10 SCIENCE REVISION NOTES
CHAPTER 5 PERIODIC CLASSIFICATION OF ELEMENTS
Several attempts were made
to classify elements based on their properties to have an orderly arrangement.
Dobereiner's
Triads:
Johann Wolfgang Dobereiner
identifed some groups having three elements and called them as triads. He
stated that when three elements are in a triad in the increasing order of
atomic masses, the atomic mass of the middle element is roughly the average of
the atomic masses of the other two elements.
Example: CI, Br, I
Atomic mass of Br
= Atomic
mass of CI + Atomic mass of Br = 35.5 +126.9
2 2
= 79.9 for Br.
‣
This system of classification into triads failed because there were only three
triads identified. Therefore this classification failed to arrange all the
elements in an orderly way:
Newlands' Laws of
Octaves:
It states that when elements
are classified in a row in the increasing order of atomic mass, every eighth
element has the properties similar to that of the first element
Example: Lithium being the
first element, the eighth element is sodium, which resembles lithium in its
properties
Li,
Be, B, C, N, O, F Ne, Na
‣
This classification failed because
★ The
law was applicable only up to calcium
★ There
were only 56 elements at that time
★ Newlands
adjusted two elements in the same slot
★ With
the discovery of noble gases, the law of octaves became irrelevant
Mendeleev's
Periodic Law:
it states: "The
properties of the elements are the periodic functions of their atomic
masses"
‣
Characteristics of Mendeleev's Periodic Table:
★ The
classification was made based on increasing order of atomic masses
★ There
were only 63 elements
★ There
were 8 groups and 6 periods
★ There
were some gaps left where the new elements discovered could fit in it.
‣ Demerits:
★ He
placed elements of slightly greater atomic mass before elements with slightly
lower atomic mass
★ There
was no fixed position for hydrogen
★ It
was not classified on electronic configuration
★ Isotopes of all elements was a challenge to Mendeleev's Periodic Law.
Modern Periodic
Table:
Modern Periodic Table is
based on atomic numbers of elements. Modern periodic law states: "The
properties of the elements are the periodic function of their atomic numbers.
All the elements are arranged in the increasing order of their atomic numbers
‣ Characteristics and
features of the Modern Periodic Table:
★ Elements
are classified in the increasing order of their atomic numbers
★There are 18 groups and 7 periods (Vertical columns are called groups and horizontal rows are called periods.
★ Elements
are classified based on their electronic configuration
★ Metals,
non-metals and metalloids are placed in their proper positions
★ All
noble gases are placed in the 18" group. These elements have stable
electronic configuration
‣ Trends in Modern Periodic
Table:
★ Valency:
Valency is determined by the number of valance electrons present in the
outermost shell
In a group: For
the elements of the groups I, 2, I3 and 14, the valency is equal to the number
of valence electrons. For the groups 15 onwards, valency =8- valency electrons.
In a period: The
valency increases from I to 4 and decreases to zero from |5 to 18.
★ Atomic
size: It is defined as the radius of an atom. It is the
distance between the centre of the nucleus and the outermost shell of an
isolated atom.
Along a group:
Atomic size increases down the group because new shells are being added
In a
period: Atomic radius decreases in moving from left to right
along a period.
★ Metallic and non-metallic character:
Metallic character decreases
across a period and increases down a group. Non-metallic character increases
across a period and decreases down a group.
★ Nature
of oxides: Along a period the basic character of oxides of the
elements decreases, while their acidic character increases.
Down a group basic character
of oxides increases and acidic character decreases.
★ Electronegativity:
Electronegative character
decreases on going down in a group and increases along a period.
★ Shells
in an atom: The maximum number of shells that can be accommodated
in a shell is given by the formula 2n²
K Shell – 2 x 12 =
2
L Shell – 2 x 22 =
8
The third, fourth, fifth,
sixth and seventh periods have 8, 18, 18, 32 and 32 elements respectively.
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