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MHT-CET Chemistry · Formula sheet

Elements of Group 16, 17 and 18 formulas

6 formulas, 2 reference tables and 8 common traps for MHT-CET Chemistry Elements of Group 16, 17 and 18, grouped by subtopic.

Full notes with worked examples

Group 16: Chalcogen Trends, Hydrides, Oxygen, Ozone and Sulphur

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The Chalcogen Family and Its Trends

Down group 16

size↑, density↑, IE↓, reactivity↓:S>Se>Te>Po (IE)\text{size}\uparrow,\ \text{density}\uparrow,\ \text{IE}\downarrow,\ \text{reactivity}\downarrow:\quad \text{S} > \text{Se} > \text{Te} > \text{Po}\ (\text{IE})

Hydrides H₂E and Oxygen Against Sulphur

Group 16 hydrides

stability: H2O>H2S>H2Se>H2Te;acidity: reverse\text{stability: } \text{H}_2\text{O} > \text{H}_2\text{S} > \text{H}_2\text{Se} > \text{H}_2\text{Te};\quad \text{acidity: reverse}

Dioxygen and Ozone

Formation of ozone

3O2→2O3;ΔH>0, ΔS<03\text{O}_2 \rightarrow 2\text{O}_3;\quad \Delta H > 0,\ \Delta S < 0

Sulphur: Allotropes, SO₂, Oleum and Ores

NameFormulaRemember
OleumH₂S₂O₇H₂SO₄ + SO₃
H₂S₂O₃ is thiosulphuric, H₂S₂O₈ peroxodisulphuric.
GalenaPbSlead ore
BaryteBaSO₄Ba, S, O
Zinc blendeZnSroasted for SO₂
Iron pyritesFeS₂roasted for SO₂
GypsumCaSO₄·2H₂O—
Sulphides and sulphates of sulphur's chapter.

Common traps

Putting astatine in group 16

Astatine sits next to polonium in period 6 but in group 17 — it is a halogen. The chalcogen list ends at Po.

Expecting stability and acidity to run the same way

They run opposite. The weak Te–H bond makes H₂Te the least stable AND the most acidic hydride.

Calling ozone paramagnetic like oxygen

O₂ has two unpaired electrons; O₃ has none. Only O₂ is paramagnetic.

Taking the S–S–S angle as 104.5° or 120°

104.5° is water's H–O–H angle and 120° a flat ring. The S₈ crown is puckered at 107°.

Group 17: Halogens, Their Oxoacids and Interhalogen Compounds

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Halogen Properties and the Hydrogen Halides

Chlorine with hot concentrated NaOH

3Cl2+6NaOH→hot, conc.5NaCl+NaClO3+3H2O3\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

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

Oxoacids of the Halogens

NameFormulaO.S. of ClCl–O bonds
HypochlorousHOCl+11 single
ChlorousHClO₂+31 single, 1 double
ChloricHClO₃+51 single, 2 double
PerchloricHClO₄+71 single, 3 double
The strongest oxoacid.
Halous = +3, not +1; 'hypo-' is +1.

Common traps

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.

Reading 'halous' as the lowest acid

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

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.

Group 18: Noble Gases, Their Uses and the Shapes of Xenon Compounds

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Noble Gas Uses and Xenon Compound Shapes

Lone pairs on xenon

lp=8−n(Xe-F)−2 n(Xe=O)2\text{lp} = \frac{8 - n(\text{Xe-F}) - 2\,n(\text{Xe=O})}{2}

Common traps

Counting an Xe=O bond as one electron

A double bond to oxygen uses two of xenon's electrons. XeOF₂ therefore has two lone pairs, not three.

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