CBSE Class 11 Chemistry Notes For Chapter 3 Mendeleev’s Periodic Table

CBSE Class 11 Chemistry Notes For Chapter 3 Mendeleev’s Periodic Table

Based on his periodic law, Mendeleev arranged the then-known elements in the form of a table (consisting of several rows and columns) which is known as Mendeleev’s periodic table. Mendeleev’s original periodic table (1871) contained only 63 elements known at that time. There were no places for inert gases because these were not discovered at the time of publication of the table.

Mendeleev, however, left several blank places in the table and predicted that there must be some unknown elements which would be discovered in due course of time. He even predicted their properties based on the properties of the adjacent elements. Mendeleev’s predictions were proved to be astonishingly correct when these elements were discovered later. Mendeleev’s table, now in use, is a modified version ofthe table originally designed by him. Important features of the modified form of Mendeleev’s periodic table are discussed below.

Periods and Groups:

In Mendeleev’s periodic table, the elements were arranged in the increasing order of their atomic weights (but in the modified form these were arranged in increasing order of their atomic numbers) into several horizontal rows. These horizontal rows were placed one below the other in such a way that chemically similar elements fell in the same vertical column. The horizontal rows are called periods and the vertical columns are called groups or families.

There is a gradual change in the properties of the elements with an increase in atomic mass across a period. However, elements belonging to the same group exhibit close chemical similarities. In the modern version of Mendeleev’s table, there are seven periods (1 to 7) and nine groups (I to VIII and 0). Gr-0 consists of the inert gases (Mendeleev’s original table did not contain this group)

Main features of Mencleleov’s Periodic Table

The first period contains only 2 elements (II and He).

  • This is called the shortest period. The second period contains only 8 dements (Li-Bc-B-C-NO-F-Nc), beginning with alkali metal Li and ending with Inert gas No. This Is called the first short period.
  • The period also contains elements (Na-Mg-Al-Si-P-SCl-Ar), beginning with the alkali metal Na and ending with the Inert gas Ar.
  • This is called the second period. The elements of these two short periods occur in nature in very large amounts and they typify the properties of all the other members of the group to which they belong. So they are called typical elements.
  • The fourth period contains 18 elements. It begins with the alkali metal K and ends with the inert gas Kr.
  • This period is called the first long period. The fourth period contains 10 additional elements than the second and third periods.
  • These 10 elements (Sc to Zn) are called the transition elements. This period consists of two series (the even and the odd series).
  • The fifth period also contains 18 elements.
  • It begins with the alkali metal Rb and ends with the inert gas Xe. This period is called the second long period.
  • 10 elements from Y to Cd are called transition elements. The fifth period also consists of two series (the even and the odd series).

CBSE Class 11 Chemistry Notes For Chapter 3 Classification Of Elements And Periodicity In Properties Periodicity Of elements

The sixth period contains 32 elements, and so it is called the longest period. It begins with the alkali metal Cs and ends with the inert gas Rn. This period contains 10 transition elements (La and Hf to Hg) and 14 lanthanide elements (Ce to Lu ). These 14 elements are also called rare earth elements because these elements were believed to be present in nature in negligible amounts.

The sixth period also consists of two series (the even and the odd series). The seventh period may contain a maximum of 32 elements (beginning with Fr), but all the elements have not yet been discovered. Till now 28 elements have been discovered. So it is an incomplete period.

All elements of this period are radioactive. The elements from Francium (Fr) to Uranium (U) are naturally occurring, while the elements beyond uranium are man-made. In this period, the 14 elements beyond actinium, Ac [i.e., the elements from thorium (Th) to lawrencium (Lr) ] are called actinides, while the elements beyond uranium (U) are called transuranic elements.

Even and odd series: Elements belonging to each of the 4th, 5th and 6th periods are divided into two series:

The even and the odd series. The three even series begin with the alkali metals K, Rb and Cs, while the three odd series begin with the coinage metals Cu, Ag and Au respectively.

Subgroups:

Except for the Gr-VHI and Gr-0, each of the other groups (Gr-I to VII) is divided into two subgroups designated as ‘A’ and ‘B! In long periods (4th, 5th and 6th), the elements of the even series are placed in subgroup A and those of the odd series are placed in subgroup B.

In short periods (2nd and 3rd), elements of Gr-I and Gr-II are placed in subgroup-A, while those of the other groups are placed in subgroup-B. Within the same group, the properties of the elements of subgroups and B are altogether different, except for their valencies. However, elements of the same subgroup exhibit more or less similar properties.

For example: 

Alkali metals of Gr-IA are closely alike. However, Gr-IA metals differ remarkably from the coinage metals of Gr-IB (Cu, Ag and Au), although they have a common valency of ‘1’.

CBSE Class 11 Chemistry Notes For Chapter 3 Classification Of Elements And Periodicity In Properties Comparision Of Properties Of The Elemens Of Subgroup 1A and 1b

Additional pieces of information about groups and sub-groups:

  • Elements of subgroup A are more electropositive than those of subgroup B.
  • For example, Gr-VIIA elements (Mn, Tc, Re) are electropositive, while Gr-VIIB elements (F, Cl, Br and I) are electronegative characters
  • .Gr-VIII has no subgroups. It contains a total of 9 elements, belonging to periods 4, 5 and 6.
  • These nine elements—Fe, Co, Ni (period-4); Ru, Rh, Pd (period-5) and Os, Ir, Pt (period-6) are arranged in this manner due to similarity in their properties and their, atomic weights are also close to v each other. Each grip, of three elements, is called Mendeleev’s triad elements. Mendeleev coined the term ‘transitional element’ for these elements.

Gr-0 has no subgroups. It contains inert gases:

  • He, Ne, Ar, Kr, Xe and Rn. These elements are chemically inert and do not exhibit any tendency to combine with other elements.
  • So they are zero-valent elements and placed in Gr-‘0′. This group acts as a bridge between highly electronegative halogens (VIIB) and highly electropositive alkali metals (IA).
  • Due to their similarity in chemical properties, La and 14 elements from Ce -Lu are placed together in Gr-3A of the 6th period.
  • The 14 elements from Ce to Lu are called lanthanoids. For similar reasons, Ac and 14 elements from Th-Lr are placed together in Gr-3A ofthe 7th period. The 14 elements from Th to Lr are ( called actinoids.

Importance & usefulness of Mendeleev’s periodic table Systematic Study of the elements:

  • Mendeleev, for the first time, arranged a vast number of elements in such a way that the elements with similar chemical properties are placed in the same group.
  • This made the study of elements quite systematic because if the properties of one element (and its compounds) in a particular group are known, then the properties of the rest of the elements (and their compounds) can be predicted.

Prediction of new elements:

  • Mendeleev left some gaps in the periodic table to accommodate new elements to be discovered in future. he even predicted the properties of those unknown elements based on their positions in the table.
  • When these elements were discovered, their properties were found to be similar as predicted By Mendeleev. For example, Mendeleev left two vacant places below b and al in gr-3 and one vacant place below.
  • He named those elements eka-boron, ca-aluminium and ca-silicon respectively as he predicted that the properties of these elements would be similar to that of boron, aluminium & silicon.
  • In 1075, de Baisbaudron discovered eka-aluminium and named it gallium. In 1079, n. l.
  • Nilson discovered eka-boron and named it scandium. In 1006, Winkler discovered eka-silicon and named it as germanium.

CBSE Class 11 Chemistry Notes For Chapter 3 Classification Of Elements And Periodicity In Properties Importance and usefulness of mandeleev's periodic table

It was observed that these newly discovered elements had properties similar to those already predicted by Mendeleev before their discovery.

CBSE Class 11 Chemistry Notes For Chapter 3 Classification Of Elements And Periodicity In Properties Importance and usefulness of mandeleev's periodic table.2

Correction of doubtful atomic weights:

  • With the help of Mendeleev’s periodic table, doubtful atomic weights of some elements are rectified.
  • For example: Be was assigned an atomic weight of 13.5 based on its equivalent weight (4.5) and valency (wrongly taken as ‘3’ because Be had certain similarities with trivalent metal Al).
  • With an atomic weight of 13.5, Be should be placed between carbon (At. weight 12) and nitrogen (At. weight 14).
  • But no vacant place was available In between C and N. Mendeleev asserted that Be must be bivalent because of its similarity with Mg, Ca etc. Thus he corrected its atomic weight as 4.5 X 2 = 9.0.

Defects Of Mendeleev’s Periodic Table

Discrepancy or anomaly in periodicity:

  • Mendeleev arranged the elements in increasing order of their atomic weights.
  • But he violated this principle in certain cases to give appropriate positions to some elements based on their properties i.e., he laid more emphasis on the properties of those elements rather than their atomic weights.
  • In the following four pairs of elements, elements with higher atomic weight have been placed before elements with lower atomic weight.

CBSE Class 11 Chemistry Notes For Chapter 3 Classification Of Elements And Periodicity In Properties Defects of mendeleev's periodic table

Position of hydrogen:

  • The controversial position of hydrogen in the periodic table also hints at discrepancies within the table.
  • Like the alkali metals ofGr-IA, it exhibits univalency, high reactivity, electropositive character, strong affinity for non-metals and reducing character.
  • On the other hand, like the halogens of Gr- VIIB, it has atomicity, high ionisation energy, non-metallic character, existence in the gaseous state at normal temperature and pressure ability to combine with milk-forming hydrides (Example: Nall).
  • Since hydrogen exhibits similarities as well as dissimilarities with both the alkali metals and the halogens, the placement of hydrogen in any one of these two groups will naturally create difficulties.
  • So it is desirable to fix a separate position for hydrogen in Mendeleev’s periodic table.

Placement of similar elements in different groups and dissimilar elements in the same group:

In some cases, elements with almost similar properties have been placed in different groups.

Example:

  • Cu and Hg resemble in properties but Cu is Gr-1B while Hg has been placed in Gr-IIB.
  • Likewise, elements like Ba (Gr-2A) and Pb (Gr – 4B) have been placed in different groups.
  • Again, some elements with dissimilar properties have been placed in the same group.
  • Highly reactive alkali metals such as Li, Na, K etc., have been placed together with almost inactive coinage metals such as Cu, Ag and Au in Gr-1.
  • Likewise, Mn, Te and Re having no similarity with F, Cl, Br etc. have been placed together.

Lack of separate positions for Gr-8 elements:

  • No proper place has been allotted to nine elements belonging to Gr-8 although they have many similarities in properties.
  • These are arranged in three triads, one in each of the 4th (Fe, Co, Ni ), 5th(Ru, Rh, Pb) and (Os, Ir, Pt)periods.

Lack of suitable positions for Lanthanoids and Actinoids:

  • The 14 elements following La from Ce to Lu (lanthanoids) and the 14 elements following Ac from Th to Lr (actinoids) have not been allotted separate positions in the main skeleton of the periodic table.
  • They have been placed in two separate rows at the bottom of the table. Besides, the number of elements in the lanthanoid and actinoid series cannot be determined from Mandeleev’s periodic table.

Position of isotopes:

  • Isotopes of an element have different atomic weights. So they should be placed at different positions in the periodic table.
  • However, all the isotopes of any specific element are placed in a single position (i.e., same period and same group)in Mendeleev’s periodic table.

Moseley’s experiment:

The atomic number determines the fundamental property of an element. In 1913, Moseley measured the frequencies of X-rays emitted by different metals when bombarded with high-speed electrons.

  • He observed that the frequencies ofthe prominent X-rays emitted by different metals were different but for each metal, there was a fixed value.
  • He observed further that the square root of the frequency (v) of the X-rays emitted by a metal was proportional to the atomic number but not to the atomic mass of the metal, Le., Jv = a(Z- b) where ‘a’ is the proportionality constant and is a constant for all the lines in a given series of X-rays.
  • Thus a plot of Tv vs Z gave a straight line but a plot of Jv vs atomic mass does not bear such a linear relationship.

This led Moseley to conclude that atomic number was a better fundamental property of an element than atomic mass.

CBSE Class 11 Chemistry Notes For Chapter 3 Classification Of Elements And Periodicity In Properties Relation betwen frequency and atomic number

Modification Of Mendeleev’s Periodic Law

  • Mendeleev regarded atomic weight as the fundamental property of an element and so he considered atomic weight as the basis of periodic classification of elements.
  • But Moseley, from his experimental results, showed clearly that atomic number is a better fundamental property of an element than its atomic weight.
  • This led Moseley to suggest that atomic number (Z) should be the basis of the classification of elements. This gave birth to a new periodic law known as the modem periodic law.

Modern Periodic Law:

  • The physical and chemical properties ofthe elements are aperiodic functions of their atomic numbers.
  • This implies that, if elements are arranged in order of increasing atomic numbers, the elements with similar chemical properties are repeated after certain regular intervals.

Rectification of the discrepancy in periodicity with the help of modern periodic law:

  • The original periodic law, based on atomic weight, was violated in the case of four pairs of elements [(Ar, K), (Co, Ni), (Te, I), (Th, Pa)].
  • In each pair, an element with a higher atomic weight is placed before the element having a lower atomic weight.
  • In the modern periodic table (based on the atomic number), this discrepancy disappears because the atomic numbers of K, Ni, I and Pa are greater than those of Ar, Co, Te and Th respectively.
  • Placement of all the isotopes of any specific element in the same position of the periodic table is quite justified as the isotopes of elements have the same atomic number although they have different atomic weights.

Theoretical justification of modern periodic law:

  • Only nuclear electrons (or more specifically valence shell electrons) take part in chemical reactions, while the atomic nucleus remains unaffected.
  • So it is understandable that the properties of the elements will depend upon their atomic numbers (equal to the number of electrons) rather than their atomic weight or mass numbers (equal to the total number of protons and neutrons).

Periodicity of elements

The periodic repetition of elements having similar properties after certain regular intervals when the elements are arranged in the increasing order of their atomic numbers is called periodicity.

Cause of periodicity:

  • According to modern periodicals, there is a repetition of properties of the elements after certain regular intervals when they are arranged in order of their increasing atomic numbers.
  • Again from a close study of electronic configurations of various elements, it is observed that with successive increases in atomic number, there occurs a repetition of similar outermost shell electronic configuration (valence shell electronic configuration) after certain regular intervals.
  • By correlating these two observations, it can be concluded that periodicity in properties is due to the recurrence of similar valence shell electronic configuration after certain regular intervals when the elements are arranged in order of increasing atomic numbers.

This can be illustrated by the following examples.

  1. Elements of Gr-1A have outermost electronic configuration ns1 (where n = outermost principal quantum number).
  2. These elements exhibit similar chemical properties due to their similarity in the valence shell electronic configuration.
  3. Elements of Gr-7B have outermost electronic configuration ns2np5.
  4. All the halogens exhibit similar chemical properties due to their similarity in valence shell electronic configuration.
  5. Inert gases belonging to this group possess similar chemical properties because they have similar valence shell electronic configurations (ns2np6).
  6. It should be noted that properties of elements get repeated only after intervals of 2, 8,18 or 32 in the atomic numbers of the elements because similar electronic configurations recur only after such intervals.
  7. The numbers 2, 8, 18 and 32 are called magic numbers. These numbers are very useful in locating elements with similar properties.

Moderntableor Long Form Of Periodic Table Bohr’s Table:

This is an improved form of the periodic table based on modern periodic law. It is also called Bohr’s table since it follows Bohr’s scheme for the classification of the element based on the outermost electronic configuration governed by the Aufbau principle. It consists of periods and 18 groups.

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