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THE PRAYAS TUTORIAL
PERIODIC CLASSIFICATION
IMPORTANCE OF CLASSIFICATION OF ELEMENTS
At present 114 elements are known. The classification of elements according to
the properties and obtain an orderly arrangement so that, it would be easy to study a
large number of elements.
DOBEREINER TRIADS
He classified certain elements into a groups of three called TRIADS.3 Elements
in a triad were written in the increasing atomic masses. The atomic mass of the middle
element was the average of the atomic mass of the other two.
LIMITATION
Only a limited number of elements could give such triads.
NEWLANDS LAW OF OCTAVES
According to him, the elements are arranged in the order of atomic weights the
eight element starting from a given one is a kind of repetition of the first.
EX:-Na is the eight element after Li.
LIMITATIONS OF NEWLAND’S LAW OF OCTAVES
It was applicable only upto calcium because after calcium, the eighth element did
not possess properties similar to the first.
The properties of newly discovered elements did not fit into the Law Of Octaves.
Newlands adjusted two elements in the same slot, but also fit unlike elements
under the same slot just to fit elements into his table. This was not acceptable.
ADVANTAGE
Newlands Law of Octaves worked well with lighter elements.
MENDELEEV’S PERIODIC TABLE
Properties of elements are a periodic function of their atomic weight. i.e., similar
elements are repeated at regular intervals.
He related the properties of elements with their atomic mass.
The hydrides and oxides formed by an element was treated as a basic property for
classification.
The elements hydrogen and oxygen were selected because they formed compounds
with most elements.
Then he arranged elements in the increasing order of atomic mass.
He found most of the elements fit into the periodic table.
He also observed periodic recurrence of elements with similar physical and chemical
properties.
ACHIEVEMENTS OF MENDELEEV’S PERIODIC TABLE
The element Cobalt (atomic mass: 58.9) appeared before Nickel(atomic mass: 58.7)
because he needs that element with similar properties would be grouped together.
He left some gaps in his periodic table because he already predicted the existence of
some elements not discovered at that time. E.g. Sc, Ga, Ge, were the elements
discovered later but the properties were similar to eka-boron, eka-aluminium and ekasilicon.
Noble gases like Helium, Neon, and Argon have been mentioned in his context though
they are highly inert and low concentration in our atmosphere. When these gases were
discovered they placed a new group without disturbing the existing order.
LIMITATIONS OF MENDELEEV’S CLASSIFICATIONS
He could not assign a correct position to hydrogen because hydrogen resembles alkali
metals in electronic configuration and also resembles halogens because of its existence
as a diatomic molecule. It reacts with metals as well as non-metals to form covalent
compounds.
The elements were arranged in the increasing atomic mass, but the atomic masses do
not increase in a regular manner.
MODERN PERIODIC LAW
The chemical and physical properties of an element depend on the number of electrons,
and their arrangement in an atom. The classification should be based on the number of
electrons and their arrangement.
This leads mostly to change the basis of classification of elements from atomic weight to
atomic number.
With this change, in the basis of classification defects in Mendeleev’s Table regarding
anomalous pairs of elements and position of isotopes disappear.
THE MODERN PERIODIC LAW
The physical and chemical properties of elements are the periodic Function of atomic
number
Group:The vertical column is a group. The No. of vertical column is 18.
Period:The horizontal row is called a period & No. of horizontal rows are 7.
Trends in the modern periodic table
Valency: The valency of the element is determined by the number of valence electrons
present in the outermost shell of an atom.
Eg:
Electronic
Element
Atomic No.
Mg
12
2,8,2
2
S
16
2,8,6
2
arrangement
Valence electron
On going down a group the valency remains the same.
On moving from left to right, the valency of the element varies
Atomic Size:
This refers to the radius of atom & it can be defined as the distance between the center
of the nucleus and outermost shell of an atom.
Along the period, size of atom decreases. This is due to an increase in nuclear charge,
which tend to pull the electrons closer to the nucleus. This reduces the size of atom.
Down the group, size of the atom increases. The new shells being added and the
distance between nucleus and the outermost electron increases and the atomic size
increases.
METALLIC AND NON-METALLIC PROPERTIES
Down the group metallic character increases. This is because, the effective nuclear
charge experienced by valance electron decrease, and these electrons can be lost
easily. Along the period, metallic character decreases (or) Non metallic character
increases. This is because the effective nuclear charge experienced by valence electron
increases along a period.
Non metals are electronegative & they have the tendency to gain electrons.
QUESTIONS
1. Give the importance of the classification of elements.
2. What is the limitation of Dobereiner’s Triad
3. State the Newland’s law of octaves
4. State the limitations of Newland’s law of octets.
5. Mention the criteria that Mendeleev’s followed while arranging the elements.
6. Mention the achievement of Mendeleev’s periodic table
7. How does the modern periodic table remove various anomalies of Mendeleev’s
periodic table
8. How does the electronic configuration of an atom relate to its position in the
modern periodic table?
9. In the modern periodic table, calcium (Atomic No. 20) is surrounded by elements
with Atomic No. 12,19,21 which of these have physical and chemical properties
resembling calcium.
10. Account for the following:
11. The size of Cs is greater than Na
12. Tin is a metal, but C is a non-metal
13. The size of B is greater than F
14. Na, K, Rb has the same valency
15. State modern periodic law.
REVIEW
1. Elements are classified on the basis of similarities in their properties.
2. Dobereiner grouped the elements into triads and Newlands gave the Law of
octaves.
3. Mendeleev arranged the elements in increasing order of their atomic masses and
according to their chemical properties.
4. Mendeleev even predicted the existence of some yet to be discovered elements
on the basis of gaps in his periodic table.
5. Anomalies in arrangement of elements based on increasing atomic mass could
be removed when the elements were arranged in order of increasing atomic
number, a fundamental property of the element discovered by Moseley.
6. Elements in the Modern Periodic Table are arranged in 18 Vertical columns
called groups and 7 horizontal row called periods.
7. Elements thus arranged show periodicity of properties including atomic size,
valency or combining capacity and metallic and not-metallic character.