy lower right corners in each box
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(ii) Prediction of new elements and their properties : Many gaps were left in this table
for undiscovered elements
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This helped in the discovery of these elements
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(iii) Correction of doubtful atomic masses :
Mendeleev corrected the atomic masses of certain elements with the help of their
expected positions and properties
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(ii) The isotopes of the same element will be given different position if atomic number is taken
as basis, which will disturb the symmetry of the periodic table
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So it was not possible to predict how many elements could be discovered between
two elements
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Modern Periodic Law : This law was given by Henry Moseley in 1913
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Cause of periodicity : Periodicity may be defined as the repetition of the similar properties of
the elements placed in a group and separated by certain definite gap of atomic numbers
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The groups have been numbered 1, 2, 3 …
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(ii) The elements belonging to a particular group make a family and usually named after the
first member
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(iii) In a period all the elements contain the same number of shells, but as we move from left to
right the number of valence shell electrons increases by one unit
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e
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In some special cases it is 2 electrons)
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The atomic radius decreases on moving from left to right along a period
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In a group atomic size decreases from top to bottom due to increase in number of shells
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In a group metallic character increases from top to bottom while non-metallic character
decrease
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In a period from left to right, the value of electronegativity increases while in a group from top
to bottom the value of electronegativity decreases
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ncerthelp

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