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best notes for class 11 chemistry ch-3
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Lecture notes of 9 pages for the course Physcis Chemistry Maths CS at Fifth year / 11th Grade (Best Notes)
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XIChemistry
Classification of Elements and Periodicity in Properties – Notes VL
1. Dobereiner's Triads: In 1817 a German chemist Doberneiner identified certain
groups of three elements. The group of three elements having similar properties
was called triad. When three elements were arranged in order of their increasing
atomic masses, the atomic mass of the middle element was roughly the mean of the
atomic masses of the other two elements. Refer NCERT book table 3.1for
examples.
2. New Lands Law of octaves: When elements were arranged in order of their
increasing relative atomic masses. The properties of every eight elements were
similar to the first one, like the eighth note of a musical scale. This repetition in the
properties of elements is just like the repetition of eighth node in an octave of
music. Refer NCERT book table 3.2 for examples.
3. Mendeleev's Periodic Law: The physical and chemical properties of elements
are the periodic function of their atomic masses.
4. Mendeleev's Periodic Table: Mendeleev's periodic table contains vertical
columns called groups and horizontal rows called periods. There were 7 periods
and 8 groups. Noble gases were not known at that time. So there was no group of
noble gases. The elements in each group of the periodic table are similar to one
another in many properties. The similar properties of the elements are
repeated periodically.
Merits of Mendeleev's classification:
a) Mendeleev's periodic law predicted the existence of some elements that had not
been discovered at that time. Refer NCERT book table 3.3 for examples.
b) He could predict the properties of several elements on the basis of their position
in the periodic table.
c) He could accommodate noble gases when they were discovered.
Limitations of Mendeleev's classification:
a) The correct position could not be assigned to the hydrogen in the periodic table.
b) Wrong order of the atomic masses of some elements could not be explained.
c) The position of isotopes could not be explained.
, Moseley, the English physicist showed that atomic number is more fundamental
property of an element than its atomic mass. Therefore, the position of an element
in the periodic table depends on its atomic number than its atomic mass.
5. Modern Periodic Law: The physical and chemical properties of elements are
the periodic functions of their atomic numbers.
6. Modern Periodic Table: This table was prepared by Bohr and is based upon the
electronic configuration of elements. The table consists of 18 vertical columns
called groups. Elements having similar outer electronic configurations in their
atoms are arranged in vertical columns, referred to as groups. According to the
recommendation of International Union of Pure and Applied Chemistry (IUPAC),
the groups are numbered from 1 to 18 and the table consists of 7 horizontal rows
called periods. The first period contains 2 elements. The subsequent periods
consists of 8, 8, 18, 18 and 32 elements, respectively. The seventh period is
incomplete. In this form of the Periodic Table, 14 elements of both sixth and
seventh periods (lanthanoids and actinoids, respectively) are placed in separate
panels at the bottom
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