Revision Notes
Class 11 Chemistry
Chapter 3 – Classification of Elements and Periodicity in Properties
Introduction
A periodic table is a table that organises all known elements into groups based on
their properties, with like elements grouped together in the same vertical column
and dissimilar elements separated.
Historical Development of Periodic Table
The atomic weights of the elements were used in all previous attempts to classify
them.
Dobereiner’s Triads
Dobereiner defined triads as groups of three elements in 1829. A triad's chemical
characteristics were comparable. The atomic weight of the middle element was
found to be approximately equal to the arithmetic mean of the other two elements
when the elements in a trio were ordered in order of increasing atomic weights.
Triad Iron Cobalt Nickel Mean of 1st and 3rd
Atomic 55.85 58.93 58.71 Atomic weights are nearly the
weight same.
Triad Lithium Sodium Potassium 7 39 23
2
Atomic 7 23 39
Weight
Triad Chlorine Bromine Iodine 35.5 127
81.25
2
Class XI Chemistry www.vedantu.com 1
,Atomic 35.5 80 127
weight
Triad Calcium Strontium Barium 40 177
88.5
2
Atomic 40 87.5 137
weight
Newland’s Law of Octaves
When the lighter elements were placed in order of increasing atomic weights, the
properties of every eighth element were comparable to those of the first, as the
eighth note of a musical scale, according to an English chemist named John
Alexander Newlands in 1865. Newlands' law of octaves was named after this
generalisation.
Lothar Meyer’s Curve
“The atomic masses of elements are periodic functions of their physical
properties.”
According to Lothar Meyer, atoms with similar properties occupy similar places in
the atomic volume, as measured by the atomic mass curve.
Class XI Chemistry www.vedantu.com 2
, Mendeleev’s Periodic Law
Mendeleev organised atoms in a table's horizontal rows and vertical columns in
order of increasing atomic weights, so that elements with comparable properties
were grouped together in the same vertical column.
Modern Periodic Law
Henry Moseley, an English physicist, discovered regularities in the distinctive X-
ray spectra of the elements in 1913. Instead of a straight line between v versus
atomic mass, a plot of v (where v is the frequency of X-rays emitted) against
the atomic number (Z) produced a straight line.
As a result, Mendeleev's Periodic Law was altered correspondingly. The Modern
Periodic Law can be stated as follows: The physical and chemical properties of
elements are periodic functions of their atomic numbers.
PERIODIC TABLE OF ELEMENTS
Class XI Chemistry www.vedantu.com 3
Class 11 Chemistry
Chapter 3 – Classification of Elements and Periodicity in Properties
Introduction
A periodic table is a table that organises all known elements into groups based on
their properties, with like elements grouped together in the same vertical column
and dissimilar elements separated.
Historical Development of Periodic Table
The atomic weights of the elements were used in all previous attempts to classify
them.
Dobereiner’s Triads
Dobereiner defined triads as groups of three elements in 1829. A triad's chemical
characteristics were comparable. The atomic weight of the middle element was
found to be approximately equal to the arithmetic mean of the other two elements
when the elements in a trio were ordered in order of increasing atomic weights.
Triad Iron Cobalt Nickel Mean of 1st and 3rd
Atomic 55.85 58.93 58.71 Atomic weights are nearly the
weight same.
Triad Lithium Sodium Potassium 7 39 23
2
Atomic 7 23 39
Weight
Triad Chlorine Bromine Iodine 35.5 127
81.25
2
Class XI Chemistry www.vedantu.com 1
,Atomic 35.5 80 127
weight
Triad Calcium Strontium Barium 40 177
88.5
2
Atomic 40 87.5 137
weight
Newland’s Law of Octaves
When the lighter elements were placed in order of increasing atomic weights, the
properties of every eighth element were comparable to those of the first, as the
eighth note of a musical scale, according to an English chemist named John
Alexander Newlands in 1865. Newlands' law of octaves was named after this
generalisation.
Lothar Meyer’s Curve
“The atomic masses of elements are periodic functions of their physical
properties.”
According to Lothar Meyer, atoms with similar properties occupy similar places in
the atomic volume, as measured by the atomic mass curve.
Class XI Chemistry www.vedantu.com 2
, Mendeleev’s Periodic Law
Mendeleev organised atoms in a table's horizontal rows and vertical columns in
order of increasing atomic weights, so that elements with comparable properties
were grouped together in the same vertical column.
Modern Periodic Law
Henry Moseley, an English physicist, discovered regularities in the distinctive X-
ray spectra of the elements in 1913. Instead of a straight line between v versus
atomic mass, a plot of v (where v is the frequency of X-rays emitted) against
the atomic number (Z) produced a straight line.
As a result, Mendeleev's Periodic Law was altered correspondingly. The Modern
Periodic Law can be stated as follows: The physical and chemical properties of
elements are periodic functions of their atomic numbers.
PERIODIC TABLE OF ELEMENTS
Class XI Chemistry www.vedantu.com 3
