Group 13 trends and the odd behaviour of boron

A can, a gem and a puddle of metal

An aluminium drinks can is light, and it does not rust. Gallium is a silvery metal that melts at $303\ \text{K}$, which is $30^\circ\text{C}$. A lump of it melts in your palm. Boron is hard, black and brittle, and it is nothing like a metal. These elements sit in one group of the periodic table.

The group has five elements: boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl). Each has three electrons in its outer shell, in the pattern $ns^2\,np^1$.

The group is a good example of how a trend can have a kink in it. Let us look at the data first, and then explain it.

The trends

• B: radius $85\ \text{pm}$, first ionisation enthalpy $801\ \text{kJ/mol}$, electronegativity $2.0$.
• Al: $143\ \text{pm}$, $577\ \text{kJ/mol}$, $1.5$.
• Ga: $135\ \text{pm}$, $579\ \text{kJ/mol}$, $1.6$.
• In: $167\ \text{pm}$, $558\ \text{kJ/mol}$, $1.7$.
• Tl: $170\ \text{pm}$, $589\ \text{kJ/mol}$, $1.8$.

The radius of boron is a covalent radius. The others are metallic radii. This is how NCERT prints them, and we use these values.

Atomic radius. Going down a group, a new shell is added, so the radius should grow. It does, but not evenly. Gallium is a little smaller than aluminium. Between Al and Ga the ten 3d electrons are filled. These d electrons shield the nucleus badly from the outer electrons. The nuclear charge rises from $+13$ to $+31$, and the outer electrons feel a stronger pull.

Ionisation enthalpy. It falls sharply from boron to aluminium, because the outer electron is in a new shell and farther away. After that it does not fall steadily. Gallium is slightly above aluminium. Thallium is clearly above indium. The reason is the same: poorly shielding d electrons, and in thallium the 4f electrons too.

Electronegativity. It falls from boron to aluminium, then rises slowly from Al to Tl. This is again the effect of the poor shielding.

Metallic character. Boron is a non-metal, or a metalloid. Its melting point, $2453\ \text{K}$, is very high, because the atoms form a hard network. All the others are soft, shiny metals. Their density grows down the group, from $2.70\ \text{g/cm}^3$ for aluminium to $11.85\ \text{g/cm}^3$ for thallium.

Move the slider from 1 to 5. Notice how the red points stay far above the blue ones. The sum of the first three energies is about 9 times the first one. A 3+ ion is costly to make, and it is paid back by the energy of bonds or of the crystal.

Oxidation states

All five elements have the oxidation state $+3$. The heavier ones also show $+1$, and the $+1$ state becomes more stable going down. Thallium is happier as Tl(I) than as Tl(III). We study this inert pair effect in the third course of this chapter.

Some reactions of the group

• With air. Aluminium, gallium and indium form oxides, $4\text{E} + 3\text{O}_{2} \to 2\text{E}_{2}\text{O}_{3}$. Aluminium is protected by a thin, strong film of its oxide. Crystalline boron does not react with air at room temperature. At a high temperature boron burns, and boron and aluminium also react with nitrogen: $2\text{E} + \text{N}_{2} \to 2\text{EN}$.
• With halogens. $2\text{E} + 3\text{X}_{2} \to 2\text{EX}_{3}$ for B, Al, Ga and In.
• With acids. Aluminium dissolves in dilute hydrochloric acid: $2\text{Al} + 6\text{HCl} \to 2\text{AlCl}_{3} + 3\text{H}_{2}$. Boron does not react with acids that do not oxidise, but concentrated nitric acid turns it into boric acid: $\text{B} + 3\text{HNO}_{3} \to \text{H}_{3}\text{BO}_{3} + 3\text{NO}_{2}$.
• With alkalis. Aluminium dissolves and gives hydrogen: $2\text{Al} + 2\text{NaOH} + 6\text{H}_{2}\text{O} \to 2\text{Na}[\text{Al}(\text{OH})_{4}] + 3\text{H}_{2}$. This happens because aluminium oxide is amphoteric, which means that it reacts with acids and with bases. Boron reacts with fused sodium hydroxide: $2\text{B} + 6\text{NaOH} \to 2\text{Na}_{3}\text{BO}_{3} + 3\text{H}_{2}$.

Why boron is the odd one out

Boron differs from the other members of its group more than any other element in the p-block. Here are the reasons.

1. Small size and high ionisation enthalpy. The B$^{3+}$ ion would be tiny and would pull strongly on any anion near it. So boron never forms an ionic compound with a 3+ ion. It shares electrons, and all its compounds are covalent. Boron trifluoride is a gas, but aluminium fluoride is a solid with ionic bonds.
2. No d orbitals in the second shell. The outer shell of boron is the second shell, which has no d orbitals. Its largest covalency is 4, as in $[\text{BF}_{4}]^{-}$. Aluminium has empty 3d orbitals nearby, so it reaches a covalency of 6, as in $[\text{AlF}_{6}]^{3-}$ and $[\text{Al}(\text{H}_{2}\text{O})_{6}]^{3+}$.
3. Electron deficiency. In $\text{BF}_{3}$ the boron atom has only six electrons in its outer shell, so the compound is called electron deficient. The boron atom has an empty p orbital and accepts a pair of electrons. A substance that accepts an electron pair is a Lewis acid. So $\text{BF}_{3}$ reacts with ammonia: $\text{BF}_{3} + \text{NH}_{3} \to \text{F}_{3}\text{B}\leftarrow \text{NH}_{3}$. The arrow shows the lone pair of nitrogen moving to boron, and the flat triangle of $\text{BF}_{3}$ becomes a tetrahedron.
4. Diagonal relationship. Boron resembles silicon, its neighbour on the lower right, more than it resembles aluminium. Both have weakly acidic oxides, volatile hydrides that burn in air, and halides that are hydrolysed by water.

Practise with new numbers

Try it in Python

The program keeps the data of the group in a table. It adds the ionisation enthalpies and looks for the places where a trend breaks.

Common mistakes

• Saying that ionisation enthalpy falls steadily down group 13. It does not. Gallium is a little above aluminium, and thallium is above indium.
• Saying that gallium is larger than aluminium because it is lower in the group. It is slightly smaller, because the 3d electrons shield poorly.
• Writing B$^{3+}$ in a compound of boron. Boron forms covalent bonds.
• Giving boron a covalency of 6, as in $[\text{BF}_{6}]^{3-}$. The second shell has no d orbitals, so the limit is 4.
• Saying that aluminium does not react. It is a reactive metal. A thin oxide film protects it.

Remember these

• The outer pattern is $ns^2\,np^1$. The usual oxidation state is $+3$, and $+1$ grows more stable down the group.
• Radius: B 85, Al 143, Ga 135, In 167 and Tl 170 pm. The kink at Ga comes from the 3d electrons.
• First ionisation enthalpy (kJ/mol): B 801, Al 577, Ga 579, In 558, Tl 589.
• Boron is a non-metal. It is small, covalent, electron deficient and has a maximum covalency of 4.
• Aluminium dissolves in acids and in alkalis, and gives hydrogen.

Check yourself

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

1. Arrange B, Al, Ga, In and Tl in order of first ionisation enthalpy, the highest first.

2. Boron makes $[\text{BF}_{4}]^{-}$, but aluminium makes $[\text{AlF}_{6}]^{3-}$. Why?

3. What forms when $\text{BF}_{3}$ meets ammonia, and why does it happen?

4. Aluminium is a reactive metal, yet a pan of it does not corrode in air. Why? What happens in strong sodium hydroxide solution?

References

• OpenStax, Chemistry 2e, Chapter 18 (Representative Metals, Metalloids and Nonmetals).
• NCERT Chemistry for Class 11, the chapter The p-Block Elements (older edition), the section on group 13.
• Housecroft and Sharpe, Inorganic Chemistry, the chapter on the group 13 elements.