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

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

The chalcogens O, S, Se, Te and Po (ns² np⁴) grow larger, denser, less reactive and easier to ionise down the group; their hydrides H₂E become less stable but more acidic; oxygen forms O₂ and the less stable, strongly oxidising O₃; and sulphur has the most allotropes and gives SO₂, sulphuric acid and oleum.

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

Why this matters

25 PYQs, none HARD. Eight are family and trends — which group is the chalcogens, which element does not belong (astatine, twice), ionisation enthalpy, density, reactivity and atomic size; six are hydrides and oxygen against sulphur — thermal stability, acidity, hybridisation, the colourless odourless hydride; four are oxygen and ozone — O₂ against O₃, the O–O bond length, ozone's properties and what depletes the layer; seven are sulphur — allotropes, the S–S–S angle, SO₂ manufacture, oleum, disulphuric acid, baryte and galena. Four cards.

Concept 1 of 4

The Chalcogen Family and Its Trends

Intuition

Group 16 is O, S, Se, Te and the radioactive Po, all ns² np⁴ with two unpaired p electrons. Going down, each atom adds a shell: atoms get LARGER, ionisation enthalpy and electronegativity FALL, density RISES and metallic character increases (O and S non-metals, Se and Te metalloids, Po a metal). Reactivity falls down the group, so oxygen is the most reactive. Across a period size shrinks, so Se (period 4, group 16) is bigger than Br beside it and bigger than S and Cl above.

Definition

Chalcogens = Group 16: O, S, Se, Te, Po. Astatine is Group 17, NOT a chalcogen.
Ionisation enthalpy decreases down: O > S > Se > Te > Po; the lowest of {Po, Te, Br, Kr} is Po.
Atomic size increases down and decreases across: largest of {S, Se, Cl, Br} is Se.
Density increases down: highest of {O, S, Se, Te} is Te.
Reactivity: O > S > Se > Te > Po.

Down group 16

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

Worked example

Arrange S, Te and Se in increasing atomic size and in decreasing first ionisation enthalpy.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Elements of Group 16, 17 and 18EASY

Which from following is a correct decreasing order of ionization enthalpies of different elements?

[Q82 · 10th May Shift 2 · 2024]

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.

Drill 7 more on the chalcogen family and its trends

Concept 2 of 4

Hydrides H₂E and Oxygen Against Sulphur

Intuition

All the hydrides H₂E have the central atom sp³ hybridised with two lone pairs, so they are bent. As E gets bigger the E–H bond gets longer and weaker. A weaker bond means LOWER thermal stability (H₂O most stable, H₂Te least) and HIGHER acidity (H₂Te gives up H⁺ most easily). Water stands apart: a colourless, odourless liquid, while H₂S, H₂Se and H₂Te are foul-smelling gases. Oxygen also differs from sulphur in oxidation states: with no d orbitals it cannot expand its octet, so it shows −2 (and −1, +2 in OF₂) but never +4 or +6, which sulphur does.

Definition

Hybridisation of E in $\text{H}_2\text{E}$ : $sp^3$ ; bond angle falls H₂O 104.5° → H₂S 92° → H₂Se 91° → H₂Te 90°.
Thermal stability: $\text{H}_2\text{O} > \text{H}_2\text{S} > \text{H}_2\text{Se} > \text{H}_2\text{Te}$ ; lowest H₂Te.
Acidity: $\text{H}_2\text{O} < \text{H}_2\text{S} < \text{H}_2\text{Se} < \text{H}_2\text{Te}$ ; most acidic H₂Te.
Colourless, odourless hydride: oxygen's (H₂O); NH₃, H₂S, H₂Se all smell.
O vs S: both have two unpaired electrons; O is a gas and S a solid; H₂O is more stable than H₂S; S shows −2, +2, +4, +6 but O does not show +4 or +6.

Group 16 hydrides

\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}

Worked example

Which of H₂S and H₂Se is the stronger acid, and which is more thermally stable?

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Elements of Group 16, 17 and 18EASY

Identify the correct order of thermal stability of hydrides of 16 group elements from the following.

[Q95 · 19 April Shift I · 2025]

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.

Drill 5 more on hydrides h₂e and oxygen against sulphur

Concept 3 of 4

Dioxygen and Ozone

Intuition

Making ozone from oxygen, 3O₂ → 2O₃, is uphill: it absorbs heat (ΔH positive) and turns three molecules into two (ΔS negative), so ozone is the LESS stable form and readily gives up an O atom — a strong oxidising and bleaching agent. O₂ has two unpaired electrons and is paramagnetic; O₃ is bent, diamagnetic, with two equal O–O bonds of 128 pm, between a single and a double bond because of resonance. In the stratosphere it absorbs harmful UV; nitric oxide and CFCs destroy it.

Definition

$3\text{O}_2 \to 2\text{O}_3$ : ΔH positive (endothermic), ΔS negative; O₃ is less stable than O₂.
O₂ paramagnetic; O₃ diamagnetic, angular (about 117°), O–O 128 pm (single 148 pm, double 121 pm).
Ozone is a strong oxidising agent (NOT reducing), a bleaching agent, and absorbs UV.
Ozone depletion: NO ( $\text{NO} + \text{O}_3 \to \text{NO}_2 + \text{O}_2$ ) and CFCs; He, CO₂ and H₂ do not.

Formation of ozone

3\text{O}_2 \rightarrow 2\text{O}_3;\quad \Delta H > 0,\ \Delta S < 0

Worked example

Explain why ozone is a stronger oxidising agent than oxygen.

Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 3Elements of Group 16, 17 and 18MODERATE

Which of the following statements is correct about

O_{2}

and

O_{3}

molecule?

[Q86 · 19 April Shift II · 2025]

Calling ozone paramagnetic like oxygen

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

Drill 3 more on dioxygen and ozone

Concept 4 of 4

Sulphur: Allotropes, SO₂, Oleum and Ores

Intuition

Sulphur has more allotropes than any other group 16 element — rhombic, monoclinic, plastic and more — and both crystalline forms are built from puckered S₈ crowns with an S–S–S angle of 107°. Industrially SO₂ comes from roasting sulphide ores (zinc blende, iron pyrites); in the lab from a sulphite and dilute acid. SO₂ → SO₃ in the contact process, and SO₃ absorbed in sulphuric acid gives oleum, H₂S₂O₇. Several ores are sulphides or sulphates: galena PbS, baryte BaSO₄.

Definition

Allotropes: sulphur has the most (rhombic α, monoclinic β, plastic…); S₈ is a puckered crown, ∠S–S–S = 107°.
SO₂ in industry: by roasting zinc sulphide and iron pyrites (4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂); in the lab from $\text{Na}_2\text{SO}_3$ + dilute acid.
Oleum (fuming sulphuric acid, pyrosulphuric/disulphuric acid) = H₂S₂O₇, made as H₂SO₄ + SO₃ → H₂S₂O₇. By the usual structure each S has two S=O and two single S–O bonds; one paper keyed 'one double and two single' — the official letter is kept in the bank.
Ores: galena PbS; baryte BaSO₄ (Ba, S, O); zinc blende ZnS; iron pyrites FeS₂; gypsum CaSO₄·2H₂O.

Name	Formula	Remember
Oleum	H₂S₂O₇	H₂SO₄ + SO₃ H₂S₂O₃ is thiosulphuric, H₂S₂O₈ peroxodisulphuric.
Galena	PbS	lead ore
Baryte	BaSO₄	Ba, S, O
Zinc blende	ZnS	roasted for SO₂
Iron pyrites	FeS₂	roasted for SO₂
Gypsum	CaSO₄·2H₂O	—

Sulphides and sulphates of sulphur's chapter.

Practice this conceptself-check · 5 quick reps

From the bank · past-year question

Example 4Elements of Group 16, 17 and 18EASY

Identify the correct molecular formula of 'Oleum' from following.

[Q69 · 19 April Shift II · 2025]

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°.

Drill 6 more on sulphur: allotropes, so₂, oleum and ores

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 (3)

The Chalcogen Family and Its Trends
Down group 16
$\text{size}\uparrow,\ \text{density}\uparrow,\ \text{IE}\downarrow,\ \text{reactivity}\downarrow:\quad \text{S} > \text{Se} > \text{Te} > \text{Po}\ (\text{IE})$
Hydrides H₂E and Oxygen Against Sulphur
Group 16 hydrides
$\text{stability: } \text{H}_2\text{O} > \text{H}_2\text{S} > \text{H}_2\text{Se} > \text{H}_2\text{Te};\quad \text{acidity: reverse}$
Dioxygen and Ozone
Formation of ozone
$3\text{O}_2 \rightarrow 2\text{O}_3;\quad \Delta H > 0,\ \Delta S < 0$

Reference tables (1)

Sulphur: Allotropes, SO₂, Oleum and Ores6 rows

Name	Formula	Remember
Oleum	H₂S₂O₇	H₂SO₄ + SO₃ H₂S₂O₃ is thiosulphuric, H₂S₂O₈ peroxodisulphuric.
Galena	PbS	lead ore
Baryte	BaSO₄	Ba, S, O
Zinc blende	ZnS	roasted for SO₂
Iron pyrites	FeS₂	roasted for SO₂
Gypsum	CaSO₄·2H₂O	—

Sulphides and sulphates of sulphur's chapter.

Watch out for (4)

Putting astatine in group 16
→ The Chalcogen Family and Its Trends
Expecting stability and acidity to run the same way
→ Hydrides H₂E and Oxygen Against Sulphur
Calling ozone paramagnetic like oxygen
→ Dioxygen and Ozone
Taking the S–S–S angle as 104.5° or 120°
→ Sulphur: Allotropes, SO₂, Oleum and Ores

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Related notes

Group 17 — the halogens beside the chalcogens →