PERIODIC CLASSIFICATION OF ELEMENTS
Early attempts at classifications of elements
Earliest attempt at classification resulted in grouping of metals and non-metals.
DOBEREINER’S TRIADS
Johann Wolfgang Dobereiner showed that when three elements in triad (group
of three) were written in increasing order of their atomic masses, the atomic
mass of the middle element is roughly the average of the atomic masses of the
other two elements.
For eg: in a triad of Li,Na and K….. the atomic mass of Na(23) is the average of
the atomic masses of Li(6.9) and K(39).
Drawback
He could only identify three triads from the elements known at the time.
Therefore, this classification was not found useful.
The three triads were: Li,Na,K……Ca,Sr,Ba…..Cl,Br,I
NEWLAND’S LAW OF OCTAVES
John Newlands arranged the elements known at the time in order of their
increasing order of their atomic masses. He found that every eighth element
had properties similar to that of the first. He compared it to the octaves found
in music and named this law as Law Of Octaves.
Drawbacks
The law of octaves was only applicable till calcium.
It was assumed that there are only 56 elements and there will be no
more elements discovered in the future, but later on several new
elements were discovered which did not fit into the law of octaves.
To fit elements in his table, he adjusted two elements in the same slot,
he also put some unlike elements in the same column. For example, Co
and Ni having different properties from F,Cl,Br were put in the same
column.
, MENDELEEV’S PERIODIC TABLE
Dmitri Ivanovich Mendeleev arranged the elements on the basis of their
fundamental property, the atomic mass and also on the similarity of chemical
properties.
Physical and chemical properties criteria for the classification
Among the chemical properties, he concentrated on the compounds formed by
elements with oxygen and hydrogen.
He selected hydrogen and oxygen because they are very reactive and form
compounds with most elements. The formula of the hydrides and the oxides
formed by the elements were one of the basic properties for its classification.
Conclusion of Mendeleev’s classification
He observed that most of the elements in the periodic table were arranged in
increasing order of their atomic mass. [exception: cobalt(atomic mass 58.9)
appeared before nickel(atomic mass 58.7) in the periodic table]
He also observed that there is a periodic recurrence of elements with similar
physical and chemical properties.
Mendeleev’s law
He formulated a Periodic Law which states that ‘the properties of elements are
the periodic function of their atomic masses.’
Achievements
He introduced groups (vertical columns) and periods (horizontal rows) of the
periodic table.
Mendeleev left some gaps in his periodic table instead of looking upon these
gaps as defects, he boldly predicted the existence of some elements thathad
not been discovered at the time.
He named these gaps as ekta along with the name of the preceding element.
Eg: Ekta-boron(scandium), Ekta-aluminium(gallium) and Ekta-
silicon(germanium)
Early attempts at classifications of elements
Earliest attempt at classification resulted in grouping of metals and non-metals.
DOBEREINER’S TRIADS
Johann Wolfgang Dobereiner showed that when three elements in triad (group
of three) were written in increasing order of their atomic masses, the atomic
mass of the middle element is roughly the average of the atomic masses of the
other two elements.
For eg: in a triad of Li,Na and K….. the atomic mass of Na(23) is the average of
the atomic masses of Li(6.9) and K(39).
Drawback
He could only identify three triads from the elements known at the time.
Therefore, this classification was not found useful.
The three triads were: Li,Na,K……Ca,Sr,Ba…..Cl,Br,I
NEWLAND’S LAW OF OCTAVES
John Newlands arranged the elements known at the time in order of their
increasing order of their atomic masses. He found that every eighth element
had properties similar to that of the first. He compared it to the octaves found
in music and named this law as Law Of Octaves.
Drawbacks
The law of octaves was only applicable till calcium.
It was assumed that there are only 56 elements and there will be no
more elements discovered in the future, but later on several new
elements were discovered which did not fit into the law of octaves.
To fit elements in his table, he adjusted two elements in the same slot,
he also put some unlike elements in the same column. For example, Co
and Ni having different properties from F,Cl,Br were put in the same
column.
, MENDELEEV’S PERIODIC TABLE
Dmitri Ivanovich Mendeleev arranged the elements on the basis of their
fundamental property, the atomic mass and also on the similarity of chemical
properties.
Physical and chemical properties criteria for the classification
Among the chemical properties, he concentrated on the compounds formed by
elements with oxygen and hydrogen.
He selected hydrogen and oxygen because they are very reactive and form
compounds with most elements. The formula of the hydrides and the oxides
formed by the elements were one of the basic properties for its classification.
Conclusion of Mendeleev’s classification
He observed that most of the elements in the periodic table were arranged in
increasing order of their atomic mass. [exception: cobalt(atomic mass 58.9)
appeared before nickel(atomic mass 58.7) in the periodic table]
He also observed that there is a periodic recurrence of elements with similar
physical and chemical properties.
Mendeleev’s law
He formulated a Periodic Law which states that ‘the properties of elements are
the periodic function of their atomic masses.’
Achievements
He introduced groups (vertical columns) and periods (horizontal rows) of the
periodic table.
Mendeleev left some gaps in his periodic table instead of looking upon these
gaps as defects, he boldly predicted the existence of some elements thathad
not been discovered at the time.
He named these gaps as ekta along with the name of the preceding element.
Eg: Ekta-boron(scandium), Ekta-aluminium(gallium) and Ekta-
silicon(germanium)
