With the discovery of new elements, scientists
tried to arrange them in a scientific and organized
manner, on the basis of their atomic masses. Dobereiner
in 1829 was the first to classify the similar elements
in the groups of three elements, known as "triads".
Newland in 1863 then classified the elements in the
increasing atomic masses and put forward the law of
octaves. In 1869 German chemist Lothar Meyer improved
the Newland's classification and plotted a graph between
the atomic volumes and atomic masses of the elements. He
included only 56 elements and discovered that the
elements with similar properties occupied the similar
positions on the curve.
In 1869 Russian scientist Mendeleev arranged the
elements in the increasing order of atomic masses and
put forward a periodic law, known as Mendeleev's
periodic law. In 1873, he published his periodic table
which consisted of eight vertical columns, known as
groups and twelve (12) horizontal rows known as periods.
Many places were left vacant for undiscovered elements
in his periodic table and predicted their properties. He
also discovered correct atomic masses of many elements.
In (1913-1914), after the discovery of atomic
numbers, the elements were then arranged in their
increasing atomic numbers and modern periodic law was
According to modern periodic law. "The physical and
chemical properties of the elements are the periodic
functions of their atomic numbers.
According to modern periodic classifications, a
long-form of periodic table has been introduced. It
contains eight (8) groups and seven (7) periods. The
groups are further subdivided into A and B sub-groups.
In the periodic table, the properties of the
elements gradually change in groups and periods. Down
the groups generally the physical properties of the
element slightly change with the increasing atomic
numbers, while chemical properties remain same to some
extent. Along the periods the physical and chemical
properties both change to larger extent with
the-increasing atomic numbers. The chemical properties
depend upon the valence electrons present. The elements
with similar electronic configurations in their valence
shells have similar chemical properties.
In the periodic table, elements on the left are
metals, while on the right side are non-metals and in
the middle, the elements are generally metalloids
showing both the characteristics of metals and
The minimum amount of energy required to remove
the most loosely bound electron from the valence
electrons in a gaseous atom to form positive ion. i.e.
cation is called first ionization energy or ionization
potential and the minimum amount of energy released from
a gaseous atom when an electron is added in the valence
shell to form negative ion i.e. anion is called Electron
affinity. Down the groups the values of ionization
potential and electron affinity both decrease with the
increasing atomic numbers due to increase in atomic
sizes. While along the periods the values of ionization
potential and electron affinity both increase with the
increasing atomic numbers.
The relative power of an atom to attract the
shared pair of electrons towards itself is called
electronegativity. The most electronegative atoms is
that of fluorine (F) with electronegativity = 4.0. The
repetition or recurrence of properties after regular
intervals is called periodicity.