Exam (elaborations)
Classification of Elements and Periodicity in Properties
Classification of Elements and Periodicity in Properties
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Unit 3Classification 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 professionals, and it provides a succinct
After studying this Unit, you will be organization of the whole of chemistry. It is a remarkable
able to demonstration of the fact that the chemical elements are
not a random cluster of entities but instead display trends
• appreciate how the concept of
and lie together in families. An awareness of the Periodic
grouping elements in accordance
Table is essential to anyone who wishes to disentangle
to their properties led to the
the world and see how it is built up from the fundamental
development of Periodic Table.
building blocks of the chemistry, the chemical elements.
• understand the Periodic Law;
• understand the significance of Glenn T. Seaborg
atomic number and electronic
configuration as the basis for
periodic classification;
In this Unit, we will study the historical development of the
• name the elements with
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
follows as a logical consequence of the electronic
• classify elements into s, p, d,
f blocks and learn their main configuration of atoms. Finally, we shall examine some of
characteristics; 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
and correlate it with their all types of matter. In 1800, only 31 elements were known.
occurrence in nature; By 1865, the number of identified elements had more than
• explain the relationship between doubled to 63. At present 114 elements are known. Of
ionization enthalpy and metallic them, the recently discovered elements are man-made.
character; Efforts to synthesise new elements are continuing. With
• use scientific vocabulary such a large number of elements it is very difficult to
appropriately to communicate study individually the chemistry of all these elements and
ideas related to certain important their innumerable compounds individually. To ease out
properties of atoms e.g., this problem, scientists searched for a systematic way to
atomic/ionic radii, ionization
organise their knowledge by classifying the elements. Not
enthalpy, electron gain enthalpy,
electronegativity, valence of only that it would rationalize known chemical facts about
elements. elements, but even predict new ones for undertaking
further study.
Rationalised 2023-24
Unit 3.indd 74 9/9/2022
, Classification of Elements and Periodicity in Properties 75
3.2 GENESIS OF PERIODIC the periodic recurrence of properties. This
CLASSIFICATION also did not attract much attention. The
Classification of elements into groups and English chemist, John Alexander Newlands
development of Periodic Law and Periodic in 1865 profounded the Law of Octaves. He
Table are the consequences of systematising arranged the elements in increasing order
the knowledge gained by a number of of their atomic weights and noted that every
scientists through their observations and eighth element had properties similar to the
experiments. The German chemist, Johann first element (Table 3.2). The relationship was
Dobereiner in early 1800’s was the first to just like every eighth note that resembles the
consider the idea of trends among properties first in octaves of music. Newlands’s Law of
of elements. By 1829 he noted a similarity Octaves seemed to be true only for elements
among the physical and chemical properties up to calcium. Although his idea was not
of several groups of three elements (Triads). In widely accepted at that time, he, for his work,
each case, he noticed that the middle element was later awarded Davy Medal in 1887 by the
of each of the Triads had an atomic weight Royal Society, London.
about half way between the atomic weights of The Periodic Law, as we know it today
the other two (Table 3.1). Also the properties owes its development to the Russian chemist,
of the middle element were in between those Dmitri Mendeleev (1834-1907) and the
of the other two members. Since Dobereiner’s German chemist, Lothar Meyer (1830-1895).
Table 3.1 Dobereiner’s Triads
Atomic Atomic Atomic
Element Element Element
weight weight weight
Li 7 Ca 40 Cl 35.5
Na 23 Sr 88 Br 80
K 39 Ba 137 I 127
relationship, referred to as the Law of Triads, Working independently, both the chemists in
seemed to work only for a few elements, it was 1869 proposed that on arranging elements in
dismissed as coincidence. The next reported the increasing order of their atomic weights,
attempt to classify elements was made by a similarities appear in physical and chemical
French geologist, A.E.B. de Chancourtois in properties at regular intervals. Lothar Meyer
1862. He arranged the then known elements plotted the physical properties such as
in order of increasing atomic weights and atomic volume, melting point and boiling
made a cylindrical table of elements to display point against atomic weight and obtained
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
Element K Ca
At. wt. 39 40
Rationalised 2023-24
Unit 3.indd 75 9/9/2022
, 76 chemistry
a periodically repeated pattern. Unlike classification if the order of atomic weight
Newlands, Lothar Meyer observed a change was strictly followed. He ignored the order
in length of that repeating pattern. By 1868, of atomic weights, thinking that the atomic
Lothar Meyer had developed a table of the measurements might be incorrect, and placed
elements that closely resembles the Modern the elements with similar properties together.
Periodic Table. However, his work was not For example, iodine with lower atomic weight
published until after the work of Dmitri than that of tellurium (Group VI) was placed
Mendeleev, the scientist who is generally in Group VII along with fluorine, chlorine,
credited with the development of the Modern bromine because of similarities in properties
Periodic Table. (Fig. 3.1). At the same time, keeping his
While Dobereiner initiated the study of primary aim of arranging the elements of
periodic relationship, it was Mendeleev who similar properties in the same group, he
was responsible for publishing the Periodic proposed that some of the elements were
Law for the first time. It states as follows : still undiscovered and, therefore, left several
gaps in the table. For example, both gallium
The properties of the elements are and germanium were unknown at the time
a periodic function of their atomic
Mendeleev published his Periodic Table.
weights.
He left the gap under aluminium and a gap
Mendeleev arranged elements in horizontal under silicon, and called these elements
rows and vertical columns of a table in order Eka-Aluminium and Eka-Silicon. Mendeleev
of their increasing atomic weights in such a predicted not only the existence of gallium and
way that the elements with similar properties germanium, but also described some of their
occupied the same vertical column or group. general physical properties. These elements
Mendeleev’s system of classifying elements were discovered later. Some of the properties
was more elaborate than that of Lothar predicted by Mendeleev for these elements
Meyer’s. He fully recognized the significance and those found experimentally are listed in
of periodicity and used broader range of Table 3.3.
physical and chemical properties to classify
the elements. In particular, Mendeleev relied The boldness of Mendeleev’s quantitative
on the similarities in the empirical formulas predictions and their eventual success
and properties of the compounds formed by made him and his Periodic Table famous.
the elements. He realized that some of the Mendeleev’s Periodic Table published in 1905
elements did not fit in with his scheme of is shown in Fig. 3.1.
Table 3.3 Mendeleev’s Predictions for the Elements Eka-aluminium (Gallium) and
Eka-silicon (Germanium)
Eka-aluminium Gallium Eka-silicon Germanium
Property
(predicted) (found) (predicted) (found)
Atomic weight 68 70 72 72.6
Density/(g/cm3) 5.9 5.94 5.5 5.36
Melting point/K Low 302.93 High 1231
Formula of oxide E2O3 Ga2O3 EO2 GeO2
Formula of chloride E Cl3 GaCl3 ECl4 GeCl4
Rationalised 2023-24
Unit 3.indd 76 9/9/2022
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