UNIT # 01: The Periodic Law and Periodicity NEED OF CLASSIFICATION During 17th Century, only few elements were known. Later on many elements were discovered in nature, the number of elements in the list of known elements went on increasing. It was thought that elements must be properly classified in order to make a systematic study of elements, otherwise it becomes impossible to understand the properties of elements. The following are the advantages of classification of elements. i) To study the elements in a systematic manner ii) To correlate the properties of elements. iii) To know the type of different compounds that different elements can form. HISTORICAL BACKGROUND OF CLASSIFICATION OF ELEMENT Earlier, an attempt was made to classify the elements into metals and non-metals. Disadvantages of classifying the elements into metals and non-metals. i) There is no justification for more active metals or non-metals. ii) There are some elements which have properties of metals and non-metal, such elements are known as Metalloids. DOBERNIER S TRIADS After the development of atomic theory and determination of the atomic masses of a large number of elements, attempts were made to classify the elements on the basis of atomic mass. Dobernier was the first to correlate the properties of elements with their atomic masses. In 1829, Dobernier German chemist classified the elements into group of three elements such that the three elements had similar properties and the atomic mass of the middle element was the arithmetic mean of the atomic masses of the first and the third elements. The group of such three elements was called Triads. Dobernier s Triads Element Atomic Mass Element Atomic Mass Element Atomic Mass Li 7 Ca 40 Cl 35.45 Na 23 Sr 88 Br 80 K 39 Ba 137 I 127 At. mass of Na = (At.mass of Li + At. Mass of K) / 2 = (7+39) / 2 = 23 Disadvantages of Dobernier s triads. i) It was not possible to group all elements into triads. ii) The elements with dissimilar properties were grouped as triads ( C, N, O) 1
NEWLAND S CLASSIFICATION In 1864, John Alexander Newland, English chemist observed that when elements are arranged in the increasing order of their atomic masses, the properties of elements repeated at every eighth element, similar to repetition of musical notes in an Octave. Newland called this as Law of Octaves. Characteristics of Newland s Classification The eighth element from lithium is sodium, eighth element from sodium is potassium, Lithium, sodium and potassium have similar properties. Similarly eighth element from beryllium is magnesium, eighth element from magnesium is calcium, beryllium, magnesium and calcium have similar properties. However this did not hold well beyond Ca. Advantages of Newland s classification. Atomic mass of the element was considered as basis of classification. It explains periodicity in the properties of elements. Disadvantages of Newland s classification. There was no proper place for hydrogen. Transition elements were not considered for Octaves. The observation made by Newland does not hold well beyond Ca. LOTHAR MEYER S GRAPH Lothar Meyer (1869) tried to classify by plotting a graph of atomic volume versus atomic masses of different elements. Similar elements occupied similar positions in the graph. Self-Assessment 1. Elements A, B and C constitute a Dobereiner s triad. What is the relationship in their atomic masses? 2. How many elements were included in the arrangement given by Newland? 3. Which property of atoms was used by Mendeleev to classify the elements? 4. How many groups were originally proposed by Mendeleev in his periodic table? 5. Where in the periodic table are chemically similar elements placed, in a group or in a period? 6. Mendeleev s periodic table had some blank spaces in it. What do they signify? 7. What name was given to the element whose properties were similar to the element ekaaluminium predicted by Mendeleev? 2
MENDELEEV S CLASSIFICATION The classification of elements made by Newland on the basis of atomic mass was continued by Mendeleev. Dmitry Mendeleev a Russian chemist while trying to classify elements discovered that on arranging in the increasing order of atomic mass, elements with similar chemical properties occurred periodically. In1869 he observed that when elements are arranged in the increasing order of their atomic masses, the properties of the elements repeated at certain intervals. He gave his observation in the form of a law, known as Mendeleev s periodic law. It states that the properties of the elements are periodic functions of their atomic masses. He arranged the element known so far in the form of periodic table. A periodic table is a chart representing the systematic arrangement of elements into vertical columns and horizontal rows such that elements with similar properties fall in the same column. He was so confident in his table that he used it to predict the physical properties of three elements that were yet unknown. After the discovery of these unknown elements between 1874 and 1885, and the fact that Mendeleev s predictions for Sc, Ga, and Ge were amazingly close to the actual values, his table was generally accepted. Characteristics of Mendeleev s periodic table The vertical columns in the periodic table are called groups. There were 8 groups numbered as I, II, III, IV, V, VI, VII & VIII and Zero group. Each of group from I & VII were divided into subgroups a & b. Thus, there were 15 vertical columns that constituted 8 groups. The horizontal rows in the periodic table are called periods. There were seven periods in the periodic table. These are numbered from 1 to 7 (Arabic numerals). Properties of elements in a particular period show regular gradation (i.e. increase or decrease) from left to right. All the elements in a particular group are chemically similar in nature. They show regular gradation in their physical properties and chemical reactivity. Merits of Mendeleev s periodic table It is based on atomic mass of the element which is considered as fundamental property of element. Mendeleev s was the first classification which successfully included all the elements. The elements with similar properties fall in the same group. There is repetition in the properties of elements at certain intervals. This is known as periodicity in the properties of elements. Thus it explains periodicity in the properties of elements. Some gaps were left for undiscovered elements. The properties of undiscovered elements were predicted by Mendeleev on the basis of other elements in the same group. Mendeleev corrected the atomic masses of some elements. 3
Mendeleev s periodic table had some blank spaces in it. These vacant spaces were for elements that were yet to be discovered. For example, he proposed the existence of an unknown element that he called eka-aluminium. The element gallium was discovered four years later and its properties matched very closely with the predicted properties of ekaaluminium. Disadvantages of Mendeleev s periodic table. In spite of being a historic achievement Mendeleev s periodic table had some defects in it. There was no proper position for hydrogen atom. It is placed in IB and VIIB in original Mendeleev s periodic table. Anomalous pairs of elements: In Mendeleev s periodic table, the elements are arranged in the increasing order of their atomic masses but some pairs of elements were placed such that elements with higher atomic masses were placed prior to those with lower atomic masses. e.g. Argon (39.91) & Potassium (39.1) Cobalt 58.93 & Nickel 58.7 Tellurium 128 & Iodine 127 The position of rare earth elements & actinides cannot be justified on the basis of atomic masses. Isotopes are the atoms of the same elements with different atomic mass & same atomic number. Different isotopes of same elements have different atomic masses, therefore, each one of them should be given a different position in the periodic table. On the other hand, because they are chemically similar, they had to be given same position. According to Mendeleev, elements of only VIII group are the transition elements. This is not justified. Superfluous relationships: Elements with different properties are placed in same group. a. Alkali metals with copper, silver and gold in group I b b. Fluorine and manganese in group VII DISCOVERY OF THE NOBLE GASES In 1895 Lord Rayleig reported the discovery of a new gaseous element named argon which proved to be chemically inert. This element did not fit any of the known periodic groups. In 1898, William Ramsey suggested that argon be placed into the periodic table between chlorine and potassium in a family with helium, despite the fact that argon's atomic weight was greater than that of potassium. This group was termed the "zero" group due to the zero valency of the elements. Ramsey accurately predicted the future discovery and properties neon. MODERN PERIODIC TABLE 4
In 1918, H.G.I. Moseley showed by X-ray analysis that the atomic number is a better fundamental property of an element than its atomic mass, therefore he classified the elements based on atomic number. He proposed his observation in the form of a law known as Modern periodic law. It states that the physical and chemical properties of all elements are periodic functions of their atomic number. Based on this, elements were classified and a periodic table was prepared. It consists of 7 periods and 18 groups. The significance of the modern periodic table is that the properties of the elements depend on the arrangement of the electrons (electronic configuration) of their atoms. It is observed in the periodic table that the elements with similar electronic configuration fall in the same group. The similar properties of the elements repeat in some intervals. This is known as periodicity in the properties of the elements. This is due to repetition of similar electronic configuration of elements in the same interval. Characteristics of Modern Periodic Table Groups There are 18 vertical columns in the periodic table. Each column is called a group. The groups have been numbered from 1 to 18 (in Arabic numerals) from left to right. Group 1 on extreme left position contains alkali metals (Li, Na, K, Rb, Cs and Fr) and group 18 on extreme right side position contains noble gases (He, Ne, Ar, Kr, Xe and Rn). All elements present in a group have similar electronic configurations and have same number of valence electrons. You can see in case of group 1 (alkali metals) and group 17 elements (halogens) that as one moves down a group, more and more shells are added. All elements of group 1 have only one valence electron. Li has electrons in two shells, Na in three, K in four while Rb has electrons in five shells. Similarly all the elements of group 17 have seven valence electrons however the number of shells is increasing from two in F to five in I. Elements present in groups 1 and 2 on left side and groups 13 to 17 on the right side of the periodic table are called normal elements or representative elements. Their outermost shells are incomplete. They are also called typical or main group elements. Elements present in groups 3 to 12 in the middle of the periodic table are called transition elements. (Although groups 11 and 12 elements are, strictly speaking, not transition elements). Their two outermost shells are incomplete. However, it should be noted here that more and more electrons are added to valence shell only in case of normal elements. In transitions elements, the electrons are added to incomplete inner shells. Elements 113, 115 and 117 are not known but included at their expected positions. Group 18 on extreme right side of the periodic table contains noble gases. Their outermost shells contain 8 electrons. Inner transition elements:14 elements with atomic numbers 58 to 71 (Ce to Lu) are called lanthanides and they are placed along with the element lanthanum (La), atomic number 57 in the same position (group 3 in period 6) because of very close resemblance 5
between them. However, for convenience sake they are shown separately below the main periodic table. 14 elements with atomic numbers 90 to103 (Th to Lr) are called actinides and they are placed along with the element actinium (Ac), atomic number 89 in the same position (group 3 in period 7) because of very close resemblance between them. They are shown also separately below the main periodic table along with lanthanides. Periods There are seven rows in the periodic table. Each row is called a period. The periods have been numbered from 1 to 7 (Arabic numerals). In each period a new shell starts filling up. The period number is also the number of shell which starts filling up in it. For example, in elements of 3rd period, the third shell (M shell) starts filling up as we move from left to right. The first element of this period sodium Na (2,8,1) has only one electron in its valence shell (third shell) while the last element of this period, argon Ar (2,8,8) has eight electrons in its valence shell. The first period is the shortest period of all and contains only 2 elements, H and He. The second and third periods are called short periods and contain 8 elements each. Fourth and fifth periods are long periods and contain 18 elements each. Sixth and seventh periods are very long periods containing 32 elements each. Although, Moseley s research was halted when the British government sent him to serve as a foot soldier in WWI. He was killed in the fighting in Gallipoli by a sniper s bullet, at the age of 28. Because of this loss, the British government later restricted its scientists to noncombatant duties during WWII. SEABORG'S WORK The last major changes to the periodic table resulted from Glenn Seaborg's work in the middle of the 20th Century. Starting with his discovery of plutonium in 1940, he discovered all the transuranic elements from 94 to 102. He reconfigured the periodic table by placing the actinide series below the lanthanide series. In 1951, Seaborg was awarded the Nobel Prize in chemistry for his work. Element 106 has been named seaborgium (Sg) in his honor in his life. Self-Assessment 1. According to the modern periodic law the properties of elements are periodic function of which property of theirs? 2. List any two defects of Mendeleev s periodic table which have been corrected in the modern periodic table? 3. How many group and periods are present in the long form of periodic table? 4. What is the name of the family of elements present in group 2 of the modern periodic table? 5. The elements that are present in the right hand portion of the periodic table are metals or non-metals? 6. How many elements are present in 6th period of the periodic table? 6
Periodic Table Timeline 440 BC Democritus and Leucippus propose the idea of the atom, an indivisible particle that all matter is made of. 330 BC Aristotle proposes the four element theory: earth, air, fire & water 360 BC Plato coins term elementsâ 1661 Robert Boyle published "The Sceptical Chymist" which was a treatise on the distinction between chemistry and alchemy. It also contained some of the earliest ideas of atoms, molecules, and chemical reaction marking the beginning of the history of modern chemistry 1754 Joseph Black isolated carbon dioxide, which he called "fixed air". 1778 Antoine Lavoisier wrote the first extensive list of elements containing 33 elements & distinguished between metals and nonmetals 1766 Henry Cavendish discovered hydrogen as a colorless, odourless gas that burns and can form an explosive mixture with air 1773-1774 Carl Wilhelm Scheele and Joseph Priestly independently isolated oxygen 1803 John Dalton proposed "Dalton's Law" describing the relationship between the components in a mixture of gases. 1828 Jakob Berzelius developed a table of atomic weights & introduced letters to symbolize elements 1828 Johann Dobereiner developed groups of 3 elements with similar properties 1864 John Newlands arranged the known elements in order of atomic weights & observed similarities between some elements 1864 Lothar Meyer develops an early version of the periodic table, with 28 elements organized by valence 1864 Dmitri Mendeleev produced a table based on atomic weights but arranged 'periodically' with elements with similar properties under each other. His Periodic Table included the 66 known elements organized by atomic weights. 1894 William Ramsay discovered the Noble Gases. 7
1898 Marie and Pierre Curie isolated radium and polonium from pitchblende. 1900 Ernest Rutherford discovered the source of radioactivity as decaying atoms 1913 Henry Moseley determined the atomic number of each of the elements and modified the 'Periodic Law'. 1940 Edwin McMillan and Philip H. Abelson identify neptunium, the lightest and first synthesized transuranium element, found in the products of uranium fission. 1940 Glenn Seaborg synthesised transuranic elements (the elements after uranium in the periodic table) 8