LNCT GROUP OF COLLEGES Name of Faculty:Dr.Preeti Gupta Designation: Associate professor Department: Engineering Chemistry Subject:Engineering Chemistry Unit: VII Topic: Periodic properties LNCT GROUP OF COLLEGES CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES Need for classification: It is very difficult to study individually the chemistry of all the elements and millions of their compounds, Hence to simplify and systematize the study of chemistry of the elements and their compounds, they are classified into groups and periods. Early attempt to classify the elements: Classification of Lavoiser Elements had been classified into two major groups by Lavoiser 1. Metals 2. Non–metals This classification was based on the differences in their properties. Dobereiner’s law of traids: It was first attempt towards classification. He arranged similar elements in a group of three elements called triad and the atomic mass of the middle elements of the traid is approximately the arithmetic mean of the other two. Newland’s law of octaves: When the lighter elements are arranged in order of their increasing atomic weights, then every eighth element is similar to the first element in its properties, similarly as the eighth node of a musical scale is similar to first one. E.g. Na 8thelement resembles in their properties with Li. Similarly, K the 8th element with Na and so on. Lother Meyer arrangement: LNCT GROUP OF COLLEGES The graphs plotting the atomic volumes against atomic weights are known as Lothar Mayer volume curves. The alkali metals have highest atomic volumes. Alkaline earth metals (Be, Mg Ca, Sr, Ba, etc.) which are relatively a little less electropositive. Occupy positions on the descending part of the curve. Halogens and the noble gases (except helium) occupy positions on the ascending part of the curve. Transition elements have very small volumes and therefore these are present at the bottoms of the curve. MENDELEEV’S PERIODIC LAW Mendeleev’s Periodic Law - The physical and chemical properties of elements are the periodic function of their atomic weight ii. Characteristic of Mendeleev’s Periodic Table (a)It is based on atomic weight (b) 63 elements were known, noble gases were not discovered. (c) 12 Horizontal rows are called periods. (d) Vertical columns are called groups and there were 8 groups in Mendeleev’s Periodic table. (e) Each group up to VIIth is divided into A & B subgroups. ‘A’sub groups element are called normal elements and ‘B’sub group’s elements are called transition elements. (f) The VIIIth group was consists of 9 elements in three rows (Transition metals group). (g) The elements belonging to same group exhibit similar properties. Merits or advantages of Mendeleev’s periodic table (a)Study of elements - First time all known elements were classified in groups according to theirsimilar properties. So study of the properties becomes easier of elements. (b) Prediction of new elements - It gave encouragement to the discovery of new elements as some gaps were left init. Sc(Scandium), Ga (Gallium), Ge (Germanium), Tc(Technetium) were the elements for whom position and properties were defined by Mendeleev even before LNCT GROUP OF COLLEGES their discoveries and he left the blank spaces for them . e.g.- Blank space at atomic weight 72 in silicon group was called Eka silicon (means properties like silicon) and element discovered later was named Germanium. Similarly, other elements discovered after Mendeleev periodic table were. Eka aluminium- Gallium(Ga) Eka Boron - Scandium (Sc) Eka Silicon - Germanium(Ge) Eka Manganese - Technetium(Tc) (c) Correction of doubtful atomic weights - Correction was done in atomic weight of some elements. Atomic Weight =Valency × Equivalent weight. Initially, it was found that equivalent weight of Be is 4.5and it is trivalent (V=3), so the weight of Be was 13.5 and there is no space in Mendeleev’s table for this element. So, after correction, it was found that Be is actually divalent (V= 2). So, the weight of Be became 2 × 4.5 = 9 and there was a space between Li and Bfor this element in Mendeleev’s table. – Corrections were done in atomic weight of elements are – U, Be, In, Au, Pt. Demerits of Mendeleev’s periodic table – (a) Position of hydrogen - Hydrogen resembles both, the alkali metals (IA) and the halogens (VIIA) in properties so Mendeleev could not decide where to place it. (b) Position of isotopes - As atomic weight of isotopes differs, they should have placed in different position in Mendeleev’s periodic table. But there were no such places for isotopes in Mendeleev’s table. (c) Anomalous pairs of elements - There were some pair of elements which did not follow the increasing order of atomic wts.eg. Ar and Co were placed before K and Ni respectively in the periodic table, but having higher atomic weights. (d) Like elements were placed in different groups. There were some elements like Platinum (Pt) and Gold (Au) which have similar properties but were placed in different groups in Mendeleev’s table. Pt – VIII Au – IB (e) Unlike elements were placed in same group. Like cu and Ag placed together. Ist group with Na. MODERN PERIODIC LAW LNCT GROUP OF COLLEGES Mosley proved that the square root of frequency of the rays, which are obtained from metal on showering high velocity electrons is proportional to the nuclear charge of the atom. This can be represented by the following expression. = a (Z–b) where Z is nuclear charge on the atom and a and b are constants. The nuclear charge on an atom is equal to the atomic number. According, to modern periodic law:”The properties of elements are the periodic functions of their atomic numbers”. Cause of periodicity: It is due to the repetition of similar outer shell electronic configuration at a certain regular intervals. Structural features of the long form of the periodic table: On the basic of the modern periodic law, a scientist named Bohr proposed a long form of periodic table that was prepared by Rang and Warner. (i) It consists of 18 vertical columns called groups and 7 horizontal rows called periods. (ii) Elements of groups, 1, 2, 13 – 17 are called normal or representative elements. (iii) Elements of groups 3 – 11 are called transition elements. (iv) The 14 elements with atomic numbers (Z) = 58 – 71 (occurring after lanthanum La in the periodic table) are called lanthanides or rare earth elements and are placed at the bottom of the periodic table. The 14 elements with atomic numbers (Z) = 90 – 103 (Occurring after actinium Ac in the periodic table) are called actinides and are placed at the bottom of the periodic table. (v) The Eleven elements with Z = 93 – 103 (93Np –Lr103) which occur in the periodic table after uranium and have been prepared from it by artificial means are called transuranics. These are all radioactive elements. (vi)The elements belonging to a particular group are said to constitute a chemical family which is usually named after the name of the first element. For example, Boron family (group 13), carbon family (group 14), nitrogen family (group 15), and oxygen family (group 16). In addition to this, some groups have typical names. For example, Elements of group 1 are called alkali metals Elements of group 2 are called alkaline earth metals Elements of group 16 are called chalcogens Elements of group 17 are called halogens LNCT GROUP OF COLLEGES LNCT GROUP OF COLLEGES Elements of group 18 are called zero group or noble gases. The long form of the periodic table contains seven periods. These are : 1st period (1H –He) contains only two elements. This is the shortest period. 2nd period (3 Li –10Ne) and third period (11Na –Ar) contain 8 elements each and are called short periods. 4th period (19K –36Kr) and 5th period (37Rb –54Xe) contain 18 elements each and are called long periods. 6th period ( 55Cs –86Rn) contains 32 elements and is the longest period. 7th period (87Li –) is, however, incomplete and contains at present only 24 elements. In yet another classification, the long form of the periodic table has been divided into four blocks (i.e. s, p, d and f ), depending upon the subshell to which the last electron enters. CLASSIFICATION IN BLOCKS s-block elements. Elements of groups 1 and 2 including He in which the last electron enters the s-orbital of the valence shell are called s-block elements. There are only 14 s-block elements in the periodic table. p-block elements. Elements of groups 13–18 in which the last electron enters the p-orbitals of the valence shell are called p-block elements. d-block elements. There are three complete series and one incomplete series of d-block elements. These are: 1st or 3dtransition series which contains ten elements which atomic numbers 21–30 (Sc- Zn). 2nd or 4d transition series contains ten elements with atomic numbers 39 – 48 (Y-Cd). 3rd or 5d transition series also contains ten elements which atomic numbers 57 and 72 –80 (57La, 72Hf –80Hg). 4th or 6d transition series is incomplete at present and contains only nine elements. Z = 111 has not been discovered so far. Thus, in all there are 39 d-block elements. f-Block elements are called inner-transition elements. In these elements, the f-subshell of the anti-penultimate is being progressively filled up. There are two series of f-block elements each containing 14 elements. The fourteen elements from 58 Ce –Lu in which 4 f-subshell is being progressively filled up are called lanthanides or rare elements. Similarly, the fourteen LNCT GROUP OF COLLEGES elements from 90Th –103Lr in which 5 f-subshell is being progressively filled up are called actinides.