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class 11 ncert chemistry chapter 3
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UNIT 3CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
Periodic Table may be defined as an arrangement of all the known elements
according to their properties so that similar elements fall within the same vertical
column and dissimilar elements are separated.
EARLIER CLASSIFICATION OF ELEMETNS
DOBEREINER’S LAW OF TRIADS
In 1829, Dobereiner arranged the known elements of at that time in the ascending
order of atomic masses.
He found out three elements group called triad.
In a triad, the properties of the middle elements are the average of the other two.
This law is known as Dobereiner’s law of triads.
Eg: Li, Na, K Cl, Br, I Ca, Sr, Ba
Atomic mass of Li = 7
Atomic mass of K = 39
7 39
Atomic mass of Na = 23
2
NEWLANDS’ LAW OF OCTAVES
In 1865, Newlands arranged all the known elements of at that time in the ascending
order of atomic masses.
He observed that the properties of the eighth elements are the simple repetitions of
the first one like eighth note in an octave in music.
This law is known as Newlands law of octaves.
MENDELEEV’S PERIODIC TABLE
In 1869, a Russian chemist Mendeleev arranged the known elements of at that time
in the ascending order of atomic masses.
He observed that same properties are repeated in regular intervals and proposed a
law known as Mendeleev’s periodic law.
The law states that “the physical and chemical properties of elements are periodic
functions of their atomic masses”.
NOTE: Gallium and Germanium were unknown at the time Mendeleev published his
periodic table.
He left a gap under aluminium and a gap under silicon. He called these elements
Eka-Aluminium and Eka-Silicon
MERITS OF MENDELEEV’S PERIODIC TABLE
It was the first comprehensive classification of elements.
He corrected the wrong atomic weights of some elements and placed them in correct
position in the periodic table.
PREPARED BY HAIZEL G. ROY, HSST (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY ERNAKULAM Page | 1
, He left vacant places for undiscovered elements and predicted some of their
properties.
Elements with similar properties are placed in the same group.
DEMERITS OF MENDELEEV’S PERIODIC TABLE
Elements with dissimilar properties are found in same group.
He could not give an exact position for hydrogen.
He could not give exact position for Lanthanoids and Actinoids and also for isotopes.
Did not strictly obey the increasing order of atomic weights.
MODERN PERIODIC TABLE
Moseley’s work on the x-ray spectra of the elements reveals that atomic number is a
more fundamental property than atomic mass.
On the basis of this, he put forward the modern periodic law.
The law states that “the physical and chemical properties of elements are periodic
functions of their atomic numbers”.
PERIODS AND GROUPS
The horizontal rows present in the modern periodic table are called periods.
There are seven periods.
The first period consists of 2 elements.
Second and third period consists of 8 elements each.
Fourth and fifth period consists of 18 elements.
Sixth period consists of 32 elements.
The last seventh period is an incomplete period.
The vertical columns present in the modern periodic table are called groups.
There are 18 vertical columns.
Therefore 18 groups are present in the modern periodic table.
MODERN CLASSIFICATION OF ELEMENTS
In the modern periodic table, elements are classified into four blocks.
They are s, p d and f block elements.
This classification is based on the orbital in which the last electron of the atom of the
element enters.
1. s-BLOCK ELEMENTS
The elements in which the last electron enters the s orbital of their valence shell are
called s block elements.
It consists of elements of group 1 and group 2.
The ground state configuration of the valence shell is ns1 or ns2.
2. p-BLOCK ELEMENTS
The elements in which the last electron enters the p orbitals of their valence shell are
called p block elements.
It consists of group 13-18 except He.
PREPARED BY HAIZEL G. ROY, HSST (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY ERNAKULAM Page | 2
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