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Q3.28:
The increasing order of reactivity among group 1 elements is $Li < Na < K < Rb < Cs$ whereas that among group 17 elements is $F > Cl > Br > I$. Explain.

Solution :

1. Theoretical Foundation: Group 1 (Alkali Metals)

Reactivity in alkali metals is governed by their ability to lose their outermost valence electron ($ns^1$) to form a unipositive ion ($M^+$). The ease of this process is measured by the Ionization Enthalpy ($\Delta_iH$).

  • Down the Group: As we move from Lithium ($Li$) to Cesium ($Cs$), the principal quantum number ($n$) increases, leading to an increase in atomic radius.
  • Shielding Effect: With each successive element, the number of inner shells increases, providing a stronger shielding effect on the valence electron.
  • Net Effect: The effective nuclear charge experienced by the valence electron decreases. Consequently, the force of attraction between the nucleus and the valence electron weakens significantly.

Therefore, $\Delta_iH$ decreases down the group ($Li > Na > K > Rb > Cs$). Since reactivity is inversely proportional to ionization energy, the ease of electron loss increases, resulting in: $Li < Na < K < Rb < Cs$. [Per the Periodic Trend of Ionization Enthalpy]

2. Theoretical Foundation: Group 17 (Halogens)

Reactivity in halogens is governed by their ability to gain an electron to achieve a stable noble gas configuration ($ns^2 np^5 + e^- \to ns^2 np^6$). This process is characterized by Electron Gain Enthalpy ($\Delta_{eg}H$) and the ability to act as an oxidizing agent.

  • Electronegativity and Size: Halogens are small in size with high effective nuclear charge, allowing them to attract shared electron pairs strongly.
  • Oxidizing Potential: Fluorine ($F$) is the strongest oxidizing agent. Although Chlorine ($Cl$) has a more negative electron gain enthalpy than Fluorine due to inter-electronic repulsions in the small $2p$ subshell of Fluorine, the reactivity of halogens in aqueous media is dominated by the Electrode Potential ($E^\circ$).
  • Hydration and Bond Dissociation: The standard reduction potential depends on:
    1. Enthalpy of dissociation of $X_2$ molecules.
    2. Electron gain enthalpy of $X$ atoms.
    3. Hydration enthalpy of $X^-$ ions.

Fluorine has a low bond dissociation energy (due to lone pair repulsion) and a very high hydration enthalpy for its fluoride ion, which compensates for other energy factors, making it the most reactive species. As atomic size increases down the group, the tendency to accept an electron decreases, resulting in: $F > Cl > Br > I$. [Per the Trends in Standard Reduction Potential]

3. Visual Representation of Atomic Radius and Reactivity Trends

Li Na K Cs Increasing Reactivity (Group 1) →

4. Comparative Summary

Feature Group 1 (Metals) Group 17 (Non-metals)
Governing Mechanism Loss of electrons (Oxidation) Gain of electrons (Reduction)
Key Periodic Factor Ionization Enthalpy ($\downarrow$) Electrode Potential ($E^\circ$) ($\uparrow$)
Reactivity Trend Increases down the group Decreases down the group

Final Solution: The increasing reactivity of Group 1 elements ($Li < Na < K < Rb < Cs$) is driven by the decreasing ionization enthalpy as atomic size increases, facilitating electron loss. The decreasing reactivity of Group 17 elements ($F > Cl > Br > I$) is driven by the decreasing ability to accept electrons and lower electrode potential as atomic size increases.


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