Dobereiner s Triads Dobereiner observed that when elements were arranged into groups of three in the order of their increasing atomic masses, the atomic mass of the middle element was the arithmetic mean of rest of the two. Limitation Could be applied only to limited number of elements. CHAPTER 5 PERIODIC CLASSIFICATION OF ELEMENTS Newlands Law of Octaves Newlands found that every eighth element has chemical properties when they are arranged in increasing order of their atomic masses. Limitations Could be valid up to calcium only Newlands assumed that only56 elements existed in nature and no more elements would be discovered. Mendeleev's Periodic Classification Mendeleev s Periodic Law states that the properties of elements are the periodic function of their atomic masses. Merits of Mendeleev s Periodic Table Mendeleev left some blank spaces for undiscoveredelements. Mendeleev predicted the discovery of some elements and named them as eka-boron, ekaaluminium and eka-silicon. Noble gases discovered later could be placed without disturbing the existing order. Limitations of Mendeleev s Periodic Table Position of Hydrogen- Could not assign a correct position to hydrogen as hydrogen resembles alkali metals as well as halogens Position of Isotopes- Isotopes are placed in same position though they have different atomic masses Separation of chemically similar elements while dissimilar elements are placed in the same group. Modern Periodic Classification Modern Periodic Law states that properties of elements are the periodic function of their atomic numbers. Groups in Modern Periodic Table: Group 1 Group 2 Alkali metals Alkaline earth metals 57
Groups 3 to 12 Group 13 Group 14 Group 15 Group 16 Group 17 Group 18 Transition elements Boron family Carbon family Nitrogen family Oxygen family Halogens Noble gases Periods in Modern Periodic Table Period No of elements 1 st period 2 (H,He) 2 nd and 3 rd period 8 (Li,Be,B,C,N,O,F,Ne) 4 th and 5 th period 18 6 th period 32 7 th period Incomplete period Trends in Modern Periodic Table: Property Variation along the group Variation along the period Valency Remains the same Increasesup to group 14 then decreases Atomic radii Increases Decreases Metallic character Increases Decreases Electropositive character Increases Decreases Electronegativity Decreases Increases 58
DIAGRAMS NEWLAND S OCTAVES 59
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CROSSWORD Question Bank: - Periodic Classification of Elements Very Short Answer Type Questions(1 mark) Q1. Give an example of Dobereiner s triad. Q2. What is the basis ofmendeleev s periodic table? Short Answer Type Questions(2 marks) Q1. State the modern periodic law for classification of elements. How many groups andperiods are there in the modern periodic table? Q2. An element 'M' has atomic number 11. (i) Write its electronic configuration. (ii) State the group to which 'M' belongs. (iii) Is 'M' a metal or a non-metal? (iv)write the formula of its chloride. Q3. Name two elements that show chemical properties similar to bromine.give reason. Q4. An atom has electronic configuration 2, 8,2. (i) What is the atomic no. of this element? (ii) Is it a metal or non-metal? 61
Short Answer Type Questions(3 marks) Q1. The elements Li(Z = 3), Na (Z = 11)and K (Z = 19) belong to group 1 (i) Predict the periods they belong. (ii)which one of them is least reactive? (iii) Which one of them has the largest atomic radius? Give reason to justify. Q2. F, Cl and Br are the elements each having seven valence electrons. answer. Pick the element (i) with the largest atomic radius (ii) which is most reactive. Justify your Q3. Nitrogen(Z = 7) and Phosphorus(Z = 15) belong to same group-15 of the periodic table. Write the electronic configuration of these two elements. Which of these two is more electronegative? Why? Long AnswerType Questions (5marks) Q1.(i) How does atomic size vary along the group? Give reason. (ii) Why are metals electropositive in nature? (iii) What are metalloids? Give an example. Q2. Name- (i) Two elements that have a single electron in their outermost shells. (ii) Two elements that have two electrons in their outermost shells. (iii) Two elements with filled outermost shell. (iv)two elements thatbelong to halogen family. (v) An element which is tetravalent and forms the basis of organic chemistry. 62