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After the discovery of atomic number by Moseley in 1913, it
was noticed ghat elements could be classified more satisfactorily by using
their atomic numbers rather than their atomic masses. This improvement
rectified a number of confusions presented in the old Periodic Table. The
Modern Periodic Law states That: If the elements are arranged in ascending
order to their atomic numbers, their chemical properties repeat in a periodic
manner.
Essential features of Periods:
§ Elements with similar properties are placed in vertical columns
called Groups. There are eight groups, which are usually numbered by Roman
numerals I to VIII. Each group is divided into two sub-groups, designated as A
and B sub-groups
§ There are 7 periods in the Periodic table numbered by Arabic
numerals 1 to 7
§ The Period 1 contains only two elements, hydrogen and helium.
§ The periods 2 and 3 contain eight elements each and are called
short periods.
§ The periods 4 and 5 are called long periods. Each long period
consists of eighteen elements. Out of these, eight are typical elements
belonging to A subgroup, whereas the other ten elements placed in the centre of
the table belong to B subgroups are known as transition Elements.
§ The period 6 is also a long period, which contains thirty two
elements. In this period there are eight typical elements, ten transition
elements and a new set of fourteen elements called Lanthanides as they start
from La.
§ The Period 7 is incomplete so far. It contains only two normal
elements 87 Fr and 88Ra, ten transition elements and fourteen inner transition
elements. The transition elements of this period are called Actinides, as they
follow 89 Ac.
§ The inner transition elements are shown at the bottom of the
Periodic Table under Lanthandies. Due to their scarcity, the inner transition
elements are also called “rare earth elements”.
Alkali metals:
Elements of the group IA are
called Alkali Metals because of their property to form strong alkalies with
water.
Alkaline earth metals:
Due to their presence in
Earth’s crust and alkaline character, the elements of group IIA are known as
Alkaline Earth Metals.
Halogens:
The name Halogens is given to
the elements of group VIIA due to their salt forming property.
Nobal gases:
As the gases of group VIIIA are
least reactive they are called Noble Gases.
s-block elements:
Elements of IA and IIA
sub-groups are called s-block element because their valence electrons are
available in s orbital.
p-block elements:
The elements of IIIA to VIIIA
subgroups (except He) are known as p-block elements as their valence electrons
are present in p orbital.
d-block elements:
In transition elements,
electrons in d orbital are responsible for their valencyhence, they are called d-block
elements.
f-block elements:
In lanthanides and Actinides
valence electrons are present in f orbital hence, these elements are called
f-block elements.
Metals:
Chemically all the elements
which have a tendency to form positive ions by losing electrons are considered
metals.
All the metals are good
conductor of heat and electricity. A characteristic property of metals is that
they form basic oxides, which give bases when dissolved in water.
Non-Metals:
The elements which gain
electrons and form negative ions are called non-metals. All the gases are
non-metals.
The non-metals are usually poor
conductor of heat and electricity. Non-metals form acidic oxides, which yield
acids on dissolving in water.
Semi Metals or metalloids:
Some elements, especially lower
members of groups, IIIA, IVA and VA have properties of both metals as well as
non-metals.
These elements are called
semi-metals or metalloids.
The oxides of these elements
are called amphoteric oxides, which have characteristic of both acidic and
basic oxides.
