NCERT Solutions for Class 10th Science
Chapter 5 – PERIODIC CLASSIFICATION OF ELEMENTS
(Complete Downloadable Chapter Solution PDF file is at the bottom of the page)
Q.1 Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Q.2 What were the limitations of Dobereiner’s classification?
Ans Limitation of Dobereiner’s triads was that he could not arrange all the then known elements in the form of triads. He could identify only three triads from then known elements.
Q.3 What were the limitations of Newlands’ Law of Octaves?
Ans Limitations of Newlands’ Law of Octaves:
1) Law of Octaves was applicable only upto calcium, as after calcium every eighth element did not possess properties similar to that of the first.
2) It was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future. But, later on, several new elements were discovered, whose properties did not fit into the Law of Octaves.
3) In order to fit elements into his Table, Newlands adjusted two elements in the same slot, but also put some unlike elements under the same note. For example cobalt and nickel are in the same slot and these are placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements.
4) Iron, which resembles cobalt and nickel in properties, has been placed far away from these elements.
Thus, we can say that Newlands’ Law of Octaves worked well with lighter elements only.
Q.4 Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, Al, Si, Ba.
Q.5 Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (any two)
Ans Scandium and Germanium
Q.6 What were the criteria used by Mendeleev in creating his Periodic Table?
Ans Mendeleev arranged the elements on the basis of their fundamental property, the atomic mass, and also on the similarity of chemical properties. He found that if the elements are arranged in the increasing order of their atomic masses, their properties get repeated after regular intervals.
Q.7 Why do you think the noble gases are placed in a separate group?
Ans Noble gases are inert elements i.e. they are highly unreactive and their properties are different from all other elements. Hence noble gases are placed in a separate group.
Q.8 How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?
Q.9 Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Ans Beryllium (Be) and Calcium (Ca)
This is because these two elements have same number of valence electrons i.e. 2 as magnesium.
Q.10 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.
Q.11 (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 a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Q.12 In the Modern Periodic Table, which are the metals among the first ten elements?
Q.13 By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?
Ga Ge As Se Be
EXERCISES
Q.1 Which of the following statements is not a 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.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.
Ans (c) The atoms lose their electrons more easily.
Q.2 Element X forms a chloride with the formula XCl2, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as
(a) Na (b) Mg (c) Al (d) Si
Ans (b) Mg
This is because Mg has a valency of 2.
Q.3 Which element has
(a) two shells, both of which are completely filled with electrons?
(b) the electronic configuration 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) Neon, Ne (Atomic no. – 10; 2,8)
(b) Magnesium, Mg (Atomic no. – 12; 2,8,2)
(c) Silicon, Si (Atomic no. – 14; 2,8,4)
(d) Boron, B (Atomic no. – 5; 2,3)
(e) Carbon, C (Atomic no. – 6; 2,4)
Q.4 (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?
Q.5 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)
Q.6 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) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?
(d) Which type of ion, cation or anion, will be formed by element A?
Q.7 Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?
Q.8 How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Ans Electronic configuration tells us about the number of shells and valence electrons of an element. Now number of shells gives us period whereas number of valence electrons gives us group number of an element which then can be placed in the Modern Periodic Table. Further, elements with same valence electrons are placed in same group. For example, Sodium (Na) has electronic configuration – 2,8,1 i.e. it has 3 shells and 1 valence electrons, hence it can be placed in 3rd period and 1st group.
Q.9 In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?
Ans Electronic configuration of calcium is 2,8,8,2 i.e. it has 2 valence electrons. Hence physical and chemical properties of elements with atomic numbers 12 and 38 will resemble calcium as they also have 2 valence electrons as shown below.
Element with atomic number 12 – 2,8,2
Element with atomic number 38 – 2,8,18,8,2
Q.10 Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.