CBSE Class 10 Science Chapter 5: Periodic Classification of Elements NCERT Solutions
This chapter delves into the fascinating world of the Periodic Classification of Elements, a cornerstone of chemistry. The NCERT Solutions for Class 10 Science, Chapter 5, provide a comprehensive understanding of how elements are organized based on their properties. Students will explore the historical development of the periodic table, the concept of atomic number, electronic configurations, and the periodic trends in physical and chemical properties across periods and groups. The solutions explain the basis of these trends, such as metallic and non-metallic character, valency, and the nature of oxides. This resource is designed to help students grasp the fundamental principles of periodicity, enabling them to predict and explain the behavior of elements. These solutions are invaluable for exam revision, offering clear explanations and step-by-step problem-solving to reinforce learning and build confidence for assessments.
Quick info
| Board | CBSE |
|---|---|
| Class | Class 10 |
| Subject | Science |
| Session | 2026 |
| Language | English |
| Type | NCERT Solutions |
| Chapter | Chapter 5: Periodic Classification of Elements |
Chapter summary
Chapter 5 of the NCERT Class 10 Science textbook focuses on the Periodic Classification of Elements. The provided NCERT Solutions cover key concepts like the evolution of the periodic table, atomic number as the basis for modern classification, electronic configurations, and the periodic trends in properties such as metallic character, valency, and the acidity of oxides. The solutions address exercises that test students' understanding of these trends across periods and within groups, aiding in the systematic study of elements.
Learning outcomes
- Understand the basis of the modern periodic table.
- Identify trends in metallic and non-metallic character across periods.
- Explain the variation in valency and electronic configuration within groups.
- Determine the chemical similarity of elements based on their electronic configuration.
- Analyze the nature of oxides formed by elements across a period.
Topics covered
Paper topics
- Periodic Classification of Elements
- History of Periodic Table
- Atomic Number
- Electronic Configuration
- Periodic Trends
- Metallic Character
- Non-metallic Character
- Valency
- Nature of Oxides
- Groups and Periods
- Chemical Similarity of Elements
Important topics
- Periodic Trends (Metallic/Non-metallic character, Valency)
- Electronic Configuration and Position in Periodic Table
- Chemical Similarity based on Valence Electrons
- Nature of Oxides across a Period
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Questions and Solutions
Exercise 1
Which of the following statements is not a correct statement about the trends when going from left to right across the periods of the 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:
The statement that is not correct is (c) The atoms lose their electrons more easily.
Explanation:
When moving from left to right across a period in the periodic table, the nuclear charge increases, and the atomic size generally decreases. This leads to an increased attraction between the nucleus and the valence electrons. Consequently, the tendency for atoms to lose electrons decreases, while the tendency to gain electrons (non-metallic character) increases. Therefore, atoms do not lose their electrons more easily; instead, they tend to hold onto them more tightly or gain them.
Let's look at the other options:
- (a) As we move from left to right, elements transition from metals to non-metals, so they become less metallic in nature. This statement is correct.
- (b) The number of valence electrons increases by one for each step across a period (e.g., from 1 in Group 1 to 8 in Group 18, excluding Helium). This statement is correct.
- (d) The oxides of elements on the left side of a period are generally basic, while those on the right side are acidic. Thus, as we move across a period, the oxides tend to become more acidic. This statement is correct.
Exercise 2
Element X forms a chloride with the formula XCl<sub>2</sub>, 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
Solution:
The correct option is (b) Mg.
Explanation:
The formula of the chloride is given as XCl<sub>2</sub>. This formula indicates that element X has a valency of +2, meaning it forms a cation with a charge of +2 (X<sup>2+</sup>). Elements that typically form ions with a +2 charge are those belonging to Group 2 of the periodic table (alkaline earth metals).
Let's examine the options:
- (a) Na (Sodium) is in Group 1 and forms NaCl (valency +1).
- (b) Mg (Magnesium) is in Group 2 and forms MgCl<sub>2</sub> (valency +2).
- (c) Al (Aluminum) is in Group 13 and forms AlCl<sub>3</sub> (valency +3).
- (d) Si (Silicon) is in Group 14 and typically forms SiCl<sub>4</sub> (valency +4).
Since X forms XCl<sub>2</sub>, it is most likely to be in the same group as Magnesium (Mg).
Furthermore, chlorides of Group 2 elements are typically solids with high melting points, consistent with the description of XCl<sub>2</sub>.
Exercise 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?
Solution:
Let's determine the element for each description based on electronic configuration:
(a) Two shells, both completely filled: The first shell (K) can hold a maximum of 2 electrons, and the second shell (L) can hold a maximum of 8 electrons. An element with two filled shells would have an electronic configuration of 2, 8. This configuration corresponds to the element Neon (Ne), which has an atomic number of 10.
(b) Electronic configuration 2, 8, 2: This configuration indicates that the element has electrons distributed in three shells. The first shell has 2 electrons, the second shell has 8 electrons, and the outermost (valence) shell has 2 electrons. The total number of electrons is 2 + 8 + 2 = 12. The element with atomic number 12 is Magnesium (Mg).
(c) Three shells, four electrons in its valence shell: This means the element has electrons in the K, L, and M shells. The K shell has 2 electrons, the L shell has 8 electrons, and the M (valence) shell has 4 electrons. The total number of electrons is 2 + 8 + 4 = 14. The element with atomic number 14 is Silicon (Si).
(d) Two shells, three electrons in its valence shell: This indicates electrons are in the K and L shells. The K shell has 2 electrons, and the L (valence) shell has 3 electrons. The total number of electrons is 2 + 3 = 5. The element with atomic number 5 is Boron (B).
(e) Twice as many electrons in its second shell as in its first shell: The first shell (K) can hold a maximum of 2 electrons. If the second shell (L) has twice the number of electrons as the first, it would have 2 * 2 = 4 electrons. The electronic configuration is 2, 4. The total number of electrons is 2 + 4 = 6. The element with atomic number 6 is Carbon (C).
Exercise 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?
Solution:
(a) Elements in the same column as Boron: Boron (B) is in Group 13 of the periodic table. Its electronic configuration is 2, 3. All elements in the same vertical column (group) of the periodic table have the same number of valence electrons. Therefore, all elements in the same column as boron have 3 valence electrons. This commonality in valence electrons leads to them having similar chemical properties and typically exhibiting a valency of 3.
(b) Elements in the same column as Fluorine: Fluorine (F) is in Group 17 (halogens) of the periodic table. Its electronic configuration is 2, 7. All elements in the same vertical column (group) have the same number of valence electrons. Thus, all elements in the same column as fluorine have 7 valence electrons. This results in them having similar chemical properties and typically exhibiting a valency of 1 (as they need to gain one electron to achieve a stable octet).
Exercise 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.)
P(15)
Ar(18)
N(7)
F(9)
Solution:
(a) Atomic number: The electronic configuration of the atom is given as 2, 8, 7. The atomic number of an element is equal to the total number of electrons in a neutral atom. Summing the electrons in each shell: 2 (in the first shell) + 8 (in the second shell) + 7 (in the third shell) = 17 electrons. Therefore, the atomic number of this element is 17.
(b) Chemical similarity: Chemical properties of an element are primarily determined by the number of valence electrons. The given element has an electronic configuration of 2, 8, 7, meaning it has 7 valence electrons. We need to find an element from the options that also has 7 valence electrons.
- P (Phosphorus, atomic number 15): Electronic configuration is 2, 8, 5. It has 5 valence electrons.
- Ar (Argon, atomic number 18): Electronic configuration is 2, 8, 8. It has 8 valence electrons (a stable octet).
- N (Nitrogen, atomic number 7): Electronic configuration is 2, 5. It has 5 valence electrons.
- F (Fluorine, atomic number 9): Electronic configuration is 2, 7. It has 7 valence electrons.
Since Fluorine (F) has the same number of valence electrons (7) as the given element, it would be chemically similar to it. Both elements belong to Group 17 (halogens).
Common mistakes
- Confusing trends in metallic/non-metallic character across periods.
- Incorrectly relating electron loss/gain tendency to metallic character.
- Misinterpreting electronic configurations for predicting group/period.
- Assuming all elements in a group have identical chemical properties without considering nuances.
Revision tips
- Memorize the electronic configurations of the first 20 elements.
- Focus on understanding the reasons behind periodic trends (e.g., nuclear charge, atomic size).
- Practice relating electronic configuration to an element's position and properties.
- Review the relationship between an element's group and its valency.
- Connect the position of an element to the nature of its oxide (acidic/basic).
Practice MCQs
Q1. Which statement about trends across a period in the periodic table is incorrect?
Explanation: Across a period, non-metallic character increases, meaning atoms gain electrons more readily and lose them less easily.
Q2. Element X forms a chloride XCl2, a solid with a high melting point. X is likely in the same group as:
Explanation: The formula XCl2 suggests X has a valency of +2. Elements with a +2 valency are typically in Group 2, like Magnesium (Mg).
Q3. Which element has two shells, both completely filled?
Explanation: Neon has electronic configuration 2, 8. The first shell (K) has 2 electrons, and the second shell (L) has 8 electrons, filling both.
Q4. An element with electronic configuration 2, 8, 2 belongs to which group?
Explanation: The number of valence electrons (2) indicates the element belongs to Group 2 of the periodic table.
Q5. Elements in the same column as Boron have:
Explanation: Elements in the same group (column) share the same number of valence electrons, leading to similar chemical properties and valency.
Q6. An element with electronic configuration 2, 8, 7 is chemically similar to:
Explanation: Chemical similarity arises from having the same number of valence electrons. Both the element (2, 8, 7) and Fluorine (2, 7) have 7 valence electrons.
Frequently asked questions
What is the main principle behind the modern periodic table?
The modern periodic table is based on the atomic number of elements, where elements are arranged in increasing order of their atomic numbers.
How do metallic properties change across a period in the periodic table?
Across a period, from left to right, the metallic character of elements generally decreases because the tendency to lose electrons reduces and non-metallic character increases.
Why are elements in the same group chemically similar?
Elements in the same group have the same number of valence electrons, which determines their chemical behavior and bonding patterns, leading to similar chemical properties.
What is the significance of electronic configuration in the periodic table?
Electronic configuration determines an element's position (period and group) in the periodic table and dictates its chemical properties and reactivity.
How can I use these NCERT solutions for revision?
These solutions provide clear explanations and step-by-step answers to textbook questions, helping you revise concepts like periodic trends and element properties effectively.
What does the formula XCl2 suggest about element X?
The formula XCl2 indicates that element X forms a compound with chlorine where X has a valency of +2, suggesting it likely belongs to Group 2 of the periodic table.
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