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NCERT Class 11 Chemistry Textbook Chapter 3 With Answer PDF Free Download
Chapter 3: Classification of Elements and Periodicity in Properties
3.1 WHY DO WE NEED TO CLASSIFY ELEMENTS?
We know by now that the elements are the basic units of all types of matter. In 1800, only 31 elements were known.
By 1865, the number of identified elements had more than doubled to 63. At present 114 elements are known. Of them, the recently discovered elements are man-made.
Efforts to synthesize new elements are continuing. With such a large number of elements, it is very difficult to study individually the chemistry of all these elements and their innumerable compounds individually.
To ease this problem, scientists searched for a systematic way to organize their knowledge by classifying the elements.
Not only that it would rationalize known chemical facts about elements, but even predict new ones for undertaking further study.
3.2 Genesis of Periodic Classification
Classification of elements into groups and development of Periodic Law and Periodic Table is the consequences of systematizing the knowledge gained by a number of scientists through their observations and experiments.
The German chemist, Johann Dobereiner in the early 1800s was the first to consider the idea of trends among properties of elements.
By 1829 he noted a similarity among the physical and chemical properties of several groups of
three elements (Triads).
In each case, he noticed that the middle element of each of the Triads had an atomic weight about halfway between the atomic weights of the other two (Table 3.1).
Also, the properties of the middle element were in between those of the other chemist, John Alexander Newlands in 1865 profounded the Law of Octaves.
He arranged the elements in increasing order of their atomic weights and noted that every eighth element had properties similar to the first element (Table 3.2).
The relationship was just like every eighth note that resembles the first in octaves of music.
Newlands’s Law of Octaves seemed to be true only for elements up to calcium.
Although his idea was not widely accepted at that time, his work was later awarded Davy Medal in 1887 by the Royal Society, London.
The Periodic Law, as we know it today owes its development to the Russian chemist, Dmitri Mendeleev (1834-1907) and the German chemist, Lothar Meyer (1830-1895).
Working independently, both the chemists in 1869 had two members. Since Dobereiner’s relationship, referred to as the Law of Triads, seemed to work only for a few elements, it was dismissed as coincidence.
The next reported attempt to classify elements was made by a French geologist, A.E.B. de Chancourtois in 1862.
He arranged the then-known elements in order of increasing atomic weights and making a cylindrical table of elements to display the periodic recurrence of properties. This also did not attract much attention.
| Author | NCERT |
| Language | English |
| No. of Pages | 26 |
| PDF Size | 2.8 MB |
| Category | Chemistry |
| Source/Credits | ncert.nic.in |
NCERT Solutions Class 11 Chemistry Chapter 3 Some Basic Concepts of Chemistry
Q-1) What is the basic theme of organization in the periodic table?
Ans.) The basic theme of organization in the periodic table is to classify the elements in periods and groups as per their properties. So, this course of action makes the study of elements and their compounds simple and systematic. In the periodic table, elements with similar properties are placed in the same group.
Q-2) Which important property did Mendeleev use to classify the elements in his periodic table and did he stick to that?
Ans.) Mendeleev organized the components in his periodic table, according to the order of their atomic weight. He organized the components in groups and periods according to the increasing atomic weight. He placed the elements with similar properties in the same group.
However, he did not stick to this arrangement for long. He discovered that if the elements were organized according to their increasing atomic weights, then some of the elements did not fit in with his scheme of classification
Hence, he ignored the order of atomic weights in some cases. For example, the atomic mass of iodine is lower than the atomic mass of tellurium.
Still, Mendeleev set tellurium (in Group 6) ahead of iodine (in Group 7) along with fluorine, chlorine, bromine because of similarities in properties.
Q-3) On the basis of quantum numbers, justify that the sixth period of the periodic table should have 32 elements.
Ans.) In a periodic table containing elements, a period shows the value of a principal quantum number (n) for the outermost shells. Every period starts with the filling of the principal quantum number (n). The value of n for the 6th period is equal to 6. Now, for n = 6, the azimuthal quantum number (l) can have “0, 1, 2, 3, 4” values.
According to Aufbau’s principle, electrons are added to various orbitals in order of their increasing energies. Here, the 6d subshell is having much higher energy than the energy of the 7s subshell.
In the sixth period, the electrons can be filled in only 6s, 4f, 5d, and 6p subshells. 6s has 1 orbital, 4f has 7 orbitals, 5d has 5 orbitals, and 6p has 3 orbitals.
Hence, there are a sum of 16 (1 + 7 + 5 + 3 = 16) orbitals available. As per Pauli’s exclusion principle, each orbital can accommodate a maximum of 2 electrons.
Therefore, sixteen orbitals can accommodate a maximum of 32 electrons.
Thus, the 6th period of the periodic table should have 32 elements.
Q-4) In terms of period and group where would you locate the element with Z =114?
Ans.) Elements with the atomic numbers from Z = 87 to Z = 114 are present in the seventh period of the periodic table. Therefore, the element with Z = 114 is present in the seventh period of the periodic table.
In the seventh period, first 2 elements with Z = 87 and Z= 88 are s-block elements, the next 14 elements except Z = 89 i.e., those with Z = 90 to Z = 103 are f – block elements, and next 10 elements with Z = 89 and Z = 104 to Z = 112 are d-block elements and the elements with Z = 113 to Z = 118 are p-block elements.
Hence, the element with Z = 114 is the second p-block element in the seventh period. Thus, the element with Z = 114 is present in the seventh period and fourteenth group of the periodic table.
Therefore,
Period = 7th, Group = 14 and Block = p-block.
Q-5) Why do elements in the same group have similar physical and chemical properties?
Ans.) The chemical and physical properties of elements rely on the number of valence electrons. In the periodic table, elements present in the same group have the same number of valence electrons. Therefore, elements present in the same group have similar chemical and physical properties.
Classification of Elements and Periodicity in Properties Textbook With Solutions PDF Free Download