PERIODIC CLASSIFICATION

Transcription

1 5 PERIODIC CLASSIFICATION OF ELEMENTS TEXTBOOK, QUESTIONS AND THEIR ANSWERS Q.1. Do Dobereiner s triads also exist in the columns of Newland s octaves? Compare and find out. Ans. Triad of Li, Na and K exists in Newland s octave. Triad of Cl, Br and I does not exist, because there are Co and Ni in between Cl and Br. Triad of Ca, Sr and Ba does not exist, because there is Zn in between Ca and Sr. Q.2. What were the limitations of Dobereiner s classification? Ans. 1. Classification into triads left room for chance. It is possible to group quite dissimilar elements into a triad. 2. Quite a large number of elements cannot be grouped into a triad. Q.3. What were the limitations of Newland s Law of octaves? Ans. 1. It did not include all the elements known at that time. 2. It did not recognise transition elements. 3. Position of hydrogen was not justified along with fluorine and chlorine. Q.4. What were the criteria used by Mendeleev in creating his Periodic Table? Ans. 1. The chemical and physical properties of an element is the function of its atomic mass. 2. The elements were arranged in periods such that their properties changed from metallic to non-metallic. 1

2 2 3. The elements were arranged in groups, such that all the elements have same, but graded physical and chemical properties. Q.5. Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (any two). Ans. 1. Eka-silicon which is Germanium in group IIIA 2. Eka-boron which is Scandium in group III B. Q.6. Why do you think, the noble gases are placed in a separate group? Ans. 1. Noble gases are chemically inactive and hence constitute a separate group. 2. Noble gases as a group offer perfect dividing line in starting with new period in the periodic table, i.e., from highly metallic element towards left to highly non-metallic element towards right in a period followed by a noble gas. Q.7. In the Modern Periodic Table which are the metals amongst the first ten elements. Ans. Lithium (Li) and Beryllium (Be) are the metals. Q.8. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristics. Ga, Ge, As, Se, Be? Ans. Be will have maximum metallic character as it has 2 valence electrons. Q.9. Choose the correct option : Which of the following statements is not correct statement about the trends when going from left to right across the periods of Periodic Table? (a) The elements become less metallic in nature.

3 3 (b) The number of valence electrons increases. (c) The atoms lose their electrons more easily. (d) The oxides become more acidic. Ans. (a) is the correct choice. It is because the atoms lose their electrons with a great difficulty. Q.10. Choose the correct option : An element X forms a chloride with formula XCl 2, which is a solid with a high melting point. X would most likely be in the same group of periodic table as : (a) sodium (b) magnesium (c) aluminium (d) silicon Ans. (b) is the correct option. It is because the ionic bond between the magnesium and chlorine is very strong. Q.11. Which element has : (a) two shells, both of which are completely filled with electrons? (b) the electronic configuration is 2, 8, 2. (c) a total of three shells, with four electrons in its valence shell? (d) a total of two shells, with three electrons in its valence shell? (e) twice as many electrons in its second shell as in its first shell? Ans. (a) Element is neon [2(K), 8(L)] (b) Element is magnesium [2(K), 8(L), 2(M)] (c) Element is silicon [2(K), 8(L), 4(M)] (d) Element is boron [2(K), 3(L)] (e) Element is carbon [2(K), 4(L)]

4 Q.12. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis of your choice? Ans. Beryllium and calcium will show chemical reactions similar to magnesium. It is because all of them belong to group 2 and hence have same number of valence electrons. Q.13. (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements? (b) Helium is an unreactive gas and neon is gas of extremely low reactivity what if anything, do their atoms have in common? Ans. (a) Yes, lithium, sodium and potassium belong to group I and hence have one electron in their valence shell. Thus, they have similar reactions with water. (b) Helium has only two electrons in its K-shell. As its K- shell is fully satisfied, therefore, it is chemically inactive. In the same way electronic configuration of neon is K(2), L(8). As, its valence shell is an octet, therefore, it is chemically inactive. Q.14. An atom has electronic configuration 2, 8, 7. (a) What is the atomic number of this element? (b) To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses). N(7), F(9), P(15), Ar(18) Ans. (a) Atomic number of element is 17. (b) F(9) will be chemically similar to element with atomic number 17. 4

5 5 Q.15. The position of three elements, A, B and C in the Periodic Table are shown below : Group 16 Group 17 A B C (a) State whether A is a metal or non-metal. (b) Sate whether C is more reactive or less reactive than A. (c) State whether C is larger or smaller in size than B. (d) Which type of ion, cation or anion, will be formed by element A? Ans. (a) A is non-metal. (b) C is less reactive than A. (c) C is smaller in size than B. (d) A forms an anion. Q.16. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to the group 15 of the Periodic Table. Write electronic configurations of these elements. Which of these will be more electronegative? Why? Ans. Electronic configuration of nitrogen is [2(K), 5(L)] Electronic configuration of phosphorus is [2(K), 8(L), 5(M)] Nitrogen is more electronegative than phosphorus. It is because, less is the number of electron shells around the nucleus, more is the tendency to attract electrons in valence shell, and hence, the element becomes more electronegative.

6 6 Q.17. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table? Ans. (a) The number of electrons in the valence shell determine the group of the element. If the element has 1 or 2 electrons, then it belongs to the group 1 or 2, respectively. If element has 3 to 8 electrons, then its group is 10 + number of valence electrons. (ii) The number of electron shells in an atom determine its period. For example, if an element has 4 electron shells, it belongs to 4th period. Q.18. In the Modern Periodic Table, calcium (atomic no : 20) is surrounded by elements with atomic numbers 12, 19, 21, 38. Which of these have physical properties resembling calcium? Ans. Calcium has electronic configuration [2(K), 8(L), 8(M), 2(N)] Element (At No. 38) has electronic configuration [2(K), 8(L), 18(M), 8(N), 2(O)] Thus, calcium resembles with element of At. No. 38, because both of them have 2 valence electrons. Q.19. Name (a) three elements that have a single electron in their outermost shells. (b) two elements that have two electrons in their outermost shells. (c) three elements with filled outermost shells.

7 7 Ans. (a) Sodium (Na), Potassium (K) and Lithium (Li) (b) Magnesium (Mg) and Calcium (Ca) (c) Helium (He), Neon (Ne) and Argon (Ar) Q.20. (a) What property do all elements in the same column of the Periodic Table as boron have in common? (b) What property do all elements in the same column of the Periodic Table as fluorine have in common? Ans. (a) All elements in the same column of the Periodic Table as boron have three electrons in their outermost orbit and have valency 3. (b) All elements in the same column of the Periodic Table have seven electrons in their outermost electrons and have valency 1. Q.21. Use Mendeleev s Periodic Table to predict the formulae for the oxides of the following elements : Ans. Elements Formula of oxides K K 2 O C CO 2 Al Al 2 O 3 Si SiO 2 Ba BaO Q.22. How could the Modern Periodic Table remove various anomalies of Mendeleev s Periodic Table. Ans. Various anomalies of Mendeleev s Periodic Table were removed by Modern Periodic Table.

8 1. Position of isotopes : Isotopes have different atomic mass but they have no different place in Mendeleev s Periodic Table. Isotopes have same atomic number so they are placed in the same position in Modern Periodic Table. 2. Position of Argon and Potassium : Argon has atomic mass higher than Potassium but it is placed before potassium in Mendeleev s Periodic Table. In Modern Periodic Table elements are placed in increasing atomic number. So, Argon is placed before calcium. Q.23. Compare and contrast the arrangement of elements in Mendeleev s Periodic Table and the Modern Periodic Table. Ans. Mandeleev s Periodic Table Modern Periodic Table 1. Elements are arranged in 1. Elements are arranged in order of increasing atomic order of increasing atomic mass. number. 2. It has eight groups. 2. It has eighteen groups. 3. Position of isotopes is 3. Isotopes have same atomic not clear. number so placed in the same period. 4. There is no separate 4. Lanthanides and actinides position of lanthanides are kept in two separate and actinides. rows and placed under the periodic table. 5. Transition elements are 5. Transition elements are placed in group VIII placed in the middle of long periods. 8