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Notes On Periodic Classification

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  1. Periodic classification of elements chapter wise important question Q.l How could the Modern Periodic Table remove various anomalies of Mendeleev's Periodic Table? Ans. Modern Periodic Table is based on the atomic number of elements, therefore (i) problem of isotopes was solved because isotopes have same atomic number Q.2 In the Modem Periodic Table, which are the metals among the first ten elements. Ans. Lithium and beryllium are metals. Q.3 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) The atomic number of the element is 17. (b) F(9) (2, 7) will be chemically similar to the given element. Q.4 How does the electronic configuration of an atom relate to its position in the Modern Periodic Table? Ans. The position of element depends upon its electronic configuration. The number of shells is equal to the period number. The valence electrons decides the group number in which it will be, elements with 1 valence electrons belong to group 1. Elements with 2 valence electrons belong to group 2. Q.5 The position of three elements A, B and C in the Periodic Table is shown below: Group 16 Group 17 c Giving reasons, explain the following: (a) Element A is a non-metal. (b) Element B has a larger atomic size than element C. (c) Element C has a valency of 1 Answer. (a) 'A' is non-metal because it can gain electron easily as it has 7 valence electrons and forms negative ion with stable electronic configuration. (b) It is because 'B' has lesser atomic number, less nuclear charge, less force of attraction between valence electrons and nucleus therefore, has larger atomic size. (c) 'C' has 7 valence electrons. It can gain one electron to become stable. So, its valency is equal to one. Q.6 The position of three elements A, B and C in the Periodic Table is shown below: 1 2 3 Giving reasons, explain the following: (a) Element A is non-metal. (b) Atom of element C has a larger size ' than atom of element A. (c) Element B has a valency of 1
  2. Answer. (a) It is because it has 7 valence electrons. It can gain one electron to form negative ion. So, it is a non-metal. (b)'C' has more number of shells than A. So, it is larger in size. (c)'B' has one valence electron. It can lose one electron to become stable. So, its valency is equal to Q.7 What physical and chemical properties of elements were used by Mendeleev in creating his periodic table? List two observations which posed a challenge to Mendeleev's Periodic Law. Answer. Atomic mass as a physical property and nature and formulae of oxide and hydride formed, and chemical property was used by Mendeleev. Following are the two observations which posed a challenge to Mendeleev's Periodic Law. (i) Increasing order of atomic weights could not be maintained while matching chemical properties. Chemical properties do not depend upon atomic mass. (ii) Isotopes have different atomic mass but same chemical properties. Q.8 (a) Why do we classify elements? (b) What were the two criteria used by Mendeleev in creating his Periodic Table? (c) Why did Mendeleev leave some gaps in his Periodic Table? (d) In Mendeleev's Periodic Table, why was there no mention of Noble gases like Helium, Neon and Argon? (e) Would you place the two isotopes ' of chlorine, Cl-35 and CI-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are the same? Justify your answer. Answer. (a) It is done so as to study the properties of elements conveniently. (b) Increasing order of atomic mass and similarities in chemical properties (especially nature and formulae of oxide and hydride formed). (c) These gaps were left for undiscovered elements. (d) Noble gases were not invented at that time. (e) They will be kept at same slot as they have same chemical properties. Q.9 State Mendeleev's periodic law. Write the achievements of Mendeleev's periodic table Answer. Mendeleev's Periodic Law: 'Properties of elements are the periodic function of their atomic masses. Achievements: It could classify all the elements discovered at that time. It helped in discovery of new elements. It helped in correction of atomic mass of some of the elements. Q. 10 (a) What is meant by periodicity in properties of elements with reference to the periodic table? (b) Why do all the elements of the same group have similar properties? (c) How will the tendency to gain electrons change as we go from left to right across a period? Why? Answer. (a) The repetition of same properties after definite interval is called periodicity in properties. (b) It is because they have same valence electrons therefore, have similar properties. (c) Tendency to gain electrons increases from left to right in a period because atomic size goes on decreasing and effective nuclear charge increases. Q. 11 (a) What are 'groups' and 'periods' in the 'periodic table'? (b) Two elements M and N belong to group I and Il respectively and are in the same period of the periodic table. How do the following properties of M and N vary? Sizes of their atoms Their metallic characters Their valencies in forming oxides Molecular formulae of their chlorides Answer. (a) The vertical columns in the periodic table are called 'groups'. The horizontal rows in the periodic table are called 'periods'.Q (b) 'M' and 'N' belong to same period but group I and ll. Therefore, 'N' will be smaller than 'M' as atomic size goes on decreasing from left to right.
  3. 'M' is more metallic than 'N'. Metallic character goes on decreasing from left to right as tendency to lose electrons decreases due to decrease in atomic size. Their valencies are 1 and 2 respectively in forming oxides. Valency goes on increasing first and then decreases. MCI, NC12 are molecular formulae of their chlorides. Q. 12 In the following table, six elements A, B, C, D, E and F (here letters are not the usual symbpls of the elements) of the Modern Periodic Table with atomic numbers 3 to 18 are given: 3 11 B 4 12 c 5 13 6 14 7 15 8 16 9 17 F 10 18 (a) Which of these halogen ?is (i) a noble gas, (ii) a halogen (b) If B combines with F, what would be the formula of the compound formed? (c)Write the electronic configurations of C and E. Answer. (a) (1) G is a noble gas. (i0 F is a halogen. (b) BF will be the formula of the compound formed. BF (c) C has atomic number 12 and electronic configuration is K = 2, L =8, M E has atomic number 8 and electronic configuration is K = 2, L = 6. Q.131n the following table, are given eight elements A, B, C, D, E, F, G and H (here letters are not the usual symbols of the elements) of the Modern Periodic Table with the atomic numbers of the elements in parenthesis. Period 2 3 4 5 What is the electronic configuration of F? Group 1 B (11) c (19) D (37) Group 2 F (12) G (20) H (38) What is the number of valence electrons in the atom of F? What is the number of shells in the atom of F? Write the size of the atoms of E, F, G and H in decreasing order, State whether F is a metal or a non-metal. Out of the three elements B, E and F, which one has the biggest atomic size? Answer. F has electronic configuration 2, 8, 2. F has 2 valence electrons. There are three shells in 'F'. H > G > F > E is decreasing order of size of atoms. 'F' is a metal. 'B' is having biggest atomic size among B, E and F. .Q. 14Na, Mg and Al are the elements having one, two and three valence electrons respectively. Which of these elements (a) has the largest atomic radius, (b) is least reactive? Justify your answer stating reason for each.
  4. Answer. (a) Na has the largest atomic radius because it has 11 protons and 11 electrons, therefore least effective nuclear charge. (b) Al is least reactive because it has smallest atomic size due to 13 protons and 13 electrons, it has greater effective nuclear charge, therefore, cannot lose electrons easily, hence it is least reactive. Q. 15 An element has atomic number 13. (a) What is the group and period number to which this element belongs? (b) Is this element a metal or a non- metal? Justify your answer. Answer. (a) It belongs to group 13 and 3rd period. (b) It is a metal because it can lose 3 electrons to become stable. Q. 16 Why do all the elements of the (a) same group have similar properties, (b) same period have different properties? Answer. (a) Elements of same group have similar properties due to same number of valence electrons, therefore, they have same valency. (b) Elements of same period have different properties as they differ in number of valence electrons. Q. 17 An element 'X' belongs to 3rd period and group 17 of the periodic table. State its (a) electronic configuration, (b) valency. Justify your answer with reasoning. Answer. (b) Valency : 1 It has atomic number 17 and therefore, electronic configuration will be 2, 8, 7. It can gain one electron to become stable therefore, its valency is equal to 1. It belongs to third period, as it has three shells. It belongs to group 1 7 because it has 7 valence electrons. Q. 18 State the Modern Periodic Law for classification of elements. How many (a) groups and (b) periods are there in the Modern Periodic Table? Answer. 'Properties of elements are the periodic function of their atomic number.' (a) There are 18 groups and (b) 7 periods in the Modern Periodic Table. Q. 19 The atomic number of an element is 16. Predict the number of valence electrons in its atom its valency its group number whether it is a metal or a non- metal the nature of oxide formed by it the formula of its chloride Answer. The electronic configuration of S(1 6) is 2, 8, 6. 6 2 16 Non-metal Acidic oxide SC12is a formula of its chloride.

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