classification-of-elements-periodicity-in-properties-for-class-xi.pptx

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1. CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES of elements

2. Why do we need classification .? Dobereniner’s triads Newlands law of octave Lother Meyer volume curve Mendeleev’s periodic table Modern periodic tab le Session Objectives

3. Lavoisier (1789) classified elements into metals, non-metals, gases and earths. During the nineteenth century, chemists began to categorize the elements according to similarities in their physical and chemical properties. The end result of these studies was our modern periodic table.

4. Dobereiner’s triads [ John Dobereiner ( 1817)] In 1829, he classified some elements into groups of three, which he called triads. The elements in a triad had similar chemical properties and orderly physical properties. Model of triads S.No Triad Atomic masses of elements of triad Arithmetic mean of atomic masses of first and third element 1 Cl,Br,I 35.5, 80, 127 35.5 + 127 2 = 81.25 3 Ca,Sr,Ba 40,87.5,137 40+137 2 = 88.5 2 Li,Na,K 7, 23, 39 7 + 39 2 = 23

5. In 1866, he suggested that elements be arranged in “octaves” because he noticed (after arranging the elements in order of increasing atomic mass) that certain properties repeated every 8th element. Newland’s law of octaves [John Newland ( 1866)]

6. Newland’s law of octaves [John Newland (1863)] Element Atomic mass Element Atomic mass Element Atomic mass I II III IV V VI VII Li Be B C N O F 7 9 11 12 14 16 19 Na Mg Al Si p S Cl 23 24 27 28 31 32 35.5 K Ca 39 40 Newland was first to publish the list of elements in increasing order of atomic masses.

7. Dmitri Mendeleev 1834 - 1907 In 1869 he published a table of the elements organized by increasing atomic mass.

8. The physical and chemical properties of elements are periodic function of their atomic masses. Mendeleev’s periodic law

9. Mendeleev’s periodic table

10. Groups 8 vertical rows. 7 groups were subdivided in A and B. 8 th group has 9 elements in the group of 3 each. Periods 7 horizontal rows. Only 63 elements were known. Mendeleev’s periodic table

11. Merits of Mendeleev’s periodic table Prediction of new elements ( Ge , Ga , Sc) 1 Systematic study of elements 2 Correction of atomic mass (Be, Au, Pt) 3

12. Mendeleev stated that if the atomic weight of an element caused it to be placed in the wrong group, then the weight must be wrong. (He corrected the atomic masses of Be, In, and U) 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.

13. Defects of Mendeleev’s periodic table Position of hydrogen . Anomalous pairs. ( Ar and K, Co and Ni, Te and I) Position of isotopes e.g. 1 H 1, 1 H 2, 1 H 3

14. Chemically dissimilar elements are grouped together. (Cu-IA and Na-IB) Chemically similar elements are placed in different groups. [Cu (I) and Hg (II)]. Defects of Mendeleev’s periodic table

15. English physicist, Henry Moseley observed regularities in the characteristic X-ray spectra . A plot of f against atomic number ( Z ) of the elements gave a straight line and not the plot of f vs atomic mass He thereby showed that the atomic number is a more fundamental property of an element than its atomic mass. MODERN PERIODIC LAW AND THE MODERN PERIODIC TABLE

16. Mendeleev’s Periodic Law was, therefore, accordingly modified. This is known as the Modern Periodic Law and can be stated as : The physical and chemical properties of the elements are periodic functions of their atomic numbers.

17. Henry Moseley In 1913, through his work with X-rays, he determined the actual nuclear charge (atomic number) of the elements*. He rearranged the elements in order of increasing atomic number. *“There is in the atom a fundamental quantity which increases by regular steps as we pass from each element to the next. This quantity can only be the charge on the central positive nucleus.” His 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.

18. Modern periodic table

19. Features of long form of periodic table Contains elements arranged in increasing order of atomic numbers. Explains the position of an element in relation to other elements. Consists of groups and periods.

20. Features of long form of periodic table Groups Vertical column Total 18. Numbered 1-18 or IA to VII A, IB to VII B, VIII and zero. Periods Horizontal column Total 7 numbered from 1 to 7. Elements in a group have similar but not identical electronic configuration and properties Contains 2,8,8,18,18,32 and 28 elements respectively.

21. ELECTRONIC CONFIGURATIONS AND TYPES OF ELEMENTS: On the basis of the nature of sub-shell in which last electron of atom enters, elements are divided into 4 blocks s-Block Element p-Block Element d-Block Element f- Block Element s-,p-,d-,f- Block Elements

22. Electronic configuration: Groups: All are metal, low ionisation energy and low melting and boiling points, electropositive elements. compounds are mostly ionic & colourless . IA (alkali metals )and IIA(alkaline earth metals ns 1 or ns 2 In these elements last electron enters the s-orbital s-Block Elements

23. Electronic configuration: Groups: Non-metals, electronegative. Form covalent compounds. ns 2 ,np 1 -6 III A to VII A and zero group (group 13-18 ). In these elements last electron enters the p-orbital p-Block Elements

24. Electronic configuration: Groups: Variable valency high melting and boiling point. Coloured compounds and catalytic property. (n-1)d 1-10 ns 1or2 I B to VII B and VIII groups ( Gr - 3-12). In these elements last electron enters the d-orbital, Also known as transition metals . d-Block Elements

25. Electronic configuration: Have high melting and boiling point. (n-2)f 1-14 (n-1)d 0-1 ns 2 Present below the periodic table in two rows Lanthanides-elements after lanthanum(Gr.-3, Pd.-6) Actinides-elements after actinium . (Gr.-3, Pd .-7) In these elements last electron enters the f-orbital, Also known as Inner-Transition Elements f-Block Elements

26. Representative elements Transition elements s and p block elements . d-block elements. Valence shell and penultimate Shell both are incomplete. Inner Transition elements f-block elements. Valence shell, penultimate shell antipenultimate shell are incomplete. Features of long form of periodic table

27. Metals Present on left hand side of periodic table. Solid,malleable,ductile and conductors . Non-metals Present on right hand side of periodic table. Solid or liquid or gas. Metalloids Present on zig-zag between metals and non-metals. e.g. B,Si,Ge,As,Sb and Te. Features of long form of periodic table

28. Based on a more fundamental basis - the atomic number Position of an element is related to the electronic configuration of its atom. Due to separation of elements into groups, dissimilar elements (e.g. alkali metals I A and coinage metals I B) do not fall together. Merits of long form of periodic table

29. Defects of long form of periodic table The problem of the position of hydrogen in the table has not been solved completely Configuration of Helium(1s 2 ) is different from inert gases (ns 2 ,np 6 ) but are placed in the same group. It is unable to include lanthanides and actinides in its main body.