MHT-CET Chemistry · Elements of Group 16, 17 and 18

Group 17: Halogens, Their Oxoacids and Interhalogen Compounds

The halogens F, Cl, Br, I and the radioactive At (ns² np⁵) change from gases to a liquid to a solid down the group, form less stable hydrogen halides as the atom grows, and give oxoacids HOX, HXO₂, HXO₃ and HXO₄ whose strength rises with the oxidation state; two different halogens combine as interhalogens XX′ₙ with n always odd.

All Elements of Group 16, 17 and 18 notes MHT-CET Chemistry strategy

Why this matters

36 PYQs, 3 HARD. Of the twenty-one set from 2021 to 2025, seven are halogen properties — the ns² np⁵ configuration, bromine the liquid, fluorine's false statement, ionic character of metal halides, the least stable hydride, boiling points of HX, and chlorine with hot NaOH; six are oxoacids — the strongest, its oxygen count and bonds, the halous acid, fluorine's only oxoacid, and the halogen with most oxoacids; eight are interhalogens — thermal stability (the HARD order), physical state at 25 °C (three times), shape of BrF₅, the oxidation state of iodine in I₂Cl₆, and the false statement about n. Three cards.

Concept 1 of 3: Halogen Properties and the Hydrogen Halides

Every halogen is one electron short of a noble gas, ns² np⁵. Down the group the molecules get heavier, so F₂ and Cl₂ are gases, Br₂ the only liquid and I₂ a solid. Fluorine is the exception in several ways: the most electronegative element, it shows only −1 and forms only one oxoacid, but its F–F bond is WEAKER than Cl–Cl because the small atoms' lone pairs repel. Down the group the H–X bond weakens, so HF is the most stable and HI the least; HF also has the highest boiling point because it hydrogen-bonds, and the rest follow size: HI > HBr > HCl. Chlorine with NaOH disproportionates: cold and dilute gives hypochlorite, hot and concentrated gives chlorate.

Definition

Configuration $ns^2np^5$ : F, Cl, Br, I, At (radioactive).
State at room temperature: F₂, Cl₂ gases; Br₂ liquid; I₂ solid.
Fluorine: most electronegative, only −1, only one oxoacid — but bond dissociation enthalpy is not the highest (Cl₂ > Br₂ > F₂ > I₂).
Metal halides: ionic character MF > MCl > MBr > MI; fluorine gives the most ionic.
H–X: thermal stability HF > HCl > HBr > HI (lowest); boiling point HF > HI > HBr > HCl.
Cl₂ + NaOH: cold, dilute → NaCl + NaOCl + H₂O; hot, concentrated → $3\text{Cl}_2 + 6\text{NaOH} \to 5\text{NaCl} + \text{NaClO}_3 + 3\text{H}_2\text{O}$ .

Chlorine with hot concentrated NaOH

3\text{Cl}_2 + 6\text{NaOH} \xrightarrow{\text{hot, conc.}} 5\text{NaCl} + \text{NaClO}_3 + 3\text{H}_2\text{O}

Worked example

Arrange HF, HCl and HBr by boiling point and by thermal stability.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 20 April Shift II · Q63Moderate

Example 1 · Elements of Group 16, 17 and 18 · Group 17 Halogens, Interhalogens and Oxoacids

Identify the correct decreasing order of boiling point for hydrides of halogens.

Putting HI above HF in boiling point

Without hydrogen bonding HI would boil highest, but HF's hydrogen bonds put it on top: HF > HI > HBr > HCl.

Drill 6 more on halogen properties and the hydrogen halides

Concept 2 of 3: Oxoacids of the Halogens

Name the oxoacid by the halogen's oxidation state: +1 hypohalous HOX, +3 halous HXO₂, +5 halic HXO₃, +7 perhalic HXO₄. The more oxygens pulling electrons off the O–H, the stronger the acid, so perchloric acid HClO₄ is the strongest; its Cl has three Cl=O bonds and one Cl–OH. Chlorine forms all four; fluorine, which cannot take a positive state, forms only HOF.

Definition

Series: hypochlorous HOCl (+1), chlorous HClO₂ (+3), chloric HClO₃ (+5), perchloric HClO₄ (+7).
Strongest: HClO₄ — acid strength rises with oxidation state; Cl is bonded to 4 O atoms: 3 double + 1 single (the Cl–OH).
Fluorine: only HOF (hypofluorous acid). Chlorine forms the most oxoacids (four).

Name	Formula	O.S. of Cl	Cl–O bonds
Hypochlorous	HOCl	+1	1 single
Chlorous	HClO₂	+3	1 single, 1 double
Chloric	HClO₃	+5	1 single, 2 double
Perchloric	HClO₄	+7	1 single, 3 double The strongest oxoacid.

Halous = +3, not +1; 'hypo-' is +1.

Practise this conceptself-check · 5 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 10th May Shift 1 · Q67Easy

Example 2 · Elements of Group 16, 17 and 18 · Group 17 Halogens, Interhalogens and Oxoacids

Identify strongest oxoacid of halogen from following.

Reading 'halous' as the lowest acid

Hypohalous (HOX) is the lowest, +1. Halous is the next one up, HXO₂ at +3.

Drill 5 more on oxoacids of the halogens

Concept 3 of 3: Interhalogen Compounds

An interhalogen XX′ₙ joins a larger halogen X to n smaller ones X′, and n is always ODD — 1, 3, 5 or 7 — so that all electrons pair. They are covalent and diamagnetic, and more reactive than the parent halogens (the X–X′ bond is weaker), fluorine excepted. The shape follows the electron pairs on X: XX′ linear, XX′₃ T-shaped, XX′₅ square pyramidal (one lone pair), IF₇ pentagonal bipyramidal. Thermal stability is highest where the bond is most polar and strong, ClF, and the paper's order is ClF > ICl > IBr > BrCl. Most are gases or volatile liquids; the iodine ones are solids except IF₅ (liquid) and IF₇ (gas).

Definition

Formula $\text{XX}'_n$ , n = 1, 3, 5, 7 (always odd); covalent, diamagnetic, more reactive than halogens (except F₂).
Shapes: XX′ linear; XX′₃ T-shaped ( $sp^3d$ ); XX′₅ square pyramidal ( $sp^3d^2$ , e.g. BrF₅, ClF₅); IF₇ pentagonal bipyramidal ( $sp^3d^3$ ).
Thermal stability: ClF > ICl > IBr > BrCl (> BrF).
State at 25 °C: gases — ClF, BrF, ClF₃, IF₇ (ClF, ClF₃ and IF₇ colourless); liquid — IF₅ (colourless); solids — ICl (red), IBr (black), IF₃ (yellow).
$\text{I}_2\text{Cl}_6$ : each I bonded to 3 Cl, so I is +3.

Interhalogen stoichiometry

\text{XX}'_n,\ n \in \{1, 3, 5, 7\};\quad \text{ClF} > \text{ICl} > \text{IBr} > \text{BrCl}\ \text{(thermal stability)}

Worked example

Predict the shape of ClF₃ and the oxidation state of Cl in it.

Practise this conceptself-check · 5 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 3rd May Shift 2 · Q57Hard

Example 3 · Elements of Group 16, 17 and 18 · Group 17 Halogens, Interhalogens and Oxoacids

Identify the correct decreasing order of thermal stability from following.

Assuming the iodine interhalogens are all solids

ICl, IBr and IF₃ are solids, but IF₅ is a liquid and IF₇ a gas at 25 °C — more fluorines, weaker intermolecular attraction.

Drill 7 more on interhalogen compounds

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Halogen Properties and the Hydrogen Halides
Chlorine with hot concentrated NaOH
$3\text{Cl}_2 + 6\text{NaOH} \xrightarrow{\text{hot, conc.}} 5\text{NaCl} + \text{NaClO}_3 + 3\text{H}_2\text{O}$
Interhalogen Compounds
Interhalogen stoichiometry
$\text{XX}'_n,\ n \in \{1, 3, 5, 7\};\quad \text{ClF} > \text{ICl} > \text{IBr} > \text{BrCl}\ \text{(thermal stability)}$

Reference tables (1)

Oxoacids of the Halogens4 rows

Name	Formula	O.S. of Cl	Cl–O bonds
Hypochlorous	HOCl	+1	1 single
Chlorous	HClO₂	+3	1 single, 1 double
Chloric	HClO₃	+5	1 single, 2 double
Perchloric	HClO₄	+7	1 single, 3 double The strongest oxoacid.

Halous = +3, not +1; 'hypo-' is +1.

Watch out for (3)

Putting HI above HF in boiling point
→ Halogen Properties and the Hydrogen Halides
Reading 'halous' as the lowest acid
→ Oxoacids of the Halogens
Assuming the iodine interhalogens are all solids
→ Interhalogen Compounds

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