NCERT Solutions for Class 10 Science Chapter 5 Intext Questions
This article on NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements provides easy and reliable answers to all the questions from the NCERT textbook. These solutions help students understand important concepts such as Dobereiner’s Triads, Newlands’ Law of Octaves, Mendeleev’s Periodic Table, the Modern Periodic Table, and the trends on the Modern Periodic Table. Whether you are preparing for your school exams, board exams, or revising key concepts, these NCERT Solutions for Chapter 5, Periodic Classification of Elements, will definitely help you. Each answer is explained in simple language to help you build a strong understanding of the chapter and score better in your upcoming Board examinations. These NCERT Solutions for Class 10 Science Chapter 5 will help you strengthen your concepts and improve your confidence in answering textbook questions.
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Questions Page Number: 81
1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Answer:
Yes, some of Döbereiner’s triads were also present in the columns of Newlands’ Law of Octaves.
For example, Lithium (Li), Sodium (Na), and Potassium (K) form a Döbereiner’s triad. These elements are also placed together in the same column in Newlands’ arrangement. According to Newlands’ Law of Octaves, every eighth element has properties similar to the first element. If we consider Lithium (Li) as the first element, then Sodium (Na) is the eighth element. If we consider Sodium as the first element, then Potassium is the eighth element.
This shows that certain Döbereiner’s triads were included in Newlands’ Octaves.
2. What were the limitations of Döbereiner’s classification?
Answer:
The main limitations of Döbereiner’s classification were:
- Döbereiner could only identify three triads from all the elements known at that time.
- The classification did not apply to all elements. It does not work for either very low atomic masses or very high atomic masses.
3. What were the limitations of Newlands’ Law of Octaves?
Answer:
The limitations of Newlands’ Law of Octaves were:
- It was applicable only up to calcium. Beyond calcium, every eighth element did not show properties similar to the first element.
- Newlands assumed that only 56 elements existed in nature and no more would be discovered. When new elements were discovered later, many of them could not be fitted into his arrangement.
- Some unlike elements were placed together. To maintain the pattern of octaves, Newlands placed elements with different properties in the same group. For instance, he put Cobalt (Co) and Nickel (Ni) in the same slot under the halogen column (Fluorine, Chlorine, Bromine), even though their properties are totally different.
- Elements with similar properties were separated. For example, iron (Fe), cobalt (Co), and nickel (Ni) have similar properties, but iron was placed far away from cobalt and nickel.
Questions Page Number: 85
1. Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, Al, Si, Ba.
Answer:
Oxygen belongs to Group VI A and has a valency of 2. Using this, we can write the formulae of oxides of different elements.
- Potassium (K) belongs to Group I A, so its valency is 1. Therefore, its oxide is K₂O.
- Carbon (C) belongs to Group IV A, so its valency is 4. Therefore, its oxide is CO₂.
- Aluminium (Al) belongs to Group III A, so its valency is 3. Therefore, its oxide is Al₂O₃.
- Silicon (Si) belongs to Group IV A, so its valency is 4. Therefore, its oxide is SiO₂.
- Barium (Ba) belongs to Group II A, so its valency is 2. Therefore, its oxide is BaO.
2. Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (Any two)
Answer:
Besides gallium, scandium (Sc) and germanium (Ge) are two elements that were later discovered and had been left as gaps by Mendeleev in his Periodic Table.
3. What were the criteria used by Mendeleev in creating his Periodic Table?
Answer:
Mendeleev used two main pillars to create his periodic table:
Atomic Mass: The elements are arranged on the basis of their increasing atomic masses.
Chemical Properties: The elements are also arranged according to their similarity of chemical properties. He focused on how elements reacted with oxygen (oxides) and hydrogen (hydrides). Elements that formed similar types of compounds with hydrogen and oxygen were placed in the same group.
4. Why do you think the noble gases are placed in a separate group?
Answer:
Noble gases are chemically inert and are present in extremely low concentrations in atmosphere. Therefore, they are kept in a separate group so that they don’t disturb the existing order.
Questions Page Number: 90
1. How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?
Answer:
The Modern Periodic Table removed many of the shortcomings of Mendeleev’s Periodic Table by arranging elements according to their atomic numbers instead of their atomic masses.
- Elements such as cobalt and nickel were arranged in the correct order by atomic number. Cobalt (27) comes before nickel (28), even though its atomic mass is slightly higher.
- All isotopes of an element have the same atomic number but different mass number. Therefore, they are placed in the same slot in the modern periodic table.
- Elements with similar chemical properties and the same number of valence electrons are placed in the same group, making the classification more accurate.
- Elements having the same number of electron shells are placed in the same period, which provides a clearer and more logical arrangement.
- The position of hydrogen is explained in the modern periodic table.
2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Answer:
Beryllium (Be) and Calcium (Ca) are two elements that are expected to show chemical reactions similar to magnesium.
Magnesium, beryllium, and calcium belong to Group 2 of the Modern Periodic Table. All of them have two valence electrons in their outermost shell. Since elements with the same number of valence electrons generally have similar chemical properties, beryllium and calcium show chemical behaviour similar to magnesium.
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 shell
(a) Three elements that have a single electron in their outermost shells.
Answer:
Lithium (Li), sodium (Na), and potassium (K) have a single electron in their outermost shells.
- Lithium (Li) – Atomic number – 3: ( 2, 1 )
- Sodium (Na) – Atomic number – 11: ( 2, 8, 1 )
- Potassium (K) – Atomic number – 19: ( 2, 8, 8, 1 )
(b) Two elements that have two electrons in their outermost shells.
Answer:
Magnesium (Mg) and Calcium (Ca) have two electrons in their outermost shells
- Magnesium (Mg) – Atomic number – 12: ( 2, 8, 2 )
- Calcium (Ca) – Atomic number – 20 : ( 2, 8, 8, 2 )
(c) Three elements with filled outermost shells.
Answer:
Helium (He), Neon (Ne) and Argon (Ar) have filled outermost shells
- Helium (He) – Atomic number – 2 : ( 2 )
- Neon (Ne) – Atomic number – 10 : ( 2, 8 )
- Argon (Ar) – Atomic number – 18 : ( 2, 8, 8 )
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 is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Answer
(a) Lithium, sodium and potassium are all metals, and all belong to the same group. The atoms of lithium, sodium and potassium all have only one electron in their outermost shells. All of these react with water to form alkalies. They are also called alkali metals.
(b) The atoms of helium and neon have completely filled outermost shells, which makes them chemically stable and very less reactive. They react only in extreme circumstances and hence are called noble gases. Helium has a duplet in its K shell, while neon has an octet in its L shell.
5. In the Modern Periodic Table, which are the metals among the first ten elements?
Solution:
Lithium and Beryllium are the metals among the first ten elements in the Modern Periodic Table.
6. 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
Answer:
Gallium (Ga) is expected to have the maximum metallic character among the given elements. Metallic character increases down a group and decreases from left to right across a period. Among the given elements, gallium (Ga) and beryllium (Be) are metals. However, Ga is placed lower in the Periodic Table and hence bigger in size, so it has a greater tendency to lose electrons than Be. Therefore, Ga is more metallic than Be.
Exercise questions Page Number: 91-92
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
Solution:
(c) The atoms lose their 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 to be in the same group of the periodic table as
(a) Na
(b) Mg
(c) Al
(d) Si
Answer:
(b) Mg
Explanation:
The formula XCl₂ shows that element X has a valency of 2.
Among the given options:
- Na has valency 1 and forms NaCl.
- Mg has valency 2 and forms MgCl₂.
- Al has valency 3 and forms AlCl₃.
- Si has valency 4 and forms SiCl₄.
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?
Answer:
(a) Neon (2, 8)
(b) Magnesium
(c) Silicon (2, 8, 4)
(d) Boron (2, 3)
(e) Carbon (2, 4)
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?
Answer:
(a) All elements in the same column as boron belong to Group 13. They have three electrons in their outermost shell and therefore show a valency of 3.
(b) All elements in the same column as fluorine belong to Group 17 (halogens). They have seven electrons in their outermost shell and therefore show a valency of 1.
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)
Solution:
(a) Given atom has electronic configuration = 2, 8, 7. So, total electrons = 2 + 8 + 7 = 17 Therefore, the atomic number of the element is 17.
(b) The given element has 7 electrons in its outermost shell. Elements having the same number of valence electrons show similar chemical properties.
Among the given elements:
- N (7) → Electronic configuration: 2, 5
- F (9) → Electronic configuration: 2, 7
- P (15) → Electronic configuration: 2, 8, 5
- Ar (18) → Electronic configuration: 2, 8, 8
Since fluorine (F) also has 7 valence electrons, it will be chemically similar to the given element.
6. The positions of three elements A, B and C in the periodic table are shown below :
(a) State whether A is a metal or non-metal.
| Group 16 | Group 17 |
| – | – |
| – | A |
| – | – |
| B | C |
(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 ?
Answer:
(a) It belongs to Group 17 (Halogens), meaning it has 7 valence electrons and thus valency is 1 and will readily gain one electron to complete its outermost shell. Thus, A is a non-metal.
(b) C is less reactive than A because, as we move down in a group, the reactivity of non-metals decreases because the tendency to gain electrons decreases.
(c) B and C are in the same period. As we move from left to right across a period, atomic size decreases from left to right due to an increase in the number of protons (nuclear charge). So, C will be smaller in size than B.
(d) Since A is a non-metal with 7 valence electrons, it tends to gain one electron to complete its octet. Therefore, it forms a negatively charged ion (anion).
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 ?