CHEMISTRY UNIT 3 CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES COMPLETE KNOWLEDGE CLASS 11th

Chemistry

                                UNIT 3

 CLASSIFICATION OF ELEMENTS AND                           PERIODICITY IN PROPERTIES

Scientists like to find patterns. About 200 years ago, scientists were discovering lots new elements . However they were struggling hard to bring order and pattern to the vast amount of information they were discovering about the elements and their properties. The periodic table is an important triumph for the nineteenth century chemists and has become an important landmark in the history of chemistry. It is used as a sorting system for chemical elements. t has become the everyday support for students and teachers and provides a concise organization of the whole of chemistry. 

        

     In  the present unit, we wiH study the historical developments of the periodic table. We will also learn how the periodic classification of elements follows as a logical consequence of the electronic configuration of atoms. We shall also study some of the important periodic trends in the physical and chemical properties of elements. 

NEED FOR CLASSIFICATION OF ELEMENTS

   Upto  the end of seventeenth century, only 31 elements were known. Therefore, it was very easy to study and remember the properties of these elements. However, during the later part of the eighteenth century, the pace of discovery of new elements quickened. Between 1800 to 1869, the number of identified elements had become nearly double to 63. With such a large number of elements, it became difficult to study individually the chemistry of all these elements and their innumerable compounds. At this stage, it was realised that there should be some simple way to study and remember the numerous properties of the elements and their compounds. This gave rise to necessity of classification of the elements ito various groups having similar properties. This has been done by arranging the elements in 8uch a way that similar elements are placed together while dissimilar elements are separated from one another. 'This is known as classification of elements. Such a classification of the elements has resulted in the formulation of the periodic table. Periodic table may be defined as

   The arrangement of the known elements according to their properties in a tabular form. 

CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES

    At present about 114 elements are know  Of these the recently discovered elements are man made. Efforts to synthesize new elements are continuing. The periodic classification of the elements has extremely simplified their study. Not only the periodic classification rationalizes the known chemical facts about elements, butit also helps to predict new ones for undertaking further study. 

HISTORICAL DEVELOPMENT OF THE PERIODIC TABLE

      Since the beginning of the nineteenth century, scientists have been trying to find a basis of grouping elements having similar properties. Lavoisier classified the elements simply as metals and non- metals. However, this classification proved to be inadequate. Some of the earlier important attempts to classify the elements are briefly summed up below:

1. Dobereiner triads

In 1817, a German scientist, Johann Dobereiner classified the elements in groups of three elements called triads. The elements in a triad had similar properties and the atomic weight of the middle member of each triad is very close to the arithmetic mean (average) of the other two elements. The common triads of Dobereiner classification were chlorine, bromine and iodine; calcium, strontium and barium bromine sodium and potassium, etc. 

2. De Chancourtois Classification

    The  next reported attempt was made by a French geologist A. E. B. de Chancourtois in 1862.

He arranged the then known elements in order of increasing atomic weight and proposed a Cylindrical table of elements to display the periodic recurrence of properties. He observed that the elements with similar properties fell in a vertical line from the centre of the spiral. However, this did not attract much attention. 

3.Newlands law of octaves in 1865, an English chemist, John Newlands proposed a new system of grouping elements of similar properties. According to him,when the elements are arranged in the increasing order of atomic weights, the properties ofevery.eighth element are similar to the first one. Newlands called this relation as the law of octaves due tO similarity with the musical scale. 

     

Important Contributions of Mendeleev's Periodic Table

Mendeleev's periodic table was one of the greatest achievements in the development of chemistry. Some of the important contributions of his periodic table are:

1. Systematic study of thee elements. The Mendeleevs periodic table simplified the study of chemistry of elements. Knowing the properties of one element in a group, the properties of other elements in the group can be easily guessed. Thus, it became very useful in studying and remembering the properties of a large number of elements. 

2. Correction of atomic masses. The Mendeleev's periodic table helped in correcting the atomic masses of some elements based on their posItIOns in the table. For example, atomic mass of beryllium was corrected from 13.5 to 9. Similarly, with the help of this table, atomic masses of indium, gold, platinum etc. were corrected. 

3. Prediction of new elements. At the time of Mendeleev, only 56 elements were known. While arranging these elements, he left some gaps. These gaps represented the undiscovered elements. Mendeleev predicted the properties of these undiscovered elements on the basis of their positions. For example he predicted the properties of gallium (eka-aluminium) and germanium (eka-silicon) which were discovered later. The observed properties of these elements were found to be similar to those predicted by Mendeleev

Defects of Mendeleev's Periodic Table 

    

   In spite of many advantages, the Mendeleev's periodic table had certain defects also. Some of these are given below

1. Position of hydrogen. Hydrogen is placed in group I. However, it resembles the elements of group I (alkali metals) as well as the elements of group  (halogens). Therefore, the position of hydrogen in the periodic table is not correctly defined. 

2. Anomalous pairs In certain pairs of elements, the increasing order of atomic masses was not obeyed. In these cases, Mendeleev placed elements according to similarities in their properties and not in increasing order of their atomic masses. For example, argon (Ar, atomic mass 39.9) is placed before potassium(K, atomic mass 39.1). Similarly, cobalt (Co, atomic mâss 58.9) is placed before nickel (Ni, atomic mas 58.6) and tellurium (Te, atomic mass 127.6) is placed before iodine (1, atomic mass 126.9). These positions were not justified

3. Position of isotopes  Isotopes are the atoms of the same element having different atomic masses but same atomic number. Therefore, according to Mendeleev's classification, these should be placed at different places depending upon their atomic masses. For example, isotopes of hydrogen with atomic masses 1,2 and 3 should be placed at three places. However, isotopes have not been given separate places in the periodic table. 

4. Cause of.periodicity, Mendeleev did nad explain the cause of periodicity among the elementa 

5. Position of lanthanoids (or lanthanides) and actinoids (or actinides). The fourteen elements following lanthanum (known as lanthanoids, from atomic number 58-71) and the fourteen elements following actinium (known as actinoids, from atomic number 90-103) have not been given separate places in Mendeleev table 

In order too cover more elements, Mendeleev modified his periodic table. 

   

  THE LONG FORM OF THE PERIODIC TABLE

  The most widely used periodic table these days is The Long Form of the Periodic Table'. Thisis Constructed on the basis of repeating electronic configuration of the atoms when the elements are arranged in the order of increasing atomic numbers. Long form of the periodic table is given  Structural Features of the Long Form of the Periodic Table 

    

    The long form of the periodic table consists of horizontal rows called periods and vertical columns called groups These are discussed below: 

Periods

A horizontal row in the periodic table is called 

Period

           In terms of electronic structure of the atom, a period constitutes a series of elements whose atoms have the same number of electron shells i.e. principal quantum number (n). Thus, the period indicates the Value of n for the outermost or valence shell of the atom of the element. Each successive period in the periodic table is associated with the filling of the next Eher principal energy level (n=1, n=2, n=8, etc.) re are seven periods and each period starts with different principal quantum number: 

    It can be seen that the number of elements in each period is twice the number of atomic orbitals available in the energy level that is being filled. Let us briefly discuss these periods.

 Elements of First Period 

The first period corresponding to n=lis unique because it contains.only two elements. This is not surprising because first energy shelf (K) has only one orbitali.e. 1s) which can accommodate only tWo electrons. This means that there can be only two elements in which one and two electrons are present in first energy level. The first period contains hydrogen (1s) and helium (1s ). 

Elements of Second Periodic

  The second period contains 8 elements because for n = 2, there are four orbitals (one2s and three 2p) in second energy shell (L). In all, these four orbitals have a capacity of eight electrons and, therefore, second period has eight elements in it. It starts with lithium (Z=3)in which one electron enters the 2s-orbital. The period ends with neon (Z = 10) in which the second shell is complete (2s* 2p ). 

Elements Third Period 

In third period corresponding to n= 3, there are nine orbitals: one 3s, three 3p and five 3d. However, we know from energy level diagram for multiple electron atoms (Unit 2) that 3d-orbitals are higher in energy than 4s-orbitals.Consequently,3d orbitals are filled after filling 4s-orbital. Hence, this period involves the filling of only four orbitals (3s and 3p) and contains eight elements from sodium (u 11) to argon (Z =18). 

Elements  of Fourth periodic

     

  The fourth period corresponding to nE4. involves the filling of one 4s-and three 4p-orbitals (4d and 4forbitals are higher in energy than 5s-orbitals and are filled later). In between 4s-and 4p-orbitals, five 3d-orbitals are filled which have energies in between these orbitals. Thus, in all nine orbitals are to be filled and, therefore, there are eighteen elements in fourth period from potassium (Z= 19) to krypton (Z = 36). In this period, we conmen across elements which involve the filling of 3d orbitals. The sep are known as transition series of elements. This starts from scandium (Z = 21) which has electronic configuration 3d' 4s* and ends at zinc (Z= 30) with 3d orbitals completely filled having electronic configuration 3d 4s. 

    

Elements of Fifth periodic

The fifth period, like the fourth period also consists of 18 elements. It begins with rubidium (Z = 37) with filling of 5s-orbital and ends at xenon (Z = 54) with the filling up of the 5p-orbitals. This period also contains ten elements of 4d transition series starting at yttrium (Z = 39). 

Elements of sixth periodic

     The sixth period contains 32 elements (Z = 55 to 86) and successive electrons enter into 6s, 4, 5d1 and 6p-orbitals, in that order. It starts with cesium (Z = 55) and ends at radon (Z = 86). In addition to ten elements of 5d transition series, this period contains 14 elements which involve filling up of 4f orbitals beginning from cerium (Z=58) to lutetium (Z=71). The series of elements are called inner transition series or lanthanoid series. 

Elements of Seventh periodic

The seventh period, is similar to the sixth period with successive filling up of the 7s, 5f, 6d and 7p orbitals and includes most of the man made radio- active elements. Though expected to have 32 elements, this period is incomplete and contains only 29 elements at present. This period is expected to end at the element with atomic number 118, which would belong to the noble gas family. This period also contains 14 elements which involve the filling of 5forbitals starting from actinium (Z=89). This also called 5f - inner

transition series or actinoid series,

This also provides a theoretical justification for periodicity occurring at regular intervals of 2, 8. 8 1 18and 32. These numbers i.e., 2, 8, 18 and 32 are al called magic numbers. The number of elements and the corresponding orbitals being filled are given in Table 8. 

    

     The first three periods containing 2, 8 and 8 elements respectively are called short periods, the next three periods containing 18, 18 and 32 elements respectively are called long periods. Groups. 

DIVISION OF ELEMENTS INTO S, p. d AND f BLOCKS 

1. The long form of the periodic table can be divided into four main blocks. These are s, p, d and f blocks. This division of elements is based upon the electronic configurations of the atoms. In this division the elements which involve the filling of a particular orbital (i.e., s, p, d or ) are grouped together as shown in Fig. 2. (These are also shown in Long form of the Periodic Table. 

i) s- block elements

    The The elements in which the last electron enters the S-orbital of their-out P most..energy level are called s-block elements. It consists of elements of groups 1 and 2 having the ground state electronic configurations of outermost shell as nsl and ns2 respectively (where n stands for outermost energy shell). The elements corresponding to ns configuration are called allkali metals while those corresponding tons configuration are called alkaline earth.metals. Thus, the general electronic configuration of s-block elements may be expressed as: 

General characteristics of s-block elements 

The general characteristics of s-block elements are 

i) They are soft metals having low melting and boiling points. 

ii) They have low 10n1sation enthalpies 

iii) They are very reactive and readily form ne univalent(alkali metals) or bivalent(alkaline earth metals) ions by losing one or two valen can Ce electrons respectively. Because of their high reactivity, they are never found pure in nature 

iv) They act as strong reducing agents. 

v) Most of them impart characteristic colours to the flame. 

ii)  p block elements

    

    

The elements in which the last electron enters tho p-orbital of their outermost energy level are called p-block elements. The elements of groups 13 to 18 involving addition of one (ns np'), two (ns np), three (ns np), four (ns2np^), five (nsnp5), and six (ns np) electrons respectively in p-orbitals and s-orbitals are already filled in their atoms constitute p-block. The general electronic configuration for the atoms of this block may be written as: 

General characteristics of p-block elements 

i) They include both metals and non-metals. The is a regular gradation from metallic to non- metallic character as we move along a period from left to right in this block. The metallie character increases as we go down the group. 

ii) Their ionisation enthalpies are relatively high as compared to s-block elements. 

iii) They form mostly covalent compounds. 

iv) Some of them show more than one oxidation states in their compounds. 

v) There occurs a gradation from reducing to oxidising properties as we move across the periods of this block. 

iii) d block elements

The elements in which the last electron.enters.the bitals of their ast but one (called penultimate) nergy leuel.constitute d-block elements. This block energy This block Cons Consists of the elements lying between s and p blocks blocks starting from fourth period and onwards. They onwards. They constitute groups 3 to 12 in the periodic table. In these elements the outermost shell contains one or two electrons in their s-orbital (ns) but the last electron enters the last but one d-subshell i.e., (n - 1) d. The general electronic configuration for the atoms of d- block may be written as 

General characteristics of d-block elements

i) They are   metals having high melting and boiling points. 

ii) Most of them form coloured compounds. 

iii) They have a good tendency to form complex compounds. 

iv) Their compounds are generally paramagnetic. 

v) They exhibit several oxidation states. 

F blocks elements

The The elements in which the last electron enters the f-orbitals of their atoms are called f block elements In these elements, the last electron is added to the third to the outermost (called antipenultimate) energy level; (n -2)f. These consist of two series of elements placed at the bottom of the periodic table 

General characteristics of f-block elements

i)  They are heavy metals. 

ii) They generally have high melting and boiling points.

iii) They exhibit variable oxidation states. 

iv) They form coloured ions. 

v) They have the tendency to form complex compounds. 

Classification of elements into General Types

Noble gasses The noble gases are found a the end of each period in group 18. Except for heliumn, the elements have completely filled s- and p-orbitals of the outermost shell i.e., ns np". Helium has 1s configuration. All these elements are highly stable and chemically inert under ordinary conditions. 

Representative elements (s and p-block  elements All the elements of s and p-block with exception of noble gases are called representative elements. They represent two groups 1 (alkali metals) and 2 (alkaline earth metals) on the extreme left and five groups from group 13 to 17 on the right hand side of the periodic table. 

Transition elements The elements of d- block elements are called transition elements. These include elementS of group 3 to 12 lying in between the representative elements (between s and p-blocks).

Inner transition elements. The elements of f block are called inner transition elements. These comprise two series of 14 elements called lanthanoids and actinoids. 

Advantage of the long  form of the periodic table

The important advantages of the long form of the periodic table are given below

i) This classification is based on the atomic number which is a more fundamental property of the elements. 

ii) Since this classification is based on the atomic number and noton the atomic mass, the position of placing isotopes at one place is fully justified.

iii) The position of elements in the periodic table is governed by the electronic configurations,which determine their properties. 

iv) It is easy to remember and reproduce. 

v) The systematic grouping of elements into four blocks; s, p, d and f has made the study of the elements more simple. 

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