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70 CHEMISTRY
UNIT 3
CLASSIFICATION OF ELEMENTS AND
PERIODICITY IN PROPERTIES
The Periodic Table is arguably the most important concept in
chemistry, both in principle and in practice. It is the everyday
support for students, it suggests new avenues of research to
After studying this Unit, you will be professionals, and it provides a succinct organization of the
able to whole of chemistry. It is a remarkable demonstration of the
fact that the chemical elements are not a random cluster of
appreciate how the concept of entities but instead display trends and lie together in families.
grouping elements in accordance to An awareness of the Periodic Table is essential to anyone who
their properties led to the wishes to disentangle the world and see how it is built up
development of Periodic Table. from the fundamental building blocks of the chemistry, the
understand the Periodic Law; chemical elements.
understand the significance of
atomic number and electronic Glenn T. Seaborg
configuration as the basis for
periodic classification;
name the elements with In this Unit, we will study the historical development of the
Z >100 according to IUPAC Periodic Table as it stands today and the Modern Periodic
nomenclature; Law. We will also learn how the periodic classification
classify elements into s, p, d, f follows as a logical consequence of the electronic
blocks and learn their main configuration of atoms. Finally, we shall examine some of
characteristics; © NCERT
the periodic trends in the physical and chemical properties
recognise the periodic trends in of the elements.
physical and chemical properties of
elements; 3.1 WHY DO WE NEED TO CLASSIFY ELEMENTS ?
compare the reactivity of elements We know by now that the elements are the basic units of all
and correlate it with their types of matter. In 1800, only 31 elements were known. By
occurrence in nature;
explain the relationship between 1865, the number of identified elements had more than
ionization enthalpy and metallic doubled to 63. At present 114 elements are known. Of
character; them, the recently discovered elements are man-made.
use scientific vocabulary Efforts to synthesise new elements are continuing. With
appropriately to communicate ideas such a large number of elements it is very difficult to study
related to certain important individually the chemistry of all these elements and their
properties of atoms e.g., atomic/ innumerable compounds individually. To ease out this
ionic radii, ionization enthalpy, problem, scientists searched for a systematic way to
electron gain enthalpy, organise their knowledge by classifying the elements. Not
not to be republished
electronegativity, valence of only that it would rationalize known chemical facts about
elements. elements, but even predict new ones for undertaking further
study.
CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES 71
3.2 GENESIS OF PERIODIC chemist, John Alexander Newlands in 1865
CLASSIFICATION profounded the Law of Octaves. He arranged
the elements in increasing order of their atomic
Classification of elements into groups and weights and noted that every eighth element
development of Periodic Law and Periodic had properties similar to the first element
Table are the consequences of systematising (Table 3.2). The relationship was just like every
the knowledge gained by a number of scientists eighth note that resembles the first in octaves
through their observations and experiments. of music. Newlands’s Law of Octaves seemed
The German chemist, Johann Dobereiner in to be true only for elements up to calcium.
early 1800’s was the first to consider the idea Although his idea was not widely accepted at
of trends among properties of elements. By that time, he, for his work, was later awarded
1829 he noted a similarity among the physical Davy Medal in 1887 by the Royal Society,
and chemical properties of several groups of London.
three elements (Triads). In each case, he
noticed that the middle element of each of the The Periodic Law, as we know it today owes
Triads had an atomic weight about half way its development to the Russian chemist, Dmitri
between the atomic weights of the other two Mendeleev (1834-1907) and the German
(Table 3.1). Also the properties of the middle chemist, Lothar Meyer (1830-1895). Working
element were in between those of the other independently, both the chemists in 1869
Table 3.1 Dobereiner’s Triads
Element Atomic Element Atomic Element Atomic
weight weight weight
Li 7 Ca 40 Cl 35.5
Na 23 Sr 88 Br 80
K 39 Ba 137 I 127
two members. Since Dobereiner’s relationship, proposed that on arranging elements in the
referred to as the Law of Triads, seemed to increasing order of their atomic weights,
work only for a few elements, it was dismissed similarities appear in physical and chemical
as coincidence. The next reported attempt to properties at regular intervals. Lothar Meyer
© NCERT
classify elements was made by a French plotted the physical properties such as atomic
geologist, A.E.B. de Chancourtois in 1862. He volume, melting point and boiling point
arranged the then known elements in order of against atomic weight and obtained a
increasing atomic weights and made a periodically repeated pattern. Unlike
cylindrical table of elements to display the Newlands, Lothar Meyer observed a change in
periodic recurrence of properties. This also did length of that repeating pattern. By 1868,
not attract much attention. The English Lothar Meyer had developed a table of the
Table 3.2 Newlands’ Octaves
Element Li Be B C N O F
At. wt. 7 9 11 12 14 16 19
Element Na Mg Al Si P S Cl
At. wt. 23 24 27 29 31 32 35.5
not to be republished
Element K Ca
At. wt. 39 40
72 CHEMISTRY
elements that closely resembles the Modern weights, thinking that the atomic
Periodic Table. However, his work was not measurements might be incorrect, and placed
published until after the work of Dmitri the elements with similar properties together.
Mendeleev, the scientist who is generally For example, iodine with lower atomic weight
credited with the development of the Modern than that of tellurium (Group VI) was placed
Periodic Table. in Group VII along with fluorine, chlorine,
While Dobereiner initiated the study of bromine because of similarities in properties
periodic relationship, it was Mendeleev who (Fig. 3.1). At the same time, keeping his
was responsible for publishing the Periodic primary aim of arranging the elements of
Law for the first time. It states as follows : similar properties in the same group, he
proposed that some of the elements were still
The properties of the elements are a undiscovered and, therefore, left several gaps
periodic function of their atomic in the table. For example, both gallium and
weights. germanium were unknown at the time
Mendeleev arranged elements in horizontal Mendeleev published his Periodic Table. He left
rows and vertical columns of a table in order the gap under aluminium and a gap under
of their increasing atomic weights in such a silicon, and called these elements Eka-
way that the elements with similar properties Aluminium and Eka-Silicon. Mendeleev
occupied the same vertical column or group. predicted not only the existence of gallium and
Mendeleev’s system of classifying elements was germanium, but also described some of their
more elaborate than that of Lothar Meyer’s. general physical properties. These elements
He fully recognized the significance of were discovered later. Some of the properties
periodicity and used broader range of physical predicted by Mendeleev for these elements and
and chemical properties to classify the those found experimentally are listed in
elements. In particular, Mendeleev relied on Table 3.3.
the similarities in the empirical formulas and The boldness of Mendeleev’s quantitative
properties of the compounds formed by the
elements. He realized that some of the elements predictions and their eventual success made
did not fit in with his scheme of classification him and his Periodic Table famous.
if the order of atomic weight was strictly Mendeleev’s Periodic Table published in 1905
followed. He ignored the order of atomic is shown in Fig. 3.1.
© NCERT
Table 3.3 Mendeleev’s Predictions for the Elements Eka-aluminium (Gallium) and
Eka-silicon (Germanium)
Property Eka-aluminium Gallium Eka-silicon Germanium
(predicted) (found) (predicted) (found)
Atomic weight 68 70 72 72.6
3
Density / (g/cm ) 5.9 5.94 5.5 5.36
Melting point /K Low 302.93 High 1231
Formula of oxide E O GaO EO GeO
2 3 2 3 2 2
not to be republished
Formula of chloride ECl GaCl ECl GeCl
3 3 4 4
PERIODIC SYSTEM OF THE ELEMENTS IN GROUPS AND SERIES CLASSIFICA
TION OF ELEMENTS AND PERIODICITY IN PROPER
© NCERT TIES
Fig. 3.1 Mendeleev’s Periodic Table published earlier
not to be republished 73
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