This document discusses the development of the periodic table. It describes Dobereiner's discovery of triads of elements with similar properties (lithium, sodium, potassium). It also discusses Newlands' law of octaves and how he arranged elements in order of increasing atomic mass and found every 8th element had similar properties. Most of the document focuses on Mendeleev's periodic table, including its key features like periods and groups, how he arranged elements and left gaps for undiscovered elements, and its merits in classifying all known elements. It notes some demerits too, like position issues caused by isotopes and atomic mass inconsistencies. Finally, it mentions Moseley discovered atomic number was a better property to use

1. SCIENCE & TECHNOLOGY
STD:-SSC(MAHRASHTRA BOARD)
 NAME:-ASHOK GANGADHAR JADHAV

2. 1.SCHOOL OF ELEMENTS
MAIN POINTS
1.1 Dobereiner’s Traids
1.2 Newland’s Octaves
1.3 Mendeleev’s Periodic Table
1.4 Modern Periodic Table

3. 1.1 Dobereiner’s Traids
 Some group of three
elements which showed
similar properties. These
group were called as
traids.
 In these traids the
atomic mass of the
middle element was
approximately the mean
of other two elements.
Element Atomic
mass
Lithium Li 6.9
Sodium
Na
23
Potassium
K
39

4. 1.2 NEWLAND’S OCTAVES
* Newland’s law states that “When the
elements are arranged in increasing order
of their atomic masses, the property of
every eighth element are similar to that of
first.”
H Li Be B C N O
F Na Mg Al Si P S
Cl K Ca

5. 1.3 MENDELEEV’S PERIODIC TABLE
 Mendeleev believed that atomic mass of element was
the fundamental property in classifying elements.
 His law is known as Mendeleev’s Periodic law:
The
physical and chemical properties of elements are a
periodic function of their atomic mass.
 The tabular arrangement of the elements are based
on the Mendeleev’s Periodic law is called as
Mendeleev’s Periodic Table.

6. Main Features Of Mendeleev’s Periodic Table
 The horizontal rows in the periodic table are
called periods. There are seven periods.
These are numbered from 1 to 7.
 Properties of elements in particular period
show regular gradation from left to right.
 Vertical columns in the periodic table are
called groups. There are eight groups
numbered from I to VIII. Groups I to VIII are
further divided into A and B subgroups.

7. Merits Of Mendeleev’s Periodic Table
 Mendeleev was the first who
successfully to classified all known
elements.
 Mendeleev kept some blank places
in his periodic table. These vacant
spaces were for elements that were
yet to be discovered. He also
predicated the properties of that
elements even before they were
discovered. Later thy were found to
be correct.
 When noble gases were discovered
later ,they were placed in
Mendeleev’s periodic table without
disturbing the positions of other
elements.
Predicated
elements
Actual element
discovered later
Eka-boron Scandium
Eka-aluminium Gallium
Eka-silicon Germenium

8. Demerits of Mendeleev’s Periodic Table
 Hydrogen resembles alkali as well as halogens. Therefore
no fixed position could be given to hydrogen in the periodic
table.
 Isotopes of same elements have different atomic masses;
therefore each one of them should be given a different
position. On the other hand isotopes are chemically similar,
they had to be given a same position.
 At a certain places, element have higher atomic mass has
been placed before an element have lower atomic mass.
For example, cobalt (Co = 58.93) is placed before nickel
( Ni = 58.71)
 Some elements placed in same subgroup had different
properties. e.g. Manganese (Mn) is placed with halogens
which is totally differ in the properties.

9. 1.4 MODERN PERIODIC TABLE
 Henry Moseley an English physicist discovered that atomic number is the
most fundamental property of an element and not atomic mass.
 The Modern Periodic Law states “The chemical and physical properties of
elements are a periodic function of their atomic numbers”.