CBSE Class 10 Science Chapter 5: Periodic Classification of Elements NCERT Solutions

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This chapter delves into the Periodic Classification of Elements, a fundamental concept in Class 10 Science. The NCERT Solutions provided here offer detailed explanations and step-by-step answers to the exercises. Students will explore the historical development of the periodic table, understand the basis of modern classification, and analyze trends in properties like metallic character, valence electrons, and acidity of oxides across periods and down groups. The solutions cover identifying elements based on electronic configuration, predicting chemical similarity, and understanding the common properties within a group. These solutions are designed to clarify complex concepts, reinforce learning, and aid students in their exam preparation by providing accurate and easy-to-understand explanations for each question.

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BoardCBSE
ClassClass 10
SubjectScience
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 5

Chapter summary

Chapter 5, Periodic Classification of Elements, focuses on organizing the elements based on their properties. These NCERT Solutions cover key concepts such as trends in the periodic table (across periods and down groups), electronic configurations, valency, and the nature of oxides. The exercises help students apply these principles to identify elements, predict their behavior, and understand the logic behind the periodic arrangement.

Learning outcomes

  • Understand the trends in properties of elements across periods and down groups.
  • Determine the position of an element in the periodic table based on its electronic configuration.
  • Predict the chemical similarity between elements.
  • Identify elements based on their electronic configuration and number of shells.
  • Explain the relationship between electronic configuration and valency.
  • Analyze the nature of oxides formed by elements.

Topics covered

Paper topics

  • Periodic Classification of Elements
  • Trends in Periodic Table
  • Metallic and Non-metallic Character
  • Valence Electrons
  • Electronic Configuration
  • Valency
  • Nature of Oxides
  • Groups and Periods
  • Atomic Number
  • Chemical Similarity

Important topics

  • Trends in properties across periods and down groups
  • Electronic configuration and its relation to position
  • Valency and its determination
  • Chemical similarity based on valence electrons
  • Nature of oxides

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Questions and Solutions

Question 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. As we move from left to right across a period, the nuclear charge increases, and the atomic radius generally decreases. This makes it harder for atoms to lose electrons. Instead, the non-metallic character increases, and the tendency to gain electrons or share them increases. Therefore, atoms do not lose their electrons more easily; they tend to hold onto them more tightly or even gain them.

Question 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 formula XCl<sub>2</sub> indicates that element X has a valency of 2. Elements with a valency of 2 typically belong to Group 2 of the periodic table, which are the alkaline earth metals. Among the given options, Magnesium (Mg) is in Group 2 and forms chlorides with the formula MgCl<sub>2</sub>. Sodium (Na) is in Group 1 (valency 1), Aluminum (Al) is in Group 13 (valency 3), and Silicon (Si) is in Group 14 (valency 4). Therefore, X would most likely be in the same group as Magnesium (Mg).

Question 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:

(a) An element with two shells, both completely filled, has 2 electrons in the first shell (K) and 8 electrons in the second shell (L). This corresponds to Neon (Ne), which has an atomic number of 10 and the electronic configuration 2, 8.

(b) An electronic configuration of 2, 8, 2 means the element has 2 electrons in the first shell, 8 in the second, and 2 in the outermost (valence) shell. This configuration belongs to Magnesium (Mg), which has an atomic number of 12.

(c) An element with three shells and four electrons in its valence shell has the electronic configuration 2, 8, 4. This corresponds to Silicon (Si), which has an atomic number of 14.

(d) An element with two shells and three electrons in its valence shell has the electronic configuration 2, 3. This corresponds to Boron (B), which has an atomic number of 5.

(e) The first shell can hold a maximum of 2 electrons, and the second shell can hold a maximum of 8 electrons. If an element has twice as many electrons in its second shell as in its first shell, it has 2 electrons in the first shell and 4 electrons in the second shell (2 * 2 = 4). The electronic configuration is 2, 4. This corresponds to Carbon (C), which has an atomic number of 6.

Question 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) All elements in the same column (group) of the Periodic Table as Boron (B) have the same number of valence electrons. Boron is in Group 13 and has 3 valence electrons. Therefore, all elements in Boron's group have 3 valence electrons, which typically leads to a valency of 3.

(b) All elements in the same column (group) of the Periodic Table as Fluorine (F) have the same number of valence electrons. Fluorine is in Group 17 and has 7 valence electrons. Therefore, all elements in Fluorine's group have 7 valence electrons, which typically leads to a valency of 1 (as they tend to gain one electron to achieve a stable octet).

Question 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) The electronic configuration 2, 8, 7 indicates that the atom has 2 electrons in the first shell, 8 electrons in the second shell, and 7 electrons in the third (valence) shell. The total number of electrons is the sum of electrons in all shells: 2 + 8 + 7 = 17. For a neutral atom, the atomic number is equal to the total number of electrons. Therefore, the atomic number of this element is 17.

(b) Chemical similarity between elements is primarily determined by the number of valence electrons. The given element has 7 valence electrons (configuration 2, 8, 7). We need to find an element from the options that also has 7 valence electrons. Let's look at the configurations of the given elements:

  • Phosphorus (P) with atomic number 15 has the configuration 2, 8, 5 (5 valence electrons).
  • Argon (Ar) with atomic number 18 has the configuration 2, 8, 8 (8 valence electrons, a noble gas).
  • Nitrogen (N) with atomic number 7 has the configuration 2, 5 (5 valence electrons).
  • Fluorine (F) with atomic number 9 has the configuration 2, 7 (7 valence electrons).

Since Fluorine (F) has 7 valence electrons, just like the element with configuration 2, 8, 7, it would be chemically similar to Fluorine.

Common mistakes

  • Confusing trends across periods with trends down groups.
  • Incorrectly calculating the number of valence electrons.
  • Misinterpreting electronic configurations leading to wrong element identification.
  • Assuming similarity based on atomic number alone, without considering electronic configuration.

Revision tips

  • Memorize the electronic configurations of the first 20 elements.
  • Draw the periodic table and mark the trends for metallic character, atomic size, and valency.
  • Practice identifying elements based on given electronic configurations and vice versa.
  • Focus on understanding the reasons behind the trends, not just memorizing them.

Practice MCQs

Q1. Which trend is NOT correct when moving from left to right across a period?

Q2. An element forms a chloride XCl2 with a high melting point. It is likely in the same group as:

Q3. Which element has two shells, both completely filled?

Q4. An element with electronic configuration 2, 8, 2 belongs to which group?

Q5. Elements in the same column as Fluorine (F) have what in common?

Frequently asked questions

What is the main focus of Chapter 5, Periodic Classification of Elements for Class 10 Science?

Chapter 5 focuses on how elements are organized in the periodic table based on their properties, exploring trends in metallic character, electron gain, atomic size, and the nature of oxides across periods and down groups.

How do these NCERT Solutions help in understanding periodic trends?

The solutions provide clear explanations and step-by-step answers to questions about trends like metallic nature, valence electrons, and oxide acidity, helping students grasp the concepts accurately.

Can I identify an element using its electronic configuration with these solutions?

Yes, the solutions demonstrate how to determine an element's atomic number, its position in the periodic table, and its chemical similarity to other elements based on its electronic configuration.

What is valency, and how is it explained in these solutions?

Valency is explained as the combining capacity of an element, directly related to its valence electrons. The solutions show how to determine valency for elements in different groups and periods.

Are the solutions for all exercises in Chapter 5 provided?

Yes, these NCERT Solutions cover all the questions from the exercises of Chapter 5, Periodic Classification of Elements, providing comprehensive coverage for your studies.

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