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अध्याय 3: तत्वों का वर्गीकरण एवं गुणधर्मों में आवर्तिता

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CBSE • कक्षा XI • Chemistry • अध्याय 3

अनुमानित समय: 45 Mins

प्रगति: अध्ययनरत

तत्वों का वर्गीकरण एवं गुणधर्मों में आवर्तिता

In Class 11 Chemistry, "Classification of Elements and Periodicity in Properties" provides an authoritative, curriculum-verified master resource aligned with the 2026–27 NCERT syllabus.

अध्याय CBT ऑनलाइन टेस्ट दें सभी अध्याय देखें

प्रस्तावना अध्याय अवधारणाएँ अध्याय सारांश स्व-मूल्यांकन अभ्यास टेस्ट दें

🗺️ Have You Ever Wondered?

How did Dmitri Mendeleev predict the existence, melting point, and atomic weight of undiscovered elements like Germanium decades before they were foun...

How did Dmitri Mendeleev predict the existence, melting point, and atomic weight of undiscovered elements like Germanium decades before they were found by leaving deliberate blank spaces in a grid? The Modern Periodic Table maps electronic orbital filling.

यह अध्याय क्यों महत्वपूर्ण है

In Class 11 Chemistry, "Classification of Elements and Periodicity in Properties" provides an authoritative, curriculum-verified master resource aligned with the 2026–27 NCERT syllabus.

अध्ययन से पूर्व (आवश्यक ज्ञान)

Mendeleev's periodic law from Class 10.
Modern periodic law.
Valency.

इस अध्याय के लक्ष्य

State Modern Periodic Law: Physical and chemical properties are periodic functions of atomic numbers.
Classify elements into $s, p, d, f$ blocks based on valence subshell electronic configuration.
Analyze periodic trends: Atomic and Ionic radii (lanthanoid contraction, screening effect).
Analyze Ionization Enthalpy ($\Delta_i H$) across periods and down groups, including anomalies (Be > B, N > O).
Evaluate Electron Gain Enthalpy ($\Delta_{eg} H$) and Electronegativity trends (Pauling scale).

अध्याय रूपरेखा एवं प्रगति

1 1. Modern Periodic Law & Blocks

2 2. Atomic Radii & Ionization Enthal...

3 3. Electron Gain Enthalpy & Electro...

सम्पूर्ण सैद्धांतिक एवं वैचारिक अध्ययन

1. Modern Periodic Law & Blocks

Moseley showed that Atomic Number ($Z$) is more fundamental than atomic mass: 'The physical and chemical properties of elements are periodic functions of their atomic numbers.' The periodic table contains 7 periods and 18 groups:
• s-block: Alkali & alkaline earth metals ($ns^1, ns^2$).
• p-block: Groups 13 to 18 ($ns^2 np^{1-6}$).
• d-block: Transition metals ($(n-1)d^{1-10} ns^{1-2}$).
• f-block: Lanthanoids & Actinoids ($(n-2)f^{1-14} (n-1)d^{0-1} ns^2$).

2. Atomic Radii & Ionization Enthalpy

Atomic Radius: Decreases across a period (due to increasing effective nuclear charge $Z_{\text{eff}}$); increases down a group (due to addition of new electron shells).
Ionization Enthalpy ($\Delta_i H$): Energy required to remove the most loosely bound electron. Increases across a period, decreases down a group.
Anomalies: $\text{Be} > \text{B}$ (stable full $2s^2$ vs $2p^1$); $\text{N} > \text{O}$ (stable half-filled $2p^3$ vs $2p^4$!).

3. Electron Gain Enthalpy & Electronegativity

Electron Gain Enthalpy: Chlorine has a more negative $\Delta_{eg} H$ than Fluorine ($-349\text{ kJ/mol}$ vs $-328\text{ kJ/mol}$) because adding an electron to compact $2p$ shell of F encounters strong inter-electronic repulsions!
Electronegativity: Qualitative tendency to pull shared electrons (Fluorine $= 4.0$ on Pauling scale).

चित्रात्मक व्याख्या एवं मॉडल

Classification of Elements and Periodicity in Properties Master Matrix

Conceptual framework, core mechanisms, and analytical relationships

Academic Architecture

1. Modern Periodic Law & Blocks • 2. Atomic Radii & Ionization Enthalpy

अध्याय का सार संक्षेप एवं 10 मुख्य निष्कर्ष

मुख्य बिंदु 1

Modern Periodic Law: Periodicity based strictly on increasing atomic numbers.

मुख्य बिंदु 2

Four Blocks ($s, p, d, f$): Direct mapping of the outer valence subshell orbital.

मुख्य बिंदु 3

Effective Nuclear Charge ($Z_{\text{eff}}$): Net positive nuclear pull experienced after inner electron shielding.

मुख्य बिंदु 4

Half-Filled Stability Anomaly: Nitrogen ionization enthalpy exceeding oxygen due to stable $2p^3$ configuration.

मुख्य बिंदु 5

Chlorine vs Fluorine Anomaly: Cl electron affinity exceeds F due to lower electron-electron crowding in $3p$.

स्व-मूल्यांकन अभ्यास (Check Your Understanding)

मूल वैचारिक स्पष्टता की जांच के लिए नैदानिक प्रश्न। पहले स्वयं हल करें, फिर उत्तर देखें।

Why is the first ionization enthalpy of Nitrogen higher than that of Oxygen?

उत्तर एवं व्याख्या देखें

उत्तर: Because Nitrogen has a stable half-filled $2p^3$ electronic configuration ($1s^2 2s^2 2p_x^1 2p_y^1 2p_z^1$), which possesses exceptional exchange stability; Oxygen ($2p^4$) has paired electrons that experience inter-electronic repulsion, making electron removal easier.
Nitrogen has a stable half-filled 2p3 configuration.

Why does Chlorine have a higher negative electron gain enthalpy than Fluorine?

उत्तर एवं व्याख्या देखें

उत्तर: Fluorine has an extremely small and compact $2p$ subshell where incoming electrons experience strong inter-electronic repulsions; Chlorine has a larger $3p$ subshell where the added electron is accommodated with much less repulsion.
F has strong inter-electronic repulsion in its tiny 2p shell.

What is the screening effect (shielding effect)? How does it affect atomic radius?

उत्तर एवं व्याख्या देखें

उत्तर: The phenomenon where inner shell electrons shield the outer valence electrons from the full electrostatic attraction of the positive nucleus. Greater screening reduces effective nuclear charge ($Z_{\text{eff}}$), increasing the atomic radius.
Inner electrons shield valence electrons, lowering Z_eff.

Define Electronegativity. How does it differ from Electron Gain Enthalpy?

उत्तर एवं व्याख्या देखें

उत्तर: Electronegativity is the relative tendency of an atom in a chemical molecule to attract the shared pair of bonding electrons towards itself (qualitative property of bonded atom); Electron Gain Enthalpy is the thermodynamic enthalpy change when an electron is added to an isolated gaseous atom.
Tendency to pull shared electrons vs thermodynamic energy change on electron addition.

What is the basic difference between the Mendeleev periodic law and the Modern periodic law?

उत्तर एवं व्याख्या देखें

उत्तर: Mendeleev periodic law arranged elements according to their atomic masses, whereas the Modern periodic law arranges elements according to their atomic numbers.
Atomic mass vs atomic number.

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