Anand Classes brings you detailed NCERT Solutions for Class 11 Chemistry Chapter 3 – Classification of Elements and Periodicity in Properties covering Questions 3.21, 3.22, 3.23, 3.24, and 3.25. These step-by-step solutions explain important concepts like electron gain enthalpy, electronegativity, atomic radius, and ionization enthalpy of isotopes. Our solutions are designed to help Class 11 students build strong conceptual clarity and prepare effectively for exams. Click the print button to download pdf study material and notes.
NCERT 3.21 : Would you expect the second electron gain enthalpy of O to be positive, more negative, or less negative than the first? Justify your answer.
Answer :
The second electron gain enthalpy of O is positive.
Explanation:
- First electron gain enthalpy ($\Delta_{eg}H_1$):
- When an electron is added to an oxygen atom to form the $O^-$ ion, energy is released.
- This makes the first electron gain enthalpy negative.
$$
O (g) + e^- (g) \rightarrow O^- (g) \quad ; \quad \Delta_{eg}H_1 = -141\ \text{kJ mol}^{-1}
$$
- Second electron gain enthalpy ($\Delta_{eg}H_2$):
- When a second electron is added to $O^-$ to form $O^{2-}$, energy must be absorbed to overcome the strong electrostatic repulsion between the negatively charged $O^-$ ion and the incoming electron.
- Therefore, the second electron gain enthalpy is positive.
$$
O^- (g) + e^- (g) \rightarrow O^{2-} (g) \quad ; \quad \Delta_{eg}H_2 = +78\ \text{kJ mol}^{-1}
$$
Conclusion:
- First electron gain enthalpy is negative (energy released).
- Second electron gain enthalpy is positive (energy absorbed due to repulsion).
NCERT 3.22 : What is the basic difference between the terms electron gain enthalpy and electronegativity?
Both electron gain enthalpy and electronegativity describe the tendency of an atom to attract electrons, but they are different concepts:
Electron Gain Enthalpy ($\Delta_{eg}H$):
- It refers to the tendency of an isolated gaseous atom to accept an additional electron.
- The process leads to the formation of a negative ion.
- It is expressed in terms of energy change (kJ mol$^{-1}$).
Electronegativity (EN):
- It refers to the tendency of an atom of an element to attract the shared pair of electrons towards itself in a covalent bond.
- It is a relative number (dimensionless, no units).
Comparison Table: Electron Gain Enthalpy vs Electronegativity
| Aspect | Electron Gain Enthalpy ($\Delta_{eg}H$) | Electronegativity (EN) |
|---|---|---|
| Definition | Tendency of an isolated gaseous atom to accept an additional electron. | Tendency of an atom in a covalent bond to attract the shared pair of electrons. |
| Nature | Measured as an energy change when an electron is added. | A relative scale (dimensionless number). |
| Units | Expressed in kJ/mol | No units (relative number). |
| Condition | Applicable to isolated atoms in gaseous state. | Applicable to bonded atoms in a molecule. |
| Positive/Negative | Can be negative (energy released) or positive (energy absorbed). | Always a relative value; no positive or negative sign. |
| Example | For oxygen: $O(g) + e^- \rightarrow O^-(g); \, \Delta_{eg}H = -141 \, \text{kJ mol}^{-1}$ | On Pauling scale: EN of F = 4.0, O = 3.5, etc. |
Key Difference:
- Electron Gain Enthalpy → deals with an isolated atom and energy change.
- Electronegativity → deals with an atom in a bonded state and has no units.
NCERT 3.23 : How would you react to the statement that the electronegativity of N on Pauling scale is 3.0 in all the nitrogen compounds?
Answer :
The electronegativity of any given atom is not constant. Therefore, the statement that the electronegativity of nitrogen on Pauling scale is 3.0 in all nitrogen compounds is incorrect.
Reasons for Variation in Electronegativity:
Effect of Hybridization
- The electronegativity increases as the percentage of $s$-character of a hybrid orbital increases.
- Order:
$$ sp^3 < sp^2 < sp $$ - Thus, nitrogen in an $sp$ hybrid orbital has a higher electronegativity than in $sp^2$, and in turn higher than in $sp^3$.
Effect of Oxidation State
- The electronegativity of nitrogen also increases with increasing oxidation state.
- Example:
- In NO (oxidation state of N = +2), electronegativity is lower.
- In NO$_2$ (oxidation state of N = +4), electronegativity is higher.
Conclusion:
Electronegativity of nitrogen is variable, depending on its state of hybridization and oxidation state, and is not fixed at 3.0 in all its compounds.
NCERT 3.24 : Describe the theory associated with the radius of an atom as it
(a) gains an electron
(b) loses an electron.
Answer :
(a) When an Atom Gains an Electron (Formation of Anion)
- When a neutral atom gains an electron to form an anion, its radius increases.
- Reason: The nuclear charge remains the same but the number of electrons increases.
- The same positive charge now attracts a greater number of electrons, so the effective nuclear charge per electron decreases.
- As a result, the force of attraction of the nucleus on each electron decreases, and the electron cloud expands.
- Hence, the ionic radius of the anion is larger than that of the parent atom.
Example:
$$ \text{Cl} + e^- \;\;\longrightarrow \;\; \text{Cl}^- $$
- Chlorine atom: 17 protons, 17 electrons, radius = 99 pm
- Chloride ion: 17 protons, 18 electrons, radius = 181 pm
(b) When an Atom Loses an Electron (Formation of Cation)
- When a neutral atom loses an electron to form a cation, its radius decreases.
- Reason: The nuclear charge remains the same but the number of electrons decreases.
- The same positive charge is now shared by fewer electrons, so the effective nuclear charge per electron increases.
- As a result, the nucleus pulls the remaining electrons closer, and the size of the cation decreases.
Example:
$$ \text{Na} \;\;\longrightarrow \;\; \text{Na}^+ + e^- $$
- Sodium atom: 11 protons, 11 electrons, radius = 156 pm
- Sodium ion: 11 protons, 10 electrons, radius = 95 pm
NCERT 3.25 : Would you expect the first ionization enthalpies for two isotopes of the same element to be the same or different? Justify your answer.
Answer :
The first ionization enthalpies of two isotopes of the same element are expected to be the same.
Ionization enthalpy depends mainly on:
- Nuclear charge (Z)
- Electronic configuration
- Atomic size (determined by number of protons and electron shells)
- Since isotopes of the same element have the same atomic number (Z) and hence the same electronic configuration, their ionization enthalpies remain almost identical.
- The only difference between isotopes lies in their number of neutrons and hence their atomic mass, which has negligible effect on ionization enthalpy.
Therefore, the first ionization enthalpies of two isotopes of the same element are practically the same.
📚 Buy Study Material & Join Our Coaching
For premium study materials specially designed for JEE, NEET, NDA, and CBSE/ICSE Classes, visit our official study material portal:
👉 https://publishers.anandclasses.co.in/
For NDA Study Material, Click Here
For SSC Study Material, Click Here
To enroll in our offline or online coaching programs, visit our coaching center website:
👉 https://anandclasses.co.in/
📞 Call us directly at: +91-94631-38669
💬 WhatsApp Us Instantly
Need quick assistance or want to inquire about classes and materials?
📲 Click below to chat instantly on WhatsApp:
👉 Chat on WhatsApp
🎥 Watch Video Lectures
Get access to high-quality video lessons, concept explainers, and revision tips by subscribing to our official YouTube channel:
👉 Neeraj Anand Classes – YouTube Channel