Group 17 Elements — Revision Notes

⚡ 30-Second Revision

Electronic Configuration: — $ns^2np^5$
Reactivity: — Decreases F > Cl > Br > I
Oxidizing Power: — Decreases $F_2 > Cl_2 > Br_2 > I_2$
Atomic/Ionic Radii: — Increases down group
Ionization Enthalpy: — Decreases down group
Electronegativity: — Decreases down group (F is highest)
Electron Gain Enthalpy: — $Cl > F > Br > I$ (Cl is most negative)
Bond Dissociation Enthalpy: — $Cl_2 > Br_2 > F_2 > I_2$ (Anomaly for $F_2$)
Acidic Strength of HX: — $HF < HCl < HBr < HI$ (HF is weak due to H-bonding)
Oxoacids Acidic Strength: — Increases with oxidation state (e.g., $HClO < HClO_2 < HClO_3 < HClO_4$)
Interhalogens: — $XX'$, $XX_3'$, $XX_5'$, $XX_7'$ (e.g., $ClF_3$ T-shaped, $IF_5$ square pyramidal)
Deacon's Process: — $4HCl + O_2 xrightarrow{CuCl_2} 2Cl_2 + 2H_2O$
Fluorine + Water: — $2F_2 + 2H_2O \rightarrow 4HF + O_2$

2-Minute Revision

Group 17 elements, the halogens (F, Cl, Br, I), are highly reactive non-metals due to their $ns^2np^5$ electronic configuration, making them strong oxidizing agents. Key trends include increasing atomic size and decreasing ionization enthalpy and electronegativity down the group.

However, fluorine exhibits significant anomalies: its electron gain enthalpy is less negative than chlorine's (due to small size and electron repulsion), and the $F-F$ bond dissociation enthalpy is lower than $Cl-Cl$ and $Br-Br$ bonds (due to lone pair repulsion).

The oxidizing power decreases from $F_2$ to $I_2$. Hydrogen halides (HX) show increasing acidic strength from HF to HI, with HF being a weak acid due to strong hydrogen bonding. Halogens form various oxoacids, where acidic strength increases with the oxidation state of the halogen.

Interhalogen compounds ($XX'$, $XX_3'$, $XX_5'$, $XX_7'$) are formed between different halogens and are generally more reactive than parent halogens (except $F_2$). Important reactions include the preparation of chlorine by Deacon's process and the unique reaction of fluorine with water to produce oxygen.

5-Minute Revision

The halogens (F, Cl, Br, I, At) are Group 17 elements, characterized by their $ns^2np^5$ valence electron configuration, which drives their high reactivity and strong tendency to gain one electron. This makes them potent oxidizing agents, with oxidizing power decreasing down the group ($F_2 > Cl_2 > Br_2 > I_2$).

Key Periodic Trends & Anomalies:

Atomic/Ionic Radii: — Increase down the group.
Ionization Enthalpy & Electronegativity: — Decrease down the group (F is the most electronegative element).
Electron Gain Enthalpy: — Cl has the most negative (most exothermic) electron gain enthalpy, not F. This is because F's small size causes significant electron-electron repulsion for the incoming electron.
Bond Dissociation Enthalpy: — The $F-F$ bond is weaker than $Cl-Cl$ and $Br-Br$ bonds ($Cl_2 > Br_2 > F_2 > I_2$). This is due to strong lone pair repulsion in the small $F_2$ molecule.
Physical State: — F and Cl are gases, Br is a liquid, I is a solid, reflecting increasing van der Waals forces.

Chemical Properties & Compounds:

1
Hydrides (HX): — Formed with hydrogen. Acidic strength increases $HF < HCl < HBr < HI$. HF is a weak acid due to strong hydrogen bonding.
2
Oxides: — Mostly unstable. Fluorine forms $OF_2$ and $O_2F_2$ (where O is positive).
3
Oxoacids: — Halogens (except F) form oxoacids (e.g., $HClO$, $HClO_2$, $HClO_3$, $HClO_4$). Acidic strength increases with the oxidation state of the halogen (e.g., $HClO < HClO_2 < HClO_3 < HClO_4$).
4
Interhalogen Compounds: — Formed between two different halogens (e.g., $ClF_3$, $IF_5$, $IF_7$). They are generally more reactive than parent halogens (except $F_2$) due to weaker $X-X'$ bonds. Their structures are predicted by VSEPR theory (e.g., $ClF_3$ is T-shaped, $IF_5$ is square pyramidal, $IF_7$ is pentagonal bipyramidal).

Important Reactions:

Chlorine Preparation (Deacon's Process): — $4HCl(g) + O_2(g) xrightarrow{CuCl_2, 723K} 2Cl_2(g) + 2H_2O(g)$
Fluorine with Water: — $2F_2(g) + 2H_2O(l) \rightarrow 4HF(aq) + O_2(g)$ (F oxidizes water)
Chlorine with Water: — $Cl_2(g) + H_2O(l) \rightleftharpoons HCl(aq) + HClO(aq)$ (Cl disproportionates)

Example: Compare the oxidizing power of $Cl_2$ and $I_2$. Since $Cl_2$ is higher in Group 17 than $I_2$, $Cl_2$ is a stronger oxidizing agent. Thus, $Cl_2$ can oxidize $I^-$ to $I_2$: $Cl_2 + 2I^- \rightarrow 2Cl^- + I_2$. However, $I_2$ cannot oxidize $Cl^-$.

Prelims Revision Notes

Group 17 elements, the halogens (F, Cl, Br, I, At), are highly reactive non-metals with $ns^2np^5$ valence electron configuration. They are strong oxidizing agents, with oxidizing power decreasing down the group ($F_2 > Cl_2 > Br_2 > I_2$).

Key Trends:

Atomic/Ionic Radii: — Increase (F to I).
Ionization Enthalpy: — Decreases (F to I).
Electronegativity: — Decreases (F is highest, 4.0).
Melting/Boiling Points: — Increase (F gas, Cl gas, Br liquid, I solid).

Anomalies of Fluorine:

Electron Gain Enthalpy: — Less negative than Cl (due to small size, interelectronic repulsion).
Bond Dissociation Enthalpy ($F_2$): — Lower than $Cl_2$ and $Br_2$ (due to lone pair repulsion).
Oxidation State: — Always -1 (most electronegative).
HF Acidic Strength: — Weak acid due to strong H-bonding.

Reactions & Compounds:

Hydrides (HX): — Acidic strength: $HF < HCl < HBr < HI$. Thermal stability: $HF > HCl > HBr > HI$.
Oxides: — Mostly unstable. F forms $OF_2$ and $O_2F_2$ (O is positive).
Oxoacids: — Halogens (except F) form $HOX$, $HOXO$, $HOXO_2$, $HOXO_3$. Acidic strength increases with oxidation state of halogen (e.g., $HClO < HClO_2 < HClO_3 < HClO_4$).
Interhalogen Compounds: — $XX'$, $XX_3'$, $XX_5'$, $XX_7'$. More reactive than parent halogens (except $F_2$). Structures: $ClF_3$ (T-shaped), $IF_5$ (square pyramidal), $IF_7$ (pentagonal bipyramidal).

Preparation of Chlorine:

Deacon's Process: — $4HCl(g) + O_2(g) xrightarrow{CuCl_2, 723K} 2Cl_2(g) + 2H_2O(g)$
Electrolytic (Brine): — $2NaCl(aq) + 2H_2O(l) xrightarrow{\text{electrolysis}} 2NaOH(aq) + Cl_2(g) + H_2(g)$

Reactions with Water:

$F_2 + H_2O \rightarrow HF + O_2$ (F oxidizes water)
$Cl_2 + H_2O \rightleftharpoons HCl + HClO$ (Cl disproportionates)

Uses: F (Teflon, refrigerants), Cl (disinfectant, bleach, PVC), Br (flame retardants, photography), I (antiseptic, thyroid hormones).

Vyyuha Quick Recall

To remember the order of acidic strength for hydrogen halides: 'Hi, HBr, HCl, HF' - Strongest to weakest. (HI is strongest, HF is weakest). For electron gain enthalpy: 'Cl For Br I' (Chlorine is most negative, then Fluorine, then Bromine, then Iodine).