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

37 PYQs, none HARD. Of the twenty-four set from 2021 to 2025, 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; six 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

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.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 10th May Shift 2 · Q82Easy

Example 1 · Elements of Group 16, 17 and 18 · Group 16 Chalcogens, Oxygen, Sulphur and Ozone

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

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

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?

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 19 April Shift I · Q95Easy

Example 2 · Elements of Group 16, 17 and 18 · Group 16 Chalcogens, Oxygen, Sulphur and Ozone

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

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

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.

Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 19 April Shift II · Q86Moderate

Example 3 · Elements of Group 16, 17 and 18 · Group 16 Chalcogens, Oxygen, Sulphur and Ozone

Which of the following statements is correct about

O_{2}

and

O_{3}

molecule?

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

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.

Practise this conceptself-check · 5 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 19 April Shift II · Q69Easy

Example 4 · Elements of Group 16, 17 and 18 · Group 16 Chalcogens, Oxygen, Sulphur and Ozone

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

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 5 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

Test yourself on Elements of Group 16, 17 and 18

15 past MHT-CET questions from this chapter, timed at 14 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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