Classification of Elements and Periodicity in Properties
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Quick Revision Notes & Formula Sheet
Class 11 Chemistry — Chapter 3: Classification of Elements and Periodicity in Properties
1. Important Historical Periodic Laws
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Mendeleev's Periodic Law (मेंडेलीव का आवर्त नियम):
"The physical and chemical properties of elements are periodic functions of their atomic masses."
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Modern Periodic Law (आधुनिक आवर्त नियम - Henry Moseley):
"The physical and chemical properties of elements are periodic functions of their atomic numbers ($Z$)."
2. Structure of the Modern Periodic Table
- Periods (आवर्त): 7 Horizontal rows.
- Period number = Principal quantum number ($n$) of the outermost shell.
- Groups (वर्ग): 18 Vertical columns.
Division of Elements into Blocks
| Block | Valence Shell Configuration | Group Numbers | Key Features |
|---|---|---|---|
| s-block | $ns^{1-2}$ | Group 1 & 2 | Reactive metals, low ionization enthalpy, form ionic compounds. |
| p-block | $ns^2 np^{1-6}$ | Group 13 to 18 | Includes metals, non-metals, and metalloids. (s + p = Representative elements). |
| d-block | $(n-1)d^{1-10} ns^{1-2}$ | Group 3 to 12 | Transition elements, variable oxidation states, form colored ions, paramagnetic. |
| f-block | $(n-2)f^{1-14} (n-1)d^{0-1} ns^2$ | Bottom 2 rows | Inner-transition elements (Lanthanoids: $Z=58-71$, Actinoids: $Z=90-103$). |
3. Nomenclature of Elements with Atomic Number $Z > 100$
| Digit | Root | Abbreviation | Digit | Root | Abbreviation |
|---|---|---|---|---|---|
| 0 | nil | n | 5 | pent | p |
| 1 | un | u | 6 | hex | h |
| 2 | bi | b | 7 | sept | s |
| 3 | tri | t | 8 | oct | o |
| 4 | quad | q | 9 | enn | e |
- Suffix added:
-ium - Example: $Z = 101 \rightarrow$ Un + nil + un + ium = Unnilunium (Symbol: Unu)
- Example: $Z = 120 \rightarrow$ Un + bi + nil + ium = Unbinilium (Symbol: Ubn)
4. Key Concepts & Formulas
Effective Nuclear Charge ($Z_{\text{eff}}$)
The net positive charge experienced by an electron in a multi-electron atom. $$\mathbf{Z_{\text{eff}} = Z - \sigma}$$
- $Z$ = Atomic Number (total nuclear charge)
- $\sigma$ (or $S$) = Shielding or Screening constant (परिरक्षण स्थिरांक)
- Shielding Power Order: $s > p > d > f$
5. Periodic Trends in Physical Properties
1. Atomic Radius (परमाणु त्रिज्या)
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Covalent Radius: Half the distance between two covalently bonded similar atoms.
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Metallic Radius: Half the internuclear distance between two adjacent metal ions in a metallic lattice.
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Van der Waals Radius: Half the distance between nuclei of two non-bonded isolated atoms of adjacent molecules. $$\text{Size Order: } \text{Van der Waals Radius} > \text{Metallic Radius} > \text{Covalent Radius}$$
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Trends:
- Across a Period (Left to Right): Decreases (due to increase in $Z_{\text{eff}}$).
- Down a Group (Top to Bottom): Increases (due to addition of new energy shells).
2. Ionic Radius (आयनिक त्रिज्या)
- Cation (धनायन): Always smaller than its parent neutral atom (e.g., $Na^+ < Na$).
- Anion (ऋणायन): Always larger than its parent neutral atom (e.g., $Cl^- > Cl$).
- Isoelectronic Species (समइलेक्ट्रॉनिक स्पीशीज): Species having the same number of electrons.
- Rule: For isoelectronic species, as positive nuclear charge increases, size decreases. $$\text{Size Order: } O^{2-} > F^- > Na^+ > Mg^{2+} > Al^{3+}$$
3. Ionization Enthalpy ($\Delta_i H$) (आयनन एन्थैल्पी)
Minimum energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state. $$M(g) + \Delta_i H \longrightarrow M^+(g) + e^-$$
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Successive Ionization Enthalpies: $\Delta_i H_1 < \Delta_i H_2 < \Delta_i H_3$
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Factors Affecting $\Delta_i H$:
- $\Delta_i H \propto Z_{\text{eff}}$
- $\Delta_i H \propto \frac{1}{\text{Atomic Radius}}$
- $\Delta_i H \propto \text{Penetration power of orbitals } (s > p > d > f)$
- Stable electronic configurations (half-filled and fully-filled orbitals) have exceptionally high $\Delta_i H$.
-
Important Anomalies:
- $\Delta_i H (\text{Be}) > \Delta_i H (\text{B})$ — Be has stable fully-filled $2s^2$ configuration.
- $\Delta_i H (\text{N}) > \Delta_i H (\text{O})$ — N has stable half-filled $2p^3$ configuration.
4. Electron Gain Enthalpy ($\Delta_{eg} H$) (इलेक्ट्रॉन लब्धि एन्थैल्पी)
Enthalpy change when an electron is added to a neutral isolated gaseous atom. $$X(g) + e^- \longrightarrow X^-(g)$$
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Trends:
- Across a Period: Becomes more negative (increases).
- Down a Group: Becomes less negative (decreases).
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Halogens have the most negative electron gain enthalpies in their respective periods.
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Noble gases have large positive $\Delta_{eg} H$ due to stable octet.
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Important Anomalies:
- $\Delta_{eg} H (\text{Cl}) > \Delta_{eg} H (\text{F})$ — Fluorine's small size leads to strong inter-electronic repulsions in the $2p$ subshell.
- $\Delta_{eg} H (\text{S}) > \Delta_{eg} H (\text{O})$ — Same reasoning ($2p$ vs $3p$ orbital size).
5. Electronegativity (ऋणविद्युत्ता)
The qualitative measure of the ability of an atom in a chemical compound to attract shared electrons to itself.
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Trends:
- Across a Period: Increases.
- Down a Group: Decreases.
- Most Electronegative Element: Fluorine ($F = 4.0$ on Pauling Scale).
- Least Electronegative / Most Electropositive: Cesium ($Cs = 0.7$).
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Mulliken's Electronegativity Scale: $$\text{Electronegativity (Pauling)} \approx \frac{IE + EA}{560} \quad \text{(when } IE \text{ and } EA \text{ are in kJ/mol)}$$ $$\text{Electronegativity (Pauling)} \approx \frac{IE + EA}{5.6} \quad \text{(when } IE \text{ and } EA \text{ are in eV)}$$
6. Periodic Trends in Chemical Properties
1. Valence / Oxidation States
- For Representative Elements:
- Valency = Number of valence electrons (for Group 1 to 4)
- Valency = $8 - \text{Number of valence electrons}$ (for Group 5 to 8)
2. Diagonal Relationship (विकर्ण संबंध)
Elements of the 2nd period show similar properties to the elements of the 3rd period situated diagonally opposite to them.
- Pairs showing diagonal relationship:
- Lithium ($\text{Li}$) & Magnesium ($\text{Mg}$)
- Beryllium ($\text{Be}$) & Aluminium ($\text{Al}$)
- Boron ($\text{B}$) & Silicon ($\text{Si}$)
- Cause: Similar ionic radii and similar charge-to-radius ratio ($\frac{\text{charge}}{\text{radius}}$).
3. Periodic Trends in Oxide Nature
$$\text{Basic Oxides} \longrightarrow \text{Amphoteric Oxides} \longrightarrow \text{Acidic Oxides}$$
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Across a Period: Metallic character decreases $\rightarrow$ Oxides change from Basic to Acidic.
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Down a Group: Metallic character increases $\rightarrow$ Oxides become more Basic.
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Examples (Period 3 Oxides):
- $\text{Na}_2\text{O}, \text{MgO}$ : Basic (क्षारीय)
- $\text{Al}_2\text{O}_3$ : Amphoteric (उभयधर्मी)
- $\text{SiO}_2, \text{P}4\text{O}{10}, \text{SO}_3, \text{Cl}_2\text{O}_7$ : Acidic (अम्लीय)
7. Master Summary Table of Periodic Trends
| Property | Across a Period (Left $\rightarrow$ Right) | Down a Group (Top $\rightarrow$ Bottom) |
|---|---|---|
| Atomic / Ionic Size | Decreases ($\downarrow$) | Increases ($\uparrow$) |
| Effective Nuclear Charge ($Z_{\text{eff}}$) | Increases ($\uparrow$) | Decreases ($\downarrow$) |
| Ionization Enthalpy ($\Delta_i H$) | Generally Increases ($\uparrow$) | Decreases ($\downarrow$) |
| Electron Gain Enthalpy ($\Delta_{eg} H$) | Becomes more negative ($\uparrow$) | Becomes less negative ($\downarrow$) |
| Electronegativity | Increases ($\uparrow$) | Decreases ($\downarrow$) |
| Metallic Character | Decreases ($\downarrow$) | Increases ($\uparrow$) |
| Non-metallic Character | Increases ($\uparrow$) | Decreases ($\downarrow$) |
| Basic Nature of Oxides | Decreases ($\downarrow$) | Increases ($\uparrow$) |
| Acidic Nature of Oxides | Increases ($\uparrow$) | Decreases ($\downarrow$) |