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CLASS - X
SUBJECT – CHEMISTRY

TOPIC-Periodic Classification of Elements
Scientists made several attempts to classify elements according to their properties and
obtain an orderly arrangement out of chaos
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Döbereiner’s Triads (1817): Groups having three elements each, written in the order of
increasing atomic masses such that the atomic mass of the middle element was roughly
the average of the atomic masses of the other two elements
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9, 23, 39)
At
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9 + 39 = 45
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Limitations of Döbereiner’s Triads:
Döbereiner could identify only three triads from the
elements known at that time
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Newlands’ Law of Octaves (1866): The then known 56 elements were arranged in the
order of increasing atomic masses and it was found that every eighth element had
properties similar to that of the first
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Therefore, he called it the ‘Law of Octaves’
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Drawbacks of Newlands’ Law of Octaves
• It was applicable only upto calcium, as after calcium every eighth element did not
possess properties similar to that of the first
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But, later on, several new elements were
discovered, whose properties did not fit into the Law of Octaves
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Ex: cobalt and nickel are
in the same slot and these are placed in the same column as fluorine, chlorine and
bromine which have very different properties than these elements
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MENDELÉEV’S PERIODIC TABLE (1872)
• When Mendeléev started his work, 63 elements were known
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• He concentrated on the compounds formed by elements with oxygen and hydrogen
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• The formulae of the hydrides and oxides formed by an element were treated as one of
the basic properties of an element for its classification
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Mendeléev Periodic Law: The properties of elements are the periodic function of their
atomic masses
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The
sequence was inverted so that elements with similar properties could be grouped
together
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He boldly predicted the existence of
some elements that had not been discovered at that time
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For instance, he named the yet to be discovered elements Eka–boron, Eka–aluminium
and Eka–silicon
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• Noble gases like helium (He), neon (Ne) and argon (Ar) were discovered very late
because they are very inert and present in extremely low concentrations in our
atmosphere
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Limitations of Mendeléev’s Classification
• Hydrogen resembles of alkali metals (in terms of Electronic configuration and
formulae of compounds with halogens, oxygen and sulphur) and also halogens (in
terms of of existence of diatomic molecules and combining with metals and nonmetals to form covalent compounds)
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• Mendeléev table could not account for the position of isotopes which have different
atomic masses but similar chemical properties
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• Atomic masses did not increase in a regular manner in going from one element to the
next in Mendeléev table
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Henry Moseley in 1913 showed that the atomic number of an element is a more
fundamental property than its atomic mass and gave The Modern Periodic Law
‘Properties of elements are a periodic function of their atomic number
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Elements, when arranged in order
of increasing atomic number Z, give the Modern Periodic Table
Modern Periodic Table
It has 18 vertical columns known as ‘groups’ and 7 horizontal rows known as ‘periods’
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For ex:,all
the group 1 elements- Li, Na, K , Rb , Cs , Fr have 1 valent electron
Elements fluorine (F) and chlorine (Cl) belong to group 17 and have 7 electrons in
their outermost shell
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• The number of shells increases as we go down the group
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Periods
On moving from left to right in a period• the elements do not have the same number of valence electrons, but they contain the
same number of shells
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• Na, Mg, Al, Si, P, S, Cl and Ar belong to the third period of the Modern Periodic
Table and hence electrons in the atoms of these elements are filled in K, L and M
shells
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• Each period marks a new electronic shell getting filled
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e the number of elements that are present in a period
The maximum number of electrons that can be accommodated in a shell depends on the
formula 2n2 where ‘n’ is the number of the given shell from the nucleus
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L Shell – 2 x (2)2 = 8, hence the second period has 8 elements
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The position of an element in the Periodic Table tells us about its chemical reactivity
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Elements in the same group have similar chemical properties because they have
same valent shell electronic configuration
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• Valency of an element is determined by the number of valence electrons present in
the outermost shell of its atom
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• For elements in groups 13 and 14 valency is equal to the number of valent electrons
which is equal to group number ─ 10
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• For elements in groups 15 - 18 valency is equal to 8─ number of valent electrons
which is equal to group number ─ 10
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For ex all elements of group 2 have valency 2
• In a period on going from left to right the valency increases from 1 to 4 (for group 14)
and then decreases to 0 (for group 18)
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No

Element/Symbol

At
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No
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No

Electronic
Configuration
K
L M

No
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2
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4
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6
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8
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10
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12
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14
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16
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18
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20
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• It is the distance between the centre of the nucleus and the outermost shell of an
isolated atom
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Valency

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•

Atomic radius decreases in moving from left to right along a period
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Atomic size increases down the group
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This increases the distance between the outermost electrons
and the nucleus so that the atomic size increases
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• In the middle, we have silicon, which is classified as a semi-metal or metalloid
because it exhibits some properties of both metals and non-metals
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The borderline elements – boron, silicon, germanium, arsenic, antimony,
tellurium and polonium – are intermediate in properties and are called metalloids
or semi-metals
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• Down the group, the effective nuclear charge experienced by valence electrons is
decreasing because the outermost electrons are farther away from the nucleus
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Hence metallic character increases down a
group
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Hence metallic
character decreases across a period
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They tend to form bonds by
gaining electrons
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• Electronegativity increases across a period from left to right
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