Group 17 Elements: The Halogens
Group 17 consists of Fluorine ($F$), Chlorine ($Cl$), Bromine ($Br$), Iodine ($I$), and Astatine ($At$). These elements are collectively known as Halogens (Greek: halos = salt, genes = born), meaning "salt producers." They are highly reactive non-metals with the general electronic configuration $ns^2 np^5$.
1. Atomic and Physical Properties
A. Electron Gain Enthalpy ($\Delta_{eg}H$)
Halogens have the maximum negative electron gain enthalpy in their respective periods.
Important Anomaly
Electron gain enthalpy of Fluorine is less negative than Chlorine.
Reason: Small size of the 2p orbital in Fluorine leads to strong inter-electronic repulsion, making it harder to add an incoming electron.
Order: $Cl > F > Br > I$
B. Electronegativity
Fluorine is the most electronegative element in the periodic table. Electronegativity decreases down the group.
C. Bond Dissociation Enthalpy
Expected trend is a decrease down the group. However, there is an anomaly.
F-F Bond Weakness
The F-F bond is weaker than the Cl-Cl bond due to high lone pair-lone pair repulsion in the small F$_2$ molecule.
Order: $Cl_2 > Br_2 > F_2 > I_2$
2. Chemical Properties
A. Oxidizing Power
Halogens are strong oxidizing agents. Oxidizing power decreases down the group.
Fluorine ($F_2$) is the strongest oxidizing agent and oxidizes other halide ions from solution ($X^- \to X_2$). Even solid iodides.
$$ F_2 + 2X^- \rightarrow 2F^- + X_2 \quad (X = Cl, Br, I) $$B. Anomalous Behavior of Fluorine
Fluorine differs from other halogens due to:
- Very small size.
- Highest electronegativity.
- Low F-F bond dissociation enthalpy.
- Absence of d-orbitals in valence shell.
Consequence: Fluorine shows only an oxidation state of -1. Other halogens show +1, +3, +5, +7 states.
C. Reaction with Water
- $F_2$: Oxidizes water to Oxygen. ($2F_2 + 2H_2O \rightarrow 4H^+ + 4F^- + O_2$)
- $Cl_2, Br_2$: React to form hypohalous acids ($X_2 + H_2O \rightarrow HX + HOX$).
- $I_2$: Reaction is non-spontaneous. In fact, $I^-$ can be oxidized by $O_2$ in acidic medium.
3. Hydrogen Halides (HX)
Acidic Strength
Acid strength increases down the group due to a decrease in H-X bond dissociation enthalpy.
Note: $HF$ is a liquid due to strong intermolecular Hydrogen Bonding, while others are gases.
4. Interhalogen Compounds
Halogens react with each other to form Interhalogen compounds ($XX'$, $XX'_3$, $XX'_5$, $XX'_7$).
Example: $ClF, BrF_3, IF_5, IF_7$.
Reactivity
Interhalogen compounds are more reactive than halogens (except $F_2$) because the $X-X'$ bond is weaker than the $X-X$ bond due to polarity differences.
Structure of $IF_7$: Pentagonal Bipyramidal ($sp^3d^3$).
5. Chlorine ($Cl_2$) & Its Compounds
- Preparation: Deacon’s process (oxidation of HCl by $O_2$ with $CuCl_2$ catalyst).
- Bleaching Action: Chlorine bleaches by oxidation in the presence of moisture.
$$ Cl_2 + H_2O \rightarrow 2HCl + [O] $$ The nascent oxygen bleaches colored organic matter. This is permanent bleaching.
Group 17 Quiz
Test your concepts on Halogens. 10 MCQs with explanations.
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