Trends in Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

Atomic/Ionic Radii: — Increase down group ($F < Cl < Br < I$).
Ionization Enthalpy: — Decrease down group ($F > Cl > Br > I$).
Electron Gain Enthalpy: — Generally less negative down group; Anomaly: $Cl > F > Br > I$ (magnitude).
Electronegativity: — Decrease down group ($F > Cl > Br > I$). F is highest.
Bond Dissociation Enthalpy ($X_2$): — Generally decrease; Anomaly: $Cl_2 > Br_2 > F_2 > I_2$.
Physical State: — Gas ($F_2, Cl_2$) $ightarrow$ Liquid ($Br_2$) $ightarrow$ Solid ($I_2$).
Melting/Boiling Points: — Increase down group.
Colour: — Pale yellow ($F_2$) $ightarrow$ Greenish-yellow ($Cl_2$) $ightarrow$ Reddish-brown ($Br_2$) $ightarrow$ Violet/Dark grey ($I_2$). Intensity increases.
Oxidation States: — 1 for all; Cl, Br, I also +1, +3, +5, +7 (due to d-orbitals). F only -1.
Reactivity: — Decrease down group ($F_2 > Cl_2 > Br_2 > I_2$).
Oxidizing Power: — Decrease down group ($F_2 > Cl_2 > Br_2 > I_2$).
Thermal Stability (HX): — Decrease down group ($HF > HCl > HBr > HI$).
Acidic Strength (HX): — Increase down group ($HF < HCl < HBr < HI$).

2-Minute Revision

Group 17 elements, halogens, exhibit systematic trends. Atomic and ionic radii increase down the group due to added electron shells. This leads to a decrease in ionization enthalpy and electronegativity.

Electron gain enthalpy generally becomes less negative, but Fluorine is an exception, having a less negative value than Chlorine due to significant inter-electronic repulsion in its small $2p$ subshell.

Similarly, the bond dissociation enthalpy of $F_2$ is anomalously low, weaker than $Cl_2$ and $Br_2$, due to lone pair repulsion. Physically, halogens transition from gases ($F_2, Cl_2$) to liquid ($Br_2$) to solid ($I_2$) at room temperature, with increasing melting and boiling points, driven by stronger van der Waals forces.

Their colours also deepen down the group. Chemically, reactivity and oxidizing power decrease from F to I. Fluorine exclusively shows a -1 oxidation state, while other halogens can exhibit positive states up to +7 due to the availability of vacant d-orbitals.

The thermal stability of hydrogen halides (HX) decreases down the group, while their acidic strength increases.

5-Minute Revision

The Group 17 elements, halogens, are highly reactive non-metals with a characteristic $ns^2np^5$ valence electron configuration. Their properties show clear periodic trends. As we move down the group from Fluorine to Iodine:

1
Atomic and Ionic Radii: — Systematically increase due to the addition of new electron shells, placing valence electrons further from the nucleus.
2
Ionization Enthalpy: — Decreases. Larger atomic size means valence electrons are less tightly held, requiring less energy to remove.
3
Electron Gain Enthalpy: — Generally becomes less negative (less exothermic). However, Fluorine is an important exception: its electron gain enthalpy is less negative than Chlorine's. This is because Fluorine's extremely small size leads to high electron density, causing significant inter-electronic repulsion for an incoming electron.
4
Electronegativity: — Decreases. Fluorine is the most electronegative element (4.0 on Pauling scale). Decreasing nuclear attraction for bonding electrons with increasing size.
5
Bond Dissociation Enthalpy ($X_2$): — Generally decreases. However, the $F-F$ bond is anomalously weak, being lower than $Cl-Cl$ and $Br-Br$. This is due to strong inter-electronic repulsion between the lone pairs on the small fluorine atoms.
6
Physical State: — Changes from gas ($F_2$, $Cl_2$) to liquid ($Br_2$) to solid ($I_2$) at room temperature. This is attributed to increasing strength of van der Waals forces (London dispersion forces) with increasing molecular size and number of electrons.
7
Melting and Boiling Points: — Increase down the group, consistent with the increasing van der Waals forces.
8
Colour: — Halogens are coloured. The colour deepens down the group (pale yellow $ightarrow$ greenish-yellow $ightarrow$ reddish-brown $ightarrow$ dark violet/grey). This is due to the decreasing energy gap between HOMO and LUMO, leading to absorption of longer wavelength (lower energy) visible light.
9
Oxidation States: — All halogens show -1. Cl, Br, and I can also show positive oxidation states (+1, +3, +5, +7) by promoting electrons to vacant d-orbitals. Fluorine, lacking d-orbitals, only exhibits -1.
10
Reactivity: — Decreases down the group. Fluorine is the most reactive.
11
Oxidizing Power: — Decreases down the group ($F_2 > Cl_2 > Br_2 > I_2$). A halogen higher in the group can oxidize halide ions of halogens lower in the group.
12
Thermal Stability of Hydrogen Halides (HX): — Decreases down the group ($HF > HCl > HBr > HI$) due to decreasing H-X bond strength.
13
Acidic Strength of Hydrogen Halides (HX): — Increases down the group ($HF < HCl < HBr < HI$). This is due to decreasing H-X bond strength and increasing size of the halide ion, making proton release easier.

Worked Mini-Example: Which of $F_2, Cl_2, Br_2, I_2$ is the strongest oxidizing agent?

Concept: — Oxidizing power decreases down the group.
Order: — $F_2 > Cl_2 > Br_2 > I_2$.
Answer: — $F_2$ is the strongest oxidizing agent.

Prelims Revision Notes

Group 17 Halogens: Key Trends for NEET

1. Atomic & Ionic Radii:

Trend: — Increase down the group ($F < Cl < Br < I$).
Reason: — Addition of new electron shells.

2. Ionization Enthalpy ($IE_1$):

Trend: — Decrease down the group ($F > Cl > Br > I$).
Reason: — Increased atomic size, increased shielding, weaker nuclear attraction for valence electrons.

3. Electron Gain Enthalpy ($Delta_{eg}H$):

General Trend: — Becomes less negative (less exothermic) down the group.
Anomaly: — Chlorine has a more negative electron gain enthalpy than Fluorine.
Order (magnitude): — $Cl > F > Br > I$.
Reason for Anomaly: — Fluorine's small size leads to high electron density and inter-electronic repulsion, hindering the addition of an electron.

4. Electronegativity:

Trend: — Decrease down the group ($F > Cl > Br > I$).
Note: — Fluorine (4.0) is the most electronegative element.

5. Bond Dissociation Enthalpy ($X-X$ bond):

General Trend: — Decreases down the group.
Anomaly: — $F-F$ bond is weaker than $Cl-Cl$ and $Br-Br$.
Order: — $Cl_2 > Br_2 > F_2 > I_2$.
Reason for Anomaly: — Strong inter-electronic repulsion between lone pairs on small F atoms.

6. Physical State at Room Temperature:

$F_2$: Pale yellow gas
$Cl_2$: Greenish-yellow gas
$Br_2$: Reddish-brown liquid
$I_2$: Dark violet solid
Reason: — Increasing van der Waals forces down the group.

7. Melting and Boiling Points:

Trend: — Increase down the group.
Reason: — Increasing van der Waals forces.

8. Colour:

Trend: — Deepens down the group.
Reason: — Decreasing energy gap between HOMO and LUMO, leading to absorption of longer wavelength visible light.

9. Oxidation States:

All halogens show -1.
$Cl, Br, I$: Can also show +1, +3, +5, +7.
Reason for positive states: — Presence of vacant d-orbitals for electron promotion.
Fluorine: — Only shows -1 (no d-orbitals, highest electronegativity).

10. Reactivity:

Trend: — Decrease down the group ($F_2 > Cl_2 > Br_2 > I_2$).

11. Oxidizing Power:

Trend: — Decrease down the group ($F_2 > Cl_2 > Br_2 > I_2$).
Displacement Reactions: — A stronger halogen can displace a weaker halogen from its halide salt (e.g., $Cl_2 + 2KBr \rightarrow 2KCl + Br_2$).

12. Hydrogen Halides (HX):

Thermal Stability: — Decrease down the group ($HF > HCl > HBr > HI$).
Reason: — Decreasing H-X bond strength.
Acidic Strength: — Increase down the group ($HF < HCl < HBr < HI$).
Reason: — Decreasing H-X bond strength and increasing size of $X^-$ ion (better stabilization of anion). HF is a weak acid due to strong H-bonding and high bond energy.

Vyyuha Quick Recall

For the anomalous bond dissociation enthalpy of halogens, remember: 'Clever Brothers Fail Instead' for the order $Cl_2 > Br_2 > F_2 > I_2$.