Class 10 Science
Chapter - 5: Periodic Classification of Elements
Textual Questions and Answers:
1. Did Dobereiner's triads also exist in the columns of Newlands' Octaves? Compare and find out.
Answer: Yes, Dobereiner's triads also exist in the columns of Newlands' Octaves. For example, Li, Na, and K.
2. What are the limitations of Dobereiner's classification?
Answer:
Limitations of Dobereiner's classification of elements
- It could identify only three triads from the known elements.
- It failed to arrange all known elements into triads with similar chemical properties.
3. What were the limitations of Newlands' Law of Octaves?
Answer: (i) It was applicable only up to calcium.
(ii) Assumed only 56 elements existed and no new elements would be discovered.
(iii) Placed two elements in the same slot.
(iv) Iron, which resembles cobalt and nickel, was placed far from them.
Textual Questions and Answers:
1. Use Mendeleev's Periodic Table to predict the formula for the oxides of the following elements: K, C, Al, Si, Ba.
Answer:
- K₂O
- CO₂
- Al₂O₃
- SiO₂
- BaO
(i) Potassium (K) is a member of group IA. Its valency is 1. Therefore, its formula is K2O.
(ii) Carbon (C) is a member of group IV A. Its valency is 4. Therefore, the formula is C2O4 or CO2.
(iii) Aluminium (Al) belongs to groups III A and its valency is 3. The formula of its oxide is Al2O3.
(iv) Silicon (Si) is present in group IV A after carbon. Its valency is also 4. The formula of oxide is Si2O4 or SiO2.
(v) Barium (Ba) belongs to group II A and the valency of the element is 2. The formula of the oxide of the element is Ba2O2 or BaO.
2. Besides gallium, which other elements have since been discovered that were left by Mendeleev in his periodic table?
Answer: Scandium and Germanium.
3. What were the criteria used by Mendeleev in creating his periodic table?
Answer:
- Elements were arranged in increasing atomic mass.
- Elements with similar properties were grouped together.
4. Why do you think noble gases are placed in a separate group?
Answer: Noble gases are chemically inert and have a stable electronic configuration.
Page - 90
1. How could the Modern Periodic Table remove various anomalies of Mendeleev's Periodic Table?
Answer: Mendeleev was unable to give fixed position to hydrogen and isotopes in the periodic table. In Mendeleev’s periodic table, the increasing manner of atomic mass of the elements is not always regular from one to its next. It was believed that a more fundamental property than atomic mass could explain periodic properties in a better manner.
It was Henry Moseley who demonstrated that atomic number of an element could explain periodic properties in a better way than atomic mass of an element and arranged the elements in increasing order of their atomic numbers. Then it was found that the various anomalies of Mendeleev’s periodic table were removed by the modern periodic table.
2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis of your choice?
Answer: Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements. And since chemical properties are due to valence electrons, they show same chemical reactions.
3. 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.
Answer:
(a) Lithium, sodium, potassium.
(b) Magnesium, calcium.
(c) Helium, Neon, Argon.
4. (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 in a gas of extremely low reactivity. What if anything, do their atoms have in common?
Answer: (a) The atoms of lithium, sodium, potassium all have only one electron in their outer most shells.
(b) The atoms of helium and neon have their outer most shell completely filled.
5. In the Modern Periodic Table, which are the metals among the first ten elements?
Answer: As, the metallic character decreases across the period so lithium and beryllium are the metals among the first ten elements in the Modern Periodic Table.
6. By considering their position in he periodic table , which one of the following elements would you expect to have maximum metallic characteristic?
Ga, Ge, As, Se, Be.
Answer: Be.
Exercises:
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) Elements become less metallic from left to right.
(b) The number of valence electrons increases.
(c) Atoms lose electrons more easily.
(d) Oxides become more acidic.
Answer: (c) Atoms lose electrons more easily.
2. Element X forms a chloride with the formula XCl₂, which is solid with a high melting point. X would most likely be in the same group as:
(a) Na
(b) Mg
(c) Al
(d) Si
Answer: (b) Mg
3.Which element has
(a) Two completely filled shells.
(b) Configuration: 2, 8, 2.
(c) Three shells, valence electrons = 4.
(d) Two shells, three valence electrons.
(e) Twice as many electrons in the second shell as in the first shell.
Answer:
(a) Ne (2,8)
(b) Mg (2,8,2)
(c) Si (2,8,4)
(d) B (2,3)
(e) C (2,4)
4. (a) What property do all elements in the same column of the periodic table as born have in common?
Answer: Same valency and metalloid properties.
(b) What property do all elements in the same column of the periodic Table as fluorine have in common?
Answer: Seven valence electrons and acidic oxide formation.
5. An atom has the electronic configuration 2,8,7.
(a) What is its atomic number?
(b) Which element would it resemble the most? (Given: N(7), F(9), P(15), Ar(18))
Answer:
(a) Atomic number = 17 (Chlorine).
(b) Resembles Fluorine (F).
6. The position of three elements A, B, and C in the Periodic Table are shown below:.
Answer:
(a) State whether A is a metal Or non metal.
(b) State whether C is more reactive or less reactive that 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?
Answer:
(a). Element A is in group 17. Now , group 17 is on the right side or the periodic table where non - metals are placed. So, element A is a non - metal.
(b). In group 17 of halogens, the chemical reactivity decreases on going down in a group. Thus element C will be less reactive than element A.
(c). On going from left to right in a period , the size of atoms, decreases. So, the atom of C will be smaller in size than an atom of B.
(d) Element A of group 17 has 7 valence electrons. So, it will accept 1 electrons to form a negatively charged ion, A. The negatively charged ion is called anion. Thus, element A will form an anion.
7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration A these two elements. Which of these will be more electronegative? Why?
Answer: Electronic configuration of Nitrogen = 2,5 Electronic configuration of phosphorous = 2,8,5
Nitrogen will be more electronegative because outermost shell is nearer to nucleus.
8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Answer: In the Modern Periodic Table, the electronic configuration of an element determines the number of valence electrons whereas the position of the atom is determined by the valence electrons.
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?
Answer: The electronic configuration of calcium = 2, 8, 8, 2 The electronic configuration of atomic number 12 = 2, 8,2
The electronic configuration of atomic number 19 = 2, 8, 8, 1
The electronic configuration of atomic number 21 = 2, 8, 8, 3
The electronic configuration of atomic number 38 = 2, 8, 18, 8, 2
Elements with atomic number 12 38 will have similar physical and chemical properties as calcium.
10. Compare and contrast the arrangement of elements in Mendeleev's Periodic Table and Modern Periodic Table.
Answer: From your chemistry lessons you have learned about the Mendeleev's Periodic Table and its modification which is known as the Modern Periodic Table. Let us compare both of them in the tabular form.
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