Group 16 trends, oxygen and its oxides

The element in every breath

A candle flame goes out under a jar. A rusting nail slowly gets heavier. Both happen because of oxygen, which makes up about one fifth of the air. Oxygen is also the most plentiful element in the Earth's crust, where it is mostly in water, rocks and sand.

Oxygen heads group 16, the chalcogens, a name from the Greek for "ore formers". The group has five elements: oxygen (O), sulphur (S), selenium (Se), tellurium (Te) and polonium (Po). Each has the outer electron arrangement $ns^2np^4$. An atom needs only two more electrons to fill its $p$ orbitals. So these elements take two electrons easily, and they form the ion $\text{E}^{2-}$ or two covalent bonds.

The family

• O: a non-metal. Covalent radius 66 pm, first ionisation enthalpy 1314 kJ/mol, electronegativity 3.5.
• S: a non-metal. Radius 104 pm, ionisation enthalpy 1000 kJ/mol, electronegativity 2.6.
• Se: a metalloid (it behaves partly like a metal). Radius 117 pm, ionisation enthalpy 941 kJ/mol, electronegativity 2.6.
• Te: a metalloid. Radius 137 pm, ionisation enthalpy 869 kJ/mol, electronegativity 2.1.
• Po: a radioactive metal. Radius 146 pm, ionisation enthalpy 812 kJ/mol, electronegativity about 2.0.

The radius grows and the ionisation enthalpy falls down the group. The electronegativity falls too, but not smoothly: selenium is a little higher than sulphur in some tables, and NCERT prints values that differ from the Pauling values given here. The character changes from non-metal to metal.

Oxidation states. The usual states are $-2$, $+2$, $+4$ and $+6$. Oxygen shows $-2$ in nearly all compounds. It is $-1$ in peroxides and $+2$ in $\text{OF}_2$. Sulphur, selenium and tellurium show $+4$ and $+6$. Going down the group, the $+6$ state becomes less stable and the $+4$ state more stable. This is the inert pair effect, as in group 15.

Why oxygen is different

Oxygen differs from sulphur much more than sulphur differs from selenium. There are four reasons.

1. Small size and high electronegativity. Oxygen forms strong hydrogen bonds. Water is a liquid at room temperature, but $\text{H}_2\text{S}$ is a gas.
2. No d orbitals in the second shell. The covalency of oxygen is at most 2, and 3 in the ion $\text{H}_3\text{O}^+$. Sulphur has a third shell, and forms $\text{SF}_6$ with six bonds. The Indian syllabus explains this by using 3d orbitals. Many modern books use other models, but sulphur can exceed eight electrons and oxygen cannot.
3. Multiple bonds. Oxygen forms a double bond with itself in $\text{O}_2$ by sideways overlap of $p$ orbitals. Sulphur prefers single bonds, so it exists as rings and chains.
4. Magnetism. The molecule $\text{O}_2$ has two unpaired electrons and is paramagnetic. Molecular orbital theory explains this. Liquid oxygen is held by a magnet.

The hydrides

All the elements form hydrides of the formula $\text{H}_2\text{E}$: water, hydrogen sulphide, hydrogen selenide and hydrogen telluride. All are bent molecules.

• Bond angle. It falls from 104.5° in water to 92.1° in hydrogen sulphide and to about 90° at the end. Water has two lone pairs that push the bonds together, and the bigger atoms bond almost with pure $p$ orbitals at 90°.
• Boiling point. Water boils at 373 K because of hydrogen bonds. After $\text{H}_2\text{S}$ (213 K) the boiling point rises with the size of the molecule.
• Acidic character. It rises from $\text{H}_2\text{O}$ to $\text{H}_2\text{Te}$. The H-E bond gets longer and weaker, so the hydrogen leaves as $\text{H}^+$ more easily.
• Thermal stability falls, and reducing power rises, from $\text{H}_2\text{O}$ to $\text{H}_2\text{Te}$, for the same reason.

Making oxygen

In industry, oxygen comes from liquid air. Air is cooled until it is liquid. Then fractional distillation separates oxygen (boiling point 90 K) from nitrogen (77 K). In the laboratory there are several routes:

• Heating potassium chlorate with manganese(IV) oxide as a catalyst: $2\text{KClO}_{3}\ \xrightarrow{\text{MnO}_{2},\ \ \text{heat}}\ 2\text{KCl}\ +\ 3\text{O}_{2}$
• Decomposing hydrogen peroxide: $2\text{H}_{2}\text{O}_{2}\ \xrightarrow{\text{MnO}_{2}}\ 2\text{H}_{2}\text{O}\ +\ \text{O}_{2}$
• Heating mercury(II) oxide: $2\text{HgO}\ \xrightarrow{\text{heat}}\ 2\text{Hg}\ +\ \text{O}_{2}$
• Heating potassium permanganate: $2\text{KMnO}_{4}\ \xrightarrow{\text{heat}}\ \text{K}_{2}\text{MnO}_{4}\ +\ \text{MnO}_{2}\ +\ \text{O}_{2}$
• Electrolysis of water: $2\text{H}_{2}\text{O}\ \xrightarrow{\text{electrolysis}}\ 2\text{H}_{2}\ +\ \text{O}_{2}$

Start with 24.5 g of the salt and a full decomposition. You collect 6.72 L. Now double the mass. The volume doubles, because the line through the origin is straight. Then lower the share that decomposes to 80 percent and see the blue line drop under the grey one.

Practise with new numbers

Sorting oxides

An oxide is a compound of oxygen with one other element. We sort oxides by what they do with water, acids and bases.

• Basic oxides are oxides of metals. They give hydroxides with water and salts with acids. $\text{CaO}\ +\ \text{H}_{2}\text{O}\ \rightarrow \ \text{Ca}(\text{OH})_{2}$ and $\text{Na}_{2}\text{O}\ +\ \text{H}_{2}\text{O}\ \rightarrow \ 2\text{NaOH}$.
• Acidic oxides are oxides of non-metals. They give acids with water and salts with bases. $\text{SO}_{2}\ +\ \text{H}_{2}\text{O}\ \rightleftharpoons \ \text{H}_{2}\text{SO}_{3}$ and $\text{CO}_{2}\ +\ 2\text{NaOH}\ \rightarrow \ \text{Na}_{2}\text{CO}_{3}\ +\ \text{H}_{2}\text{O}$.
• Amphoteric oxides react with both acids and bases. Aluminium oxide and zinc oxide are the standard cases:
• $\text{Al}_{2}\text{O}_{3}\ +\ 6\text{HCl}\ \rightarrow \ 2\text{AlCl}_{3}\ +\ 3\text{H}_{2}\text{O}$ and $\text{Al}_{2}\text{O}_{3}\ +\ 2\text{NaOH}\ +\ 3\text{H}_{2}\text{O}\ \rightarrow \ 2\text{Na}[\text{Al}(\text{OH})_{4}]$.
• $\text{ZnO}\ +\ 2\text{HCl}\ \rightarrow \ \text{ZnCl}_{2}\ +\ \text{H}_{2}\text{O}$ and $\text{ZnO}\ +\ 2\text{NaOH}\ +\ \text{H}_{2}\text{O}\ \rightarrow \ \text{Na}_{2}[\text{Zn}(\text{OH})_{4}]$.
• Neutral oxides react with neither. $\text{CO}$, $\text{NO}$, $\text{N}_2\text{O}$ and $\text{H}_2\text{O}$ are neutral.

Across a period, the oxides go from basic to amphoteric to acidic. The more electronegative the element, the more acidic its oxide. For one element, a higher oxidation state gives a more acidic oxide: $\text{SO}_3$ is more acidic than $\text{SO}_2$.

Peroxides and superoxides. In a peroxide, such as sodium peroxide, the ion $\text{O}_2^{2-}$ holds two oxygen atoms joined by a single bond. Each oxygen is $-1$. It makes oxygen with water: $2\text{Na}_{2}\text{O}_{2}\ +\ 2\text{H}_{2}\text{O}\ \rightarrow \ 4\text{NaOH}\ +\ \text{O}_{2}$. In a superoxide, such as potassium superoxide, the ion is $\text{O}_2^{-}$, with oxygen at $-\tfrac{1}{2}$. Space crews use it to remove carbon dioxide and make oxygen: $4\text{KO}_{2}\ +\ 2\text{CO}_{2}\ \rightarrow \ 2\text{K}_{2}\text{CO}_{3}\ +\ 3\text{O}_{2}$.

Try it in Python

The program uses the bond enthalpies of the H-E bond. It shows how fast the bond weakens down the group, which is why the hydrides become more acidic.

A second program: oxygen from potassium chlorate

The program finds the moles, the mass and the volume of oxygen from a mass of the salt. It has a line for a partial decomposition.

Common mistakes

• Giving oxygen a covalency of 6. Oxygen has no d orbitals, so it cannot form six bonds. Sulphur forms $\text{SF}_6$.
• Saying that water boils high because the O-H bond is strong. It boils high because of hydrogen bonds between molecules. Breaking the bond is not part of boiling.
• Calling $\text{CO}$ an acidic oxide. It is neutral. It does not react with water or with bases in the cold.
• Forgetting the ratio 3 : 2. Three moles of oxygen come from two moles of potassium chlorate.
• Writing the amphoteric oxide with only an acid. An amphoteric oxide reacts with both an acid and a base.

Remember these

• Group 16: $ns^2np^4$, from non-metal (O, S) to metalloid (Se, Te) to metal (Po). States $-2$, $+2$, $+4$, $+6$.
• Oxygen: small, no d orbitals, covalency at most 2, hydrogen bonding, paramagnetic $\text{O}_2$.
• $\text{H}_2\text{E}$: the angle falls to about 90°, acidity and reducing power rise, stability falls.
• Oxides: basic (metals), amphoteric ($\text{Al}_{2}\text{O}_{3}$, $\text{ZnO}$), acidic (non-metals), neutral ($\text{CO}$, $\text{NO}$, $\text{N}_{2}\text{O}$).
• Peroxide has $\text{O}_2^{2-}$ and superoxide has $\text{O}_2^{-}$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Water is a liquid and hydrogen sulphide is a gas at room temperature. Explain.

2. Why is $\text{H}_2\text{Te}$ a stronger acid than $\text{H}_2\text{S}$?

3. Aluminium oxide dissolves in hydrochloric acid and in sodium hydroxide solution. What kind of oxide is it?

4. What volume of oxygen at 273 K and 1 atm comes from 0.20 mol of hydrogen peroxide, $2\text{H}_{2}\text{O}_{2}\ \rightarrow \ 2\text{H}_{2}\text{O}\ +\ \text{O}_{2}$?

References

• NCERT, Chemistry Part I for Class XII, the chapter The p-Block Elements, on group 16, oxygen and its oxides.
• OpenStax, Chemistry 2e, Chapter 18, on the occurrence, preparation and compounds of oxygen.