NCERT Solutions For Class 11 Chemistry Chapter 3 Classification Of Elements And Periodicity In Properties - 2026-27

NCERT Solutions for Class 11 Chemistry Chapter 3 – Classification of Elements and Periodicity in Properties are available here to support CBSE Class 11 students in mastering key concepts. These solutions are carefully structured as per the latest CBSE syllabus (2026–27) and provide clear, step-by-step answers to all in-text and exercise questions from the chapter.

Designed to simplify learning, the explanations help students build a strong conceptual foundation while making it easier to tackle complex questions with confidence. Each solution is presented in a student-friendly format, ensuring better understanding and improved answer-writing skills for exams.

At Infinity Learn, the focus is on making learning intuitive and effective. These solutions are crafted by experienced educators to guide students on how to approach problems strategically an essential skill for both board exams and competitive tests. Additionally, students can conveniently download the NCERT Solutions for class 11 chemistry chapter 3 PDF format for quick access and revision anytime.

NCERT Solutions Class 11 Chemistry Chapter 3 – Free PDF Download

Explore the fascinating concepts of Classification of Elements and Periodicity in Properties with NCERT Solutions for Class 11 Chemistry Chapter 3 PDF. This chapter takes you from Mendeleev’s groundbreaking periodic table to the modern periodic law, helping you understand how elements are organized and why their properties follow predictable trends.

With simple, student-friendly explanations, these solutions break down complex topics like atomic structure, electronegativity, periodic trends, and chemical reactivity into easy-to-understand concepts. Whether you're preparing for school exams or competitive tests, these solutions help you build clarity and confidence step by step.

At Infinity Learn, the focus is on making learning engaging and effective. These NCERT solutions for class 11 are designed to help you grasp concepts quickly, practice smarter, and improve your answer-writing skills. Download the NCERT Solutions for Class 11 Chemistry Chapter 3 PDF now and start mastering periodic trends with ease.

Access Class 11 Chemistry Chapter 3 Questions with Answers PDF

Q1. What is the basic theme of organisation in the periodic table?

Ans: The primary idea of the periodic table's organisation is to organise elements into periods and groups based on their qualities. This arrangement simplifies and organises the study of elements and their compounds. Elements with comparable characteristics are grouped together in the periodic table.

Q2. Which important property did Mendeleev use to classify the elements in his periodic table and did he stick to that?

Ans:
In his periodic chart, Mendeleev organized the elements by atomic weight or mass. In order of increasing atomic weight, he divided the elements into periods and groups. He grouped elements that have comparable qualities together.

He did not, however, keep this arrangement strictly. In certain cases, he neglected atomic weight order. For example, iodine was placed after tellurium despite having lower atomic mass because of similar properties.

Q3. What is the basic difference in approach between Mendeleev's Periodic Law and the Modern Periodic Law?

Ans: The physical and chemical properties of elements are periodic functions of their atomic weights (Mendeleev), whereas in modern law they are periodic functions of atomic number.

Q4. On the basis of quantum numbers, justify that the sixth period of the Periodic Table should have 32 elements.

Ans: For n = 6, subshells are 6s, 4f, 5d, 6p.

Orbitals = 1 + 7 + 5 + 3 = 16

Each orbital holds 2 electrons → total = 32 elements.

Q5. What does atomic radius and ionic radius really mean to you?

Ans:

• Atomic radius → size of atom
• Ionic radius → size of ion

Example:

• Metallic radius of Cu = 128 pm
• Covalent radius of Cl = 99 pm

Cations are smaller, anions are larger than parent atoms.

Also Check: NCERT Solutions for Class 11 Chemistry Chapter 1

Q6. How does atomic radius vary in a period and in a group?

Ans:

• Across period → decreases (due to increase in nuclear charge)
• Down group → increases (due to increase in shells)

Q7. Consider N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺

(a) What is common?

All are isoelectronic (10 electrons).

(b) Increasing ionic radii: Al³⁺ < Mg²⁺ < Na⁺ < F⁻ < O²⁻ < N³⁻

Q8. Calculate ionization enthalpy of hydrogen

Ans: 2.18 × 10⁻¹⁸ × 6.02 × 10²³

= 1.31 × 10⁶ J mol⁻¹

Q9. Ionization enthalpy values of Group 13 elements

Ans: Deviation occurs due to poor shielding by d and f electrons.

Q10. What are the major differences between metals and non-metals?

Q11. Ionization and electron gain enthalpy data

Answers:

• Least reactive → V
• Most reactive metal → II
• Most reactive non-metal → III
• Least reactive non-metal → V

12. Which of the following pairs of elements would have a more negative electron gain enthalpy?

(i) O or F (ii) F or Cl

Ans.

(i) O and F belong to the same period of the periodic table. Fluorine has one more proton and one more electron than oxygen. Since the additional electron is added to the same shell, fluorine has a smaller atomic size compared to oxygen. Due to its higher nuclear charge, fluorine attracts the incoming electron more strongly. Moreover, fluorine requires just one electron to complete its octet and attain a stable noble gas configuration. Therefore, the electron gain enthalpy of fluorine is more negative than that of oxygen.

(ii) F and Cl are in the same group of the periodic table. Generally, electron gain enthalpy becomes less negative down a group. However, chlorine has a more negative electron gain enthalpy than fluorine. This is because fluorine is very small in size, and the incoming electron is added to the second shell (n = 2), where electron-electron repulsion is relatively high. In chlorine, the electron is added to the third shell (n = 3), which is larger and experiences less repulsion. As a result, chlorine can accommodate the additional electron more easily, making its electron gain enthalpy more negative than that of fluorine.

Also Read: class 11th chemistry chapter 2 question answers NCERT

13. What is the basic difference between the terms electron gain enthalpy and electronegativity?

Ans.

14. How would you react to the statement that the electronegativity of N on the Pauling scale is 3.0 in all the nitrogen compounds?

Ans. The electronegativity of an element is not fixed; it varies depending on its hybridisation and oxidation state in a compound. In other words, electronegativity changes from one compound to another. For example, in nitrogen, the electronegativity increases with the change in hybridisation as: sp³–N < sp²–N < sp–N. Hence, the given statement is incorrect.

15. Would you expect the first ionization enthalpies for two isotopes of the same element to be the same or different? Justify your answer.

Ans. The ionization enthalpy of an atom depends on its nuclear charge (number of protons) and the number of electrons present. Since isotopes of an element have the same number of protons and electrons, their ionization enthalpy remains unchanged. Therefore, the first ionization enthalpy of two isotopes of the same element is the same.

16. Describe the theory associated with the radius of an atom as it

(a) gains an electron

(b) loses an electron

Ans. 

(a) When an atom gains an electron, its size increases. The addition of an extra electron raises electron–electron repulsion, while the number of protons remains unchanged. This reduces the effective nuclear charge experienced by the electrons, leading to an increase in atomic radius.

(b) When an atom loses an electron, its size decreases. The number of electrons reduces, which lowers electron–electron repulsion, while the nuclear charge remains the same. This increases the effective nuclear charge, pulling the electrons closer to the nucleus and decreasing the atomic radius.

17. Use the periodic table to answer the following questions.

(a) Identify an element with five electrons in the outer subshell.

(b) Identify an element that would tend to lose two electrons.

(c) Identify an element that would tend to gain two electrons.

(d) Identify the group having metal, non-metal, liquid as well as gaseous elements at the room temperature.

Ans. 

(a) An element with 5 electrons in its outermost subshell has the electronic configuration ns²np⁵. This corresponds to the halogen group, so the element could be F, Cl, Br, I, or At.

(b) An element with two valence electrons tends to lose both electrons to attain a stable noble gas configuration. Its general electronic configuration is ns², which represents group 2 elements such as Be, Mg, Ca, Sr, and Ba.

(c) An element that requires two additional electrons to achieve a stable noble gas configuration will have the electronic configuration ns²np⁴. These elements belong to the oxygen family (group 16).

(d) Group 17 elements include a variety of substances that exist as metals, non-metals, liquids, and gases at room temperature.

18. Write the general outer electronic configuration of s-, p-, d- and f- block elements.

Ans. The General outer electronic configuration of s-, p-, d- and f- block elements:

(i) s- Block elements:

ns^1-2 where n = 2 to 7

(ii) p- Block elements:

ns²np^1-6 where n = 2 to 6

(iii) d- Block elements:

(n-1)d^1-10ns^0-2 where n = 4 to 7

(iv) f- Block elements:

(n-2)f^0-14 (n-1)d^0-1 ns² where n = 6 to 7

19. In the modern periodic table, the period indicates the value of which of the following?

(a) atomic number

(b) atomic mass

(c) principal quantum number

(d) azimuthal quantum number.

Ans. In the modern periodic table, each period starts with the filling of a new electron shell. Therefore, the period number represents the value of the principal quantum number (n). Hence, option (c) is correct.

20. Which of the following statements related to the modern periodic table is incorrect?

(a) The p-block has 6 columns because a maximum of 6 electrons can occupy all the orbitals in a p-shell.

(b) The d-block has 8 columns because a maximum of 8 electrons can occupy all the orbitals in a d-subshell.

(c) Each block contains a number of columns equal to the number of electrons that can occupy that subshell.

(d) The block indicates the value of an azimuthal quantum number (l) for the last subshell that received electrons in building up the electronic configuration.

Ans. (b) The statement is incorrect. The d-block does not have 8 columns; it actually has 10 columns, as a d-subshell can accommodate a maximum of 10 electrons across its orbitals.

Chemistry Class 11 Chapter 3 Important Formulas and Concepts

Class 11 NCERT Solutions make it easier for students to understand key formulas and concepts in a simple and structured way. Below are the important formulas and concepts from Chapter 3 – Classification of Elements and Periodicity in Properties that will help strengthen your preparation for exams.

Mendeleev’s Periodic Table

Mendeleev arranged elements in order of increasing atomic mass and noticed a periodic repetition in their properties.

Modern Periodic Law

It states that the physical and chemical properties of elements are periodic functions of their atomic numbers.

Periodic Trends

• Atomic Radius: Decreases across a period and increases down a group.
• Ionization Energy: Increases across a period and decreases down a group.
• Electron Affinity: Generally increases across a period and decreases down a group.
• Electronegativity: Increases across a period and decreases down a group.

Effective Nuclear Charge (Zₑff)

Zₑff = Z − S

Where:

• Z = Atomic number
• S = Shielding constant

It represents the net positive charge experienced by an electron in an atom.

Periodic Classification of Elements

Elements are systematically arranged in:

• Periods (horizontal rows)
• Groups (vertical columns)

This classification is based on similarities in their physical and chemical properties, making it easier to study and compare elements.

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