Class 10 Science Chapter 5 Periodic Classification of Elements
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In this chapter, you will learn about the need for classifying elements, Dobereiner’s Triads, Newlands’ Law of Octaves, Mendeleev’s Periodic Table, and the Modern Periodic Table. You will also understand periodic trends such as valency, atomic size, metallic and non-metallic properties, and how these properties change across periods and groups. These concepts are essential for building a strong foundation in Chemistry.
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Whether you are preparing for class tests, annual examinations, or the HSLC Board Examination, this chapter will help you improve your understanding, strengthen your concepts, and score higher marks in Science.
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- ❓ Complete Textbook Question Answers
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Periodic Classification of Elements
Questions &Answers
Q: Were there Dobereiner’s triads in the columns of Newlands’ octaves? Compare and find out.
Ans: Yes, there were. For example—H, F, Cl, Li, Na, K, Be, etc.
Q: Write Mendeleev’s periodic law.
Ans: “The properties of elements change periodically with their atomic masses.”
Q: Which element discovered at that time could not be placed inMendeleev’s periodic table?
Ans: Hydrogen.
Q: Write one limitation of Mendeleev’s periodic table.
Ans: The increase in atomic mass from one element to the next is not uniform.
Q: Why are noble gases placed in a separate group?
Ans: Noble gases do not react with other elements, nor do they lose or gain electrons from the outermost shell. Therefore, they are placed in a separate group.
Q: Write the modern periodic law.
Ans: “The properties of elements change periodically with their atomic numbers.”
Q: Why does the atomic size decrease from left to right in a period of the periodic table?
Ans: Due to the increased nuclear charge, electrons are pulled closer to the nucleus, resulting in a decrease in atomic size.
Q: What were the limitations of Newlands’ law of octaves? HSLC ’20
Ans: The limitations of Newlands’ law of octaves are—
(a) It was applicable only up to calcium.
(b) Sometimes two elements were placed in the same position, and dissimilar elements were grouped together.
(c) It was not applicable to noble gases.
Q: Using Mendeleev’s periodic table, determine the formulae of oxides of the following elements—K, C, Al, Si, Ba.
Ans: K = K₂O, C = CO₂, Al = Al₂O₃, Si = SiO₂, Ba = BaO.
Q: Why are noble gases placed in a separate group? HSLC ’18
Ans: Noble gases do not take part in chemical reactions. Therefore, they are placed in a separate group from other elements.
Q: How did the modern periodic table remove the defects ofMendeleev’s periodic table? HSLC ’20
Ans: The modern periodic table removed the defects as follows—
(a) Position of hydrogen: Mendeleev could not assign a proper position to hydrogen. In the modern periodic table, hydrogen is placed at the top of Group 1 above alkali metals.
(b) Position of isotopes: Elements having the same number of protons have the same atomic number. Therefore, isotopes are placed in the same position based on their properties.
Q: Among the following statements about changes across a period in the periodic table from left to right, which one is incorrect? HSLC ’18
(a) Elements become less metallic.
(b) The number of valence electrons increases.
(c) Atoms lose their valence electrons more easily.
(d) Oxides become more acidic.
Ans: (c) Atoms lose their valence electrons more easily.
Q: Identify the elements with the following electronic configurations.
(a) 2, 8→Neon (Ne)
(b) 2, 8, 2→Magnesium (Mg)
(c) 2, 8, 4 →Silicon (Si)
(d) 2, 3 →Boron (B)
(e) 2, 4 →Carbon (C)
Q: Besides gallium, for which other elements did Mendeleev leave spaces before their discovery? (Any two) HSLC ’18
Ans: (a) Scandium (b) Germanium
Q: What principles did Mendeleev follow while constructing his periodic table?
Ans: (a) He arranged elements in increasing order of their atomic masses.
(b) He gave importance to the formulae of hydrides and oxides formed by elements for classification.
Q: Name two elements which would show chemical reactions similar to magnesium. What is the basis of your selection?
Ans: (a) Beryllium and (b) Calcium. These elements have the same number of electrons in their valence shell as magnesium; hence, they show similar reactions.
Q: (a) Lithium, sodium, potassium—these are all metals. They react with water to release hydrogen gas. Is there any similarity in their atomic structure?
(b) Helium is an inert gas, and neon is also nearly inert. What similarity is observed in their atoms?
Ans:
(a) The similarity is that each has one electron in the outermost shell.
(b) Both have completely filled outermost shells.
Q: (a) What are the common properties of elements in the same group as boron?
(b) What are the common properties of elements in the same group as fluorine?
Ans:
(a) All have valency 3.
(b) All have valency 1.
Q: Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to Group 15 of the periodic table. Write their electronic configurations. Which one is more electronegative and why? HSLC ’18
Ans: Nitrogen: 2.5; Phosphorus: 2.8.5. Nitrogen is more electronegative than phosphorus because its atomic size is smaller.
Q: An atom has the electronic configuration 2, 8, 7. HSLC ’20
(a) What is its atomic number?
(b) With which of the following elements will it show chemical similarity? N(7), F(9), P(15), Ar(18)
Ans:
(a) Its atomic number is 17.
(b) It will be similar to fluorine (F), whose atomic number is 9 and configuration is 2.7.
Q: How is the electronic configuration of an atom related to its position in the modern periodic table?
Ans: The atomic number and electronic configuration determine the position of an element in the periodic table. For example, sodium has configuration 2.8.1, so it has one valence electron and is placed in Group 1. Since it has three shells, it belongs to Period 3.
Q: Besides gallium, which elements did Mendeleev predict and leave spaces for before their discovery?
Ans: Scandium and Germanium, which he predicted as “eka-boron” and“eka-silicon.”
Q: What criteria did Mendeleev follow while preparing his periodic table?
Ans: First, he arranged elements in increasing order of atomic mass.
Second, he grouped elements with similar physical and chemical properties together.
Q: Compare the arrangement of elements in Mendeleev’s periodic table and the modern periodic table.
Ans:
In Mendeleev’s periodic table—
(a) There were only 8 groups.
(b) No definite place for isotopes.
(c) Noble gases were not included.
In the modern periodic table—
(a) There are 18 groups.
(b) Isotopes occupy the same position.
(c) Noble gases are placed in a separate group.
Q: Select the metalloid from the following.
(a) P (b) S (c) Cl (d) Si
Ans: (d) Si
Q: Which of the following elements belong to the same group?
(a) Li, Na, K
(b) Li, Be, B
(c) Na, Mg, Al
(d) P, S, Cl
Ans: (a) Li, Na, K
Q: What is meant by a period in the periodic table?
Ans: The horizontal rows in the periodic table are called periods.
Q: Define the modern periodic law.
Ans: The physical and chemical properties of elements vary periodically with their atomic numbers.
Q: An element X is placed in a group of the periodic table. Its chloridehas the formula ______.
(a) XCl₃
(b) XCl
(c) X₂Cl
(d) None of these
Ans: (a) XCl₃
Q: What is the valency of an element?
Ans: Valency is the number of electrons an atom gains, loses, or shares to attain the electronic configuration of the nearest noble gas.
Q: Write the correct answer. HSLC ’16
In a period of the periodic table, from left to right, atomic radius—
(a) increases
(b) decreases
(c) remains same
(d) none of these
Ans: (b) decreases
Q: Write the correct answer. HSLC ’17
An atom has electronic configuration 2, 8, 7. With which element will it show chemical similarity? HSLC ’20
(a) N (7)
(b) F (9)
(c) P (15)
(d) Ar (18)
Ans: (b) F (9)
Q: Write the names of elements in the second period of the periodic table. HSLC ’16
Ans: Li, Be, B, C, N, O, F, Ne
Q: Which element has the electronic configuration 2.8.3? HSLC ’17
Ans: Aluminium (Al)
Q: Two elements A and B have atomic numbers 12 and 17 respectively. HSLC ’17
(a) Write the electronic configurations of A and B.
(b) What type of bond will be formed between A and B?
(c) Write the formula of the compound formed between A and B.
Ans:
(a) Electronic configuration of A = 2.8.2
Electronic configuration of B = 2.8.7
(b) An ionic bond will be formed between A and B.
(c) The formula of the compound formed is AB2.
Q: Lithium, sodium, and potassium are all metals. They react with water to release hydrogen gas. Is there any similarity in the atoms of these elements? HSLC ’19
Ans: Yes. These elements have similarity in their atoms. Each of them has one electron in the outermost shell.
Q: Among the following statements about the nature of changes across a period in the periodic table from left to right, which one is not correct? HSLC ’19
(a) Elements become less metallic.
(b) The number of valence electrons increases.
(c) Atoms lose their valence electrons more easily.
(d) Oxides become more acidic.
Ans: The incorrect statement is: (c) Atoms lose their valence electrons more easily.
Q: For which of the following elements is the octet rule applicable?
(a) Oxygen
(b) Calcium
(c) Cobalt
(d) Sodium
Ans: (d) Sodium
Q: Element X forms a chloride with the formula XCl₂. It is a solid with a high melting point. With which of the following elements is X most likely to be in the same group? HSLC ’19
(a) Na
(b) Mg
(c) Al
(d) Si
Ans: X is most likely to be in the same group as Mg.
Multiple Choice Questions (MCQ)
Q: Using Mendeleev’s table, determine the formula of the oxide of element ‘C’—
(a) CO
(b) C₂O
(c) C₂O₂
(d) CO₂
Ans: (d) CO₂
Q: Using Mendeleev’s table, determine the formula of the oxide of element ‘K’—
(a) K₂O
(b) K²O₂
(c) KO
(d) K₂O₃
Ans: (a) K₂O
Q: Using Mendeleev’s table, determine the formula of the oxide of element ‘Al’—
(a) AlO
(b) Al₂O₂
(c) Al₂O₃
(d) AlO₂
Ans: (c) Al₂O₃
Q: “The atomic number of an element is more fundamental than its atomic mass”—this fact was shown by—
(a) J. W. Döbereiner
(b) John Newlands
(c) Henry Moseley
(d) D. I. Mendeleev
Ans: (c) Henry Moseley
Q: Dobereiner first highlighted the catalytic property of—
(a) Nickel
(b) Platinum
(c) Hydrogen
(d) None of the above
Ans: (b) Platinum
Q: The law of octaves was proposed by—
(a) Mendeleev
(b) Döbereiner
(c) Isaac Newton
(d) Newlands
Ans: (d) Newlands
Q: During Newlands’ time, the number of known elements was—
(a) 36
(b) 56
(c) 46
(d) 66
Ans: (b) 56
Q: Up to which element was Newlands’ law of octaves applicable?
(a) Carbon
(b) Thorium
(c) Calcium
(d) Copper
Ans: (c) Calcium
Q: During Mendeleev’s time, the number of known elements was—
(a) 63
(b) 53
(c) 73
(d) 83
Ans: (a) 63
Q: Across a period in the periodic table, from left to right, atomic radius—
(a) increases
(b) decreases
(c) remains the same
(d) none of these
Ans: (b) decreases
Q: When nuclear charge increases, atomic size—
(a) increases
(b) decreases
(c) remains the same
(d) none of these
Ans: (b) decreases
Q: Which of the following is not a metalloid?
(a) Boron
(b) Silicon
(c) Germanium
(d) Sodium
Ans: (d) Sodium
Q: While studying compound formation, Mendeleev gave more importance to which two elements?
(a) Hydrogen and Oxygen
(b) Hydrogen and Nitrogen
(c) Carbon and Oxygen
(d) Nitrogen and Oxygen
Ans: (a) Hydrogen and Oxygen
Q: On what basis did Mendeleev classify elements?
(a) Atomic number
(b) Valence electrons
(c) Atomic mass
(d) None of these
Ans: (c) Atomic mass
Q: Elements in the second period of the periodic table are—
(a) Carbon, Hydrogen
(b) Lithium, Beryllium
(c) Zinc, Arsenic
(d) Cadmium, Fermium
Ans: (b) Lithium, Beryllium
Q: In a group of the periodic table, the number of valence electrons—
(a) increases
(b) decreases
(c) remains the same
(d) none of these
Ans: (c) remains the same
Q: Across a period in the periodic table, atomic number—
(a) increases
(b) decreases
(c) remains the same
(d) none of these
Ans: (a) increases
