PERIODIC CLASSIFICATION OF ELEMENTS
• Elements are classified on the basis of similarities in their properties.
• Döbereiner grouped the elements into triads and Newlands gave the Law of
• Mendeléev arranged the elements in increasing order of their atomic masses and
according to their chemical properties.
• Mendeléev even predicted the existence of some yet to be discovered elements
on the basis of gaps in his Periodic Table.
• Anomalies in arrangement of elements based on increasing atomic mass could be
removed when the elements were arranged in order of increasing atomic number, a
fundamental property of the element discovered by Moseley.
• Elements in the Modern Periodic Table are arranged in 18 vertical columns
called groups and 7 horizontal rows called periods.
• Elements thus arranged show periodicity of properties including atomic size,
valency or combining capacity and metallic and non-metallic character.
Periodic Law: ‘Properties of elements are a periodic function of their
Atomic number gives us the number of protons in the nucleus of an atom and this
number increases by one in going from one element to the next. Elements, when
arranged in order of increasing atomic number Z, lead us to the classification
known as the Modern Periodic Table. Prediction of properties of elements could
be made with more precision when elements were arranged on the basis of
increasing atomic number.
Position of Elements in the Modern Periodic Table :
The Modern Periodic Table has 18 vertical columns known as ‘groups’ and 7
horizontal rows known as ‘periods’. Let us see what decides the placing of an
element in a certain group and period. All elements of a group contain same
number of valence electrons, which justifies similar chemical properties.
The atomic radius decreases in moving from left to right along a period. This is
due to an increase in nuclear charge which tends to pull the electrons closer to
the nucleus and reduces the size of the atom.
Atoms of different elements with the same number of occupied shells are placed
in the same period. Na, Mg, Al, Si, P, S, Cl and Ar belong to the third period
of the Modern Periodic Table, since the electrons in the atoms of these elements
are filled in K, L and M shells.
Metallic & Non-metallic Properties:
Metals like Na and Mg are towards the left-hand side of the Periodic Table while
the non-metals like sulphur and chlorine are found on the right-hand side. In
the middle, we have silicon, which is classified as a semi-metal or metalloid
because it exhibits some properties of both metals and non-metals.
In the Modern Periodic Table, a zig-zag line separates metals from non-metals.
The borderline elements – boron, silicon, germanium, arsenic, antimony,
tellurium and polonium – are intermediate in properties and are called
metalloids or semi-metals. Metals tend to lose electrons while forming bonds,
that is, they are electropositive in nature.
As the effective nuclear charge acting on the valence shell electrons increases
across a period, the tendency to lose electrons will decrease. Down the group,
the effective nuclear charge experienced by valence electrons is decreasing
because the outermost electrons are farther away from the nucleus. Therefore,
these can be lost easily. Hence metallic character decreases across a period and
increases down a group.
As the trends in the electronegativity show, non-metals are found on the
right-hand side of the Periodic Table towards the top. These trends also help us
to predict the nature of oxides formed by the elements because it is know that
the oxides of metals are basic and that of non-metals are acidic in general.