The arrangement of elements on the basis of their atomic
masses left many anomalies in the position of different
elements in the periodic table. Moreover the existence
of isotopes showed that the atomic mass of an element is
not the fundamental property of an element.
The modern periodic table is the result of discovery of
atomic number by Moseley in 1914.
Based on the concept of atomic number Bohr, Werner and
Bury proposed the
Modern Periodic Law
which states that,
"The physical and chemical properties of all elements
are periodic Junctions of their atomic numbers.”
In modern periodic table, also known as Bohr's Long Form
Of Periodic Table, elements are arranged in order of
their increasing atomic number. The elements having
similar properties are repeated at regular intervals. As
atomic number is related to the number of protons in an
atom, so the real basis of periodicity of properties is
due to recurrence of similar valency shell configuration
of the next element in the same group. The modern
periodic table is shown in table 4.4
The modern periodic table contains seven horizontal rows
called periods and sixteen vertical columns called
The elements within a period have dissimilar properties
from left to right across any period, the physical and
chemical properties of elements change from metallic to
non metallic along a period.
All periods except the first, start with an alkali metal
with one electron in their valence shell and end up with
zero group element with valence shell having 8
electrons, except He which has only 2 electrons.
The First Period:
It contains only two elements i.e. H and He. This period
signifies the competition of K-shell or first orbit. It
is the shortest period with two elements.
The Second and Third Period (Short Periods):
Each of these periods contains 8 elements. They signify
the filling up of L-shell and M-shell respectively.
The second period starts with Li and ends up with Ne;
whereas the third period starts with Na and ends at Ar.
The Fourth and Fifth Period (Long Periods):
Each of these periods contains 18 elements. In these
periods the electrons fill M and N shells. Fourth period
starts from K and ends at Kr. Fifth period starts from
Rb and ends at Xe.
The Sixth Period (Longest Period):
It contains 32 elements. It starts from Cs and ends with
Besides, fourteen elements called Lanthanides, are
placed at the bottom
of periodic table.
The Seventh Period (Incomplete Period):
It starts with Francium (Fr). This period is incomplete
as to date about 109 elements have been discovered.
This period also includes a group of fourteen elements
(Ac). These elements are called
They are also placed at the bottom of the table.