Trends in Chemical Properties of Group 16 Elements and Their Compounds

Lessons

Senior 4 · Chemistry

Trends in Chemical Properties of Group 16 Elements and Their Compounds

4 lessons

Lesson 1 of 4

The Oxygen Group: Occurrence, Allotropes and Physical Properties

25 min

Two elements we cannot do without

Oxygen is almost half of the Earth's crust by mass, about a fifth of the air, and eight-ninths of the mass of water; every cell in your body burns food with it. Sulphur, directly below it, is a yellow solid that has been dug from volcanic deposits for thousands of years and is now made in huge amounts from natural gas and crude oil, most of it to be turned into sulphuric acid. Like nitrogen and phosphorus in Unit 10, the two top members of Group 16 are strikingly different: oxygen is a gas of small $\mathrm{O_2}$ molecules, sulphur a solid of eight-membered rings. This lesson explains the difference and the trends down the group.

Definition

Group 16 (the chalcogens): oxygen (O), sulphur (S), selenium (Se), tellurium (Te), polonium (Po) and the artificial element livermorium (Lv). Each atom has the outer configuration $ns^2\,np^4$ and needs two electrons to complete its octet, so the common oxidation state in compounds with metals is −2 ( $\mathrm{O^{2-}}$ , $\mathrm{S^{2-}}$ ). Sulphur and the heavier elements also show +4 and +6 (as in $\mathrm{SO_2}$ and $\mathrm{H_2SO_4}$ ).

Allotropy: the existence of an element in two or more different structural forms. Example: dioxygen $\mathrm{O_2}$ and ozone $\mathrm{O_3}$ ; rhombic and monoclinic sulphur.

Where they occur

Oxygen: 46.1% of the Earth's crust by mass (in silicates, oxides and carbonates), about 21% of the air by volume, and in water. It is obtained by fractional distillation of liquid air.
Sulphur: native sulphur near volcanoes; sulphide ores such as pyrite ( $\mathrm{FeS_2}$ ) and galena (PbS); sulphates such as gypsum; and hydrogen sulphide in natural gas, from which most sulphur is now recovered.
Selenium and tellurium: by-products of copper refining.
Polonium: a rare, intensely radioactive element found in traces in uranium ores.

Physical properties down the group

Table 11.1: The Group 16 elements

Property	O	S	Se	Te	Po
Atomic number	8	16	34	52	84
Outer configuration	$2s^2\,2p^4$	$3s^2\,3p^4$	$3d^{10}\,4s^2\,4p^4$	$4d^{10}\,5s^2\,5p^4$	$4f^{14}\,5d^{10}\,6s^2\,6p^4$
Covalent radius / pm	63	103	116	136	145
First ionisation energy / kJ mol⁻¹	1313.9	999.6	941.0	869.3	812.1
Electronegativity (Pauling)	3.44	2.58	2.55	2.1	2.0
Melting point / °C	−218.8	115.2	220.5	449.5	254
Boiling point / °C	−183.0	444.6	685	988	962
Structure	$\mathrm{O_2}$ molecules	$\mathrm{S_8}$ rings	long chains (grey Se)	long chains	metallic
Type	non-metal	non-metal	non-metal (grey form semiconducts)	metalloid	metal

Ionisation energy and electronegativity fall down the group (Figure 11.1). Oxygen, the second most electronegative element after fluorine, is in a class of its own, which is why it forms hydrogen bonds in water (Lesson 2).
Metallic character rises: non-metals at the top, the metalloid tellurium, and polonium, a metal.
Melting points rise from $\mathrm{O_2}$ to tellurium, as the structures change from small molecules to rings and then long chains with stronger forces between them, then fall for metallic polonium (Figure 11.2).

Figure 11.1: First ionisation energies down Group 16.

Figure 11.2: Melting points down Group 16.

Why oxygen is $\mathrm{O_2}$ but sulphur is $\mathrm{S_8}$

The same idea you met in Units 9 and 10 applies. A small oxygen atom forms a strong π bond, so two oxygen atoms complete their octets with one O=O double bond, and $\mathrm{O_2}$ molecules attract each other only weakly: oxygen is a gas. Sulphur's larger 3p orbitals overlap poorly sideways, so each sulphur atom forms two single bonds to two different atoms, giving rings of eight (Figure 11.3) with stronger London forces between them: sulphur is a solid.

Figure 11.3: $\mathrm{O_2}$ , $\mathrm{O_3}$ and $\mathrm{S_8}$ .

Allotropes of oxygen and sulphur

Ozone, $\mathrm{O_3}$ , is a pale blue, sharp-smelling, toxic gas made when an electric discharge or ultraviolet light passes through oxygen: $\mathrm{3O_2 \rightleftharpoons 2O_3}$ . It is a far stronger oxidising agent than $\mathrm{O_2}$ . High in the atmosphere the ozone layer absorbs harmful ultraviolet radiation from the Sun.
Rhombic (α) sulphur, made of $\mathrm{S_8}$ rings, is the stable form at room temperature. Above 95.3 °C it slowly changes to monoclinic (β) sulphur, also made of $\mathrm{S_8}$ rings packed differently, which forms long needle-shaped crystals; below 95.3 °C the change reverses.
Plastic sulphur: when molten sulphur is heated further, the rings break open and join into long chains, and the liquid becomes very thick. Poured into cold water, it sets as a rubbery, stretchy solid that slowly reverts to rhombic sulphur.

A quarter of a disc of tellurium. Its silvery, metallic shine and brittle, crystalline fracture are typical of a metalloid; tellurium is a semiconductor. Photo: W. Oelen, CC BY-SA 3.0, via Wikimedia Commons

Worked example 1: reading a trend and an anomaly

Use Table 11.1 to (a) explain why the first ionisation energy of sulphur (999.6 kJ mol⁻¹) is lower than that of phosphorus (1011.8 kJ mol⁻¹, Unit 10), even though sulphur has one more proton, and (b) explain the large drop from oxygen to sulphur.

Solution. (a) Phosphorus is $3s^2\,3p^3$ : three unpaired 3p electrons, one in each orbital. Sulphur is $3s^2\,3p^4$ : one 3p orbital holds a pair, and the two electrons in it repel each other, so one is removed more easily. This outweighs the extra proton. (b) Sulphur's outer electron is in the third shell, further from the nucleus and shielded by eight more inner electrons than oxygen's second-shell electron, so it is much more easily removed (1313.9 down to 999.6 kJ mol⁻¹).

Worked example 2: how much ozone?

An ozone generator converts 6.0% of the oxygen molecules passing through it into ozone, $\mathrm{3O_2 \rightarrow 2O_3}$ . How many moles of ozone are made from 50.0 dm³ of oxygen at room temperature and pressure (24.0 dm³ mol⁻¹)?

Solution. $n(\mathrm{O_2}) = \frac{50.0}{24.0} = 2.08\ \mathrm{mol} \qquad n(\mathrm{O_2}\text{ converted}) = 0.060 \times 2.08 = 0.125\ \mathrm{mol}$ $n(\mathrm{O_3}) = 0.125 \times \frac{2}{3} = 0.0833\ \mathrm{mol}$ (about 0.083 mol; two significant figures, because 6.0% has two).

Warning

Common mistakes. (1) Writing sulphur as S or $\mathrm{S_2}$ when asked for the formula of the molecule: it is $\mathrm{S_8}$ (S alone is fine in most equations). (2) Calling ozone "a compound of oxygen": it is an allotrope, an element. (3) Expecting the first ionisation energy to rise from P to S: the paired 3p electrons in S make it lower. (4) Saying sulphur melts at 115 °C "because its covalent bonds break": melting separates $\mathrm{S_8}$ molecules; the S–S bonds stay intact.

Safety. Liquid oxygen makes almost anything burn fiercely and causes frostbite. Ozone irritates the lungs. Molten sulphur causes severe burns, and burning sulphur gives toxic sulphur dioxide, so plastic sulphur is made only as a teacher demonstration in a fume cupboard. Polonium is intensely radioactive and never handled outside specialist laboratories.

Tip

Across Groups 14 to 16 the same rule appears three times: Period 2 makes π bonds and small molecules ( $\mathrm{CO_2}$ , $\mathrm{N_2}$ , $\mathrm{O_2}$ ); Period 3 makes single bonds and bigger structures ( $\mathrm{SiO_2}$ , $\mathrm{P_4}$ , $\mathrm{S_8}$ ).

Professor Dave Explains: Periodic Table Part 7, the Chalcogens (9 min). Watch for the allotropes of oxygen and sulphur, how the elements become more metallic down the group, and the uses of selenium and tellurium.

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Practice

Write the full electron configuration of sulphur and of the sulphide ion, $\mathrm{S^{2-}}$ .
Name the main sources of oxygen and of sulphur.
Explain why oxygen is a gas at room temperature but sulphur is a solid.
Describe the difference between rhombic and monoclinic sulphur, and state the temperature at which one changes into the other.
What is plastic sulphur, and why is molten sulphur so viscous at about 200 °C?
Explain why electronegativity decreases from oxygen to polonium.
Ozone decomposes on warming: $\mathrm{2O_3 \rightarrow 3O_2}$ . What volume of oxygen forms from 2.40 dm³ of ozone (same temperature and pressure)?
Classify each Group 16 element as non-metal, metalloid or metal, giving one property in support.

Show answersHide answers

S: $1s^2\,2s^2\,2p^6\,3s^2\,3p^4$ ; $\mathrm{S^{2-}}$ : $1s^2\,2s^2\,2p^6\,3s^2\,3p^6$ .
Oxygen: fractional distillation of liquid air (also in water, oxides, silicates). Sulphur: recovered from hydrogen sulphide in natural gas and oil; native sulphur near volcanoes; sulphide ores.
Small O atoms form a strong π bond, so $\mathrm{O_2}$ molecules are small with weak London forces between them. Larger S atoms form single bonds in $\mathrm{S_8}$ rings, which are bigger molecules with stronger London forces.
Both are made of $\mathrm{S_8}$ rings, packed differently: rhombic is stable below 95.3 °C, monoclinic (needle-shaped crystals) above it.
A rubbery form made by pouring very hot molten sulphur into cold water; at about 200 °C the $\mathrm{S_8}$ rings have opened and joined into long tangled chains, which make the liquid viscous.
The atoms get bigger and the outer shell is further from the nucleus and better shielded, so bonding electrons are attracted less strongly.
By Avogadro's law, volume ratio = mole ratio: $2.40 \times \frac{3}{2} = 3.60\ \mathrm{dm^3}$ .
O, S, Se non-metals (O and S insulators with molecular structures; grey Se is a semiconductor); Te metalloid (shiny but brittle semiconductor); Po metal.

Quick check

Why is oxygen a gas at room temperature while sulphur is a solid?

Key points

Group 16 atoms are $ns^2\,np^4$ ; the common state with metals is −2, and S, Se and Te also show +4 and +6.
Oxygen (46.1% of the crust, 21% of the air) is $\mathrm{O_2}$ ; its allotrope ozone, $\mathrm{O_3}$ , is a strong oxidant that shields the Earth from UV light.
Sulphur forms $\mathrm{S_8}$ rings: rhombic below 95.3 °C, monoclinic above; long chains form in hot molten sulphur (plastic sulphur).
Down the group ionisation energy and electronegativity fall and metallic character rises (O, S, Se non-metals; Te metalloid; Po metal); melting points rise to Te, then fall.

Sources and credits

Image: Liquid oxygen in a beaker (cropped and retouched).jpg by Staff Sgt. Nika Glover, U.S. Air Force, via Wikimedia Commons, public domain.
Image: Tellurium element 2.jpg by W. Oelen, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Data: ionisation energies from Wikipedia's ionisation energies data page; covalent radii from the atomic radii data page; crustal abundance from Abundance of elements in Earth's crust; the sulphur transition from Allotropes of sulfur; the ozone angle from Ozone; all CC BY-SA 4.0, adapted by Ganzaa. Melting and boiling points and electronegativities from the PubChem Periodic Table.
Video: Periodic Table Part 7: Chalcogens (O, S, Se, Te, Po, Lv), Professor Dave Explains, embedded from YouTube.
The text, charts, structure diagrams, table, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Two elements we cannot do without

2Where they occur

3Physical properties down the group

4Why oxygen is O2 \mathrm{O_2} O2​ but sulphur is S8 \mathrm{S_8} S8​

5Allotropes of oxygen and sulphur

Two elements we cannot do without

Where they occur

Physical properties down the group

Why oxygen is $\mathrm{O_2}$ but sulphur is $\mathrm{S_8}$

Allotropes of oxygen and sulphur