Trends in Chemical Properties of Group 13 Elements and Their Compounds

Lessons

Senior 4 · Chemistry

Trends in Chemical Properties of Group 13 Elements and Their Compounds

4 lessons

Lesson 4 of 4

Boron, the Odd One Out, and Uses of Group 13 Elements

25 min

Glass that does not crack

Pour boiling water into a cold jam jar and it may crack. Heat a laboratory beaker over a Bunsen burner and it survives, year after year. The difference is boron: laboratory glassware is borosilicate glass, in which some of the silicon dioxide is replaced by boron oxide. That boron behaves like silicon, two places away in the periodic table, rather than like aluminium directly below it, is the clearest sign of how different boron is from the rest of its group.

Borosilicate (Duran) laboratory glassware: a 1000 cm³ beaker, a 600 cm³ beaker and a wide test tube. Borosilicate glass expands only about a third as much as ordinary window glass when heated, so it survives sudden heating. Photo: Skatebiker at English Wikipedia, public domain, via Wikimedia Commons

Definition

Anomalous behaviour: the way the first element of a group differs from the elements below it, because its atom is very small, it is highly electronegative compared with the rest of the group, and its outer shell has no d orbitals (n = 2). Example: boron forms no $\mathrm{B^{3+}}$ ion and at most four bonds, while aluminium forms $\mathrm{Al^{3+}}$ and the six-coordinate ion $\mathrm{[AlF_6]^{3-}}$ .

Diagonal relationship: the similarity between an element of Period 2 and the element one group to the right in Period 3, for example boron and silicon. Moving right raises charge density and electronegativity; moving down lowers them; the two effects roughly cancel along the diagonal.

Lewis acid: an electron-pair acceptor. Lewis base: an electron-pair donor. Example: $\mathrm{BF_3}$ (acid) accepts the lone pair of $\mathrm{NH_3}$ (base).

How boron differs from the rest of Group 13

It is a non-metal (metalloid). Boron is a hard (it scratches glass), high-melting semiconductor; aluminium to thallium are metals.
All its compounds are covalent. Removing three electrons costs 6887.4 kJ mol⁻¹ (Lesson 1), and a tiny $\mathrm{B^{3+}}$ would polarise any anion completely.
Its maximum covalency is four. Boron has only the 2s and three 2p orbitals to bond with, so it can form at most four bonds, as in $\mathrm{[BF_4]^-}$ and $\mathrm{[B(OH)_4]^-}$ . Aluminium can use more orbitals and is large enough to hold six neighbours, as in $\mathrm{[AlF_6]^{3-}}$ (cryolite) and $\mathrm{[Al(H_2O)_6]^{3+}}$ .
Its oxide and hydroxide are acidic. $\mathrm{B_2O_3}$ dissolves in water to give boric acid, $\mathrm{B(OH)_3}$ , a very weak acid; $\mathrm{Al_2O_3}$ is amphoteric.
Its halides are monomers and strong Lewis acids. $\mathrm{BF_3}$ and $\mathrm{BCl_3}$ are gases with only six electrons round the boron. They do not dimerise like $\mathrm{Al_2Cl_6}$ , because the small boron atom cannot fit four large halogen atoms; instead they accept lone pairs from other molecules (Figure 8.7).
It forms many volatile hydrides. The boranes, such as diborane, $\mathrm{B_2H_6}$ (a gas boiling at −92.5 °C), are electron-deficient molecules held together partly by B–H–B bridges in which three atoms share one pair of electrons. Aluminium hydride is a polymeric solid.

Figure 8.7: $\mathrm{BF_3}$ is a Lewis acid: the empty p orbital on boron accepts the lone pair of ammonia, and the boron changes from trigonal planar to tetrahedral.

Boric acid: an acid that does not give up a proton

Boric acid is monobasic, but not because it loses one of its own hydrogen atoms. The boron atom, with only six outer electrons, accepts a lone pair from a water molecule, which then releases a proton:

\mathrm{B(OH)_3(aq) + 2H_2O(l) \rightleftharpoons [B(OH)_4]^-(aq) + H_3O^+(aq)} \qquad \mathrm{p}K_a = 9.24

So boric acid is a Lewis acid, and a very weak one: weak enough to be used, dilute, as a mild antiseptic.

Boron and silicon: a diagonal pair

Table 8.5: Boron resembles silicon more than aluminium

Property	Boron	Silicon	Aluminium
Type of element	metalloid, semiconductor	metalloid, semiconductor	metal, good conductor
Electronegativity	2.04	1.90	1.61
Oxide	$\mathrm{B_2O_3}$ , acidic, forms glasses	$\mathrm{SiO_2}$ , acidic, forms glasses	$\mathrm{Al_2O_3}$ , amphoteric
Chloride with water	hydrolysed completely: $\mathrm{BCl_3 + 3H_2O \rightarrow B(OH)_3 + 3HCl}$	hydrolysed completely: $\mathrm{SiCl_4 + 4H_2O \rightarrow SiO_2{\cdot}2H_2O + 4HCl}$	$\mathrm{Al_2Cl_6}$ fumes, then dissolves as $\mathrm{[Al(H_2O)_6]^{3+}}$
Hydrides	many volatile boranes	many volatile silanes	one polymeric solid
Carbide	$\mathrm{B_4C}$ , extremely hard	SiC, extremely hard	$\mathrm{Al_4C_3}$ , reacts with water

Unit 9 returns to diagonal relationships across Groups 1, 2, 13 and 14.

Worked example 1: how weak is boric acid?

Calculate the pH of 0.100 mol dm⁻³ boric acid. $\mathrm{p}K_a = 9.24$ .

Solution. $K_a = 10^{-9.24} = 5.75 \times 10^{-10}\ \mathrm{mol\,dm^{-3}}$ $[\mathrm{H_3O^+}] = \sqrt{K_a \times c} = \sqrt{5.75 \times 10^{-10} \times 0.100} = 7.59 \times 10^{-6}\ \mathrm{mol\,dm^{-3}}$ $\mathrm{pH} = -\log_{10}(7.59 \times 10^{-6}) = 5.12$ Compare 0.100 mol dm⁻³ aluminium chloride (pH about 3.0, Lesson 3): the hydrated aluminium ion is a far stronger acid than boric acid.

Worked example 2: hydrolysis of boron trichloride

1.00 g of boron trichloride is added to water. Write the equation and calculate the mass of hydrogen chloride formed. $A_{r}$ : B 10.81, Cl 35.45, H 1.01.

Solution. $\mathrm{BCl_3 + 3H_2O \rightarrow B(OH)_3 + 3HCl}$ . $M(\mathrm{BCl_3}) = 10.81 + 3(35.45) = 117.16\ \mathrm{g\,mol^{-1}} \qquad n = \frac{1.00}{117.16} = 8.54 \times 10^{-3}\ \mathrm{mol}$ $n(\mathrm{HCl}) = 3 \times 8.54 \times 10^{-3} = 0.0256\ \mathrm{mol} \qquad m = 0.0256 \times 36.46 = 0.934\ \mathrm{g}$ Water attacks the boron because it is electron-deficient: a lone pair on a water molecule forms a bond to the empty orbital first, then HCl is lost. Carbon tetrachloride, whose carbon has a full octet and no empty orbital, is not hydrolysed in this way.

Uses of Group 13 elements and compounds

Table 8.6: Uses, and the property behind each

Element or compound	Use	Property that makes it suitable
Aluminium and its alloys	aircraft, vehicle bodies, window frames, cooking pots, drink cans, foil	low density (2.70 g cm⁻³); protective oxide film resists corrosion; non-toxic
Aluminium	overhead power cables, on a steel core	good conductor: for the same current it weighs about half as much as copper
Aluminium powder	thermite welding of rails	very strong affinity for oxygen; highly exothermic reduction of $\mathrm{Fe_2O_3}$
Aluminium oxide	abrasives, furnace linings; ruby and sapphire are coloured forms	very hard, high melting (2072 °C), unreactive
Aluminium sulphate and alum	water treatment; mordant in dyeing	$\mathrm{Al^{3+}}$ hydrolyses to a floc of $\mathrm{Al(OH)_3}$ that traps fine particles
Aluminium hydroxide	antacids; flame retardant in plastics	a weak base that neutralises stomach acid; decomposes on heating, absorbing heat and releasing water
Borax and boric acid	borosilicate glass, glazes, mild antiseptic, boron fertiliser	$\mathrm{B_2O_3}$ forms glasses with low thermal expansion; boron is an essential trace nutrient for plants
Boron-10 and boron carbide	neutron absorbers in nuclear reactors; armour and abrasives	boron-10 captures neutrons; $\mathrm{B_4C}$ is one of the hardest materials known
Gallium nitride and gallium arsenide	blue and white LEDs, laser diodes, solar cells in satellites	semiconductors that emit or absorb light efficiently
Indium tin oxide (ITO)	touchscreens and flat-panel displays	transparent yet electrically conducting coating on glass

The invention of efficient blue LEDs based on gallium nitride won the 2014 Nobel Prize in Physics for Isamu Akasaki, Hiroshi Amano and Shuji Nakamura, because blue light, combined with a yellow phosphor, makes the white light of the energy-saving lamps now common in homes and street lights.

Worked example 3: an antacid tablet

An antacid tablet contains 0.250 g of aluminium hydroxide. Calculate the volume of stomach acid, taken as 0.100 mol dm⁻³ hydrochloric acid, that one tablet can neutralise. $M(\mathrm{Al(OH)_3}) = 78.0\ \mathrm{g\,mol^{-1}}$ .

Solution. $\mathrm{Al(OH)_3 + 3HCl \rightarrow AlCl_3 + 3H_2O}$ . $n(\mathrm{Al(OH)_3}) = \frac{0.250}{78.0} = 3.21 \times 10^{-3}\ \mathrm{mol} \qquad n(\mathrm{HCl}) = 3 \times 3.21 \times 10^{-3} = 9.62 \times 10^{-3}\ \mathrm{mol}$ $V = \frac{9.62 \times 10^{-3}\ \mathrm{mol}}{0.100\ \mathrm{mol\,dm^{-3}}} = 0.0962\ \mathrm{dm^3} = 96.2\ \mathrm{cm^3}$ Aluminium hydroxide is chosen because it is insoluble, so it does not make the stomach alkaline, and it reacts only as fast as acid reaches it.

Warning

Common mistakes. (1) Saying boric acid is monobasic because "only one H is acidic": it acts as a Lewis acid, accepting $\mathrm{OH^-}$ from water. (2) Writing $\mathrm{[BF_6]^{3-}}$ by analogy with $\mathrm{[AlF_6]^{3-}}$ : boron's covalency stops at four. (3) Giving "aluminium is a good conductor" as the only reason for power cables: copper conducts better; the point is conductivity per kilogram. (4) Listing uses without the property that explains them: examiners award the mark for the link.

Safety. Boric acid and borax are harmful if swallowed: never taste them, and keep them away from children. Boron trichloride and boron trifluoride are toxic, corrosive gases handled only in industry. Diborane catches fire in air. Wash your hands after handling any boron compound.

Tip

Boron's whole personality comes from small size + no d orbitals: covalent, a maximum of four bonds, an empty p orbital (Lewis acid), and a silicon-like diagonal twin. For uses, always write use → property in one sentence.

Periodic Videos: Boron (4 min). Watch for the appearance of the element itself and for the bright green flame that a volatile, covalent boron compound gives when it burns.

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Practice

Give three ways in which boron differs from aluminium, and the cause of each.
Explain why the formula $\mathrm{[AlF_6]^{3-}}$ exists but $\mathrm{[BF_6]^{3-}}$ does not.
Draw the product of the reaction of boron trichloride with trimethylamine, $\mathrm{N(CH_3)_3}$ , and label the dative bond.
Write the equation that shows why boric acid is a Lewis acid, not a proton donor.
Calculate the percentage by mass of boron in borax, $\mathrm{Na_2B_4O_7{\cdot}10H_2O}$ . $A_{r}$ : Na 22.99, B 10.81, O 16.00, H 1.01.
Give two properties of boron that are shared with silicon but not with aluminium.
For each use, name the Group 13 element or compound and the property that makes it suitable: (a) overhead power cables; (b) touchscreen coatings; (c) antacid tablets; (d) laboratory glassware.
Evaluate this claim: "Aluminium cooking pots are safe to use because aluminium is unreactive."

Show answersHide answers

Any three: boron is a metalloid, aluminium a metal (boron's small atom holds electrons tightly); boron compounds are all covalent ( $\mathrm{B^{3+}}$ would need 6887 kJ mol⁻¹ and would be extremely polarising); boron forms at most four bonds (no d orbitals, small size); $\mathrm{B_2O_3}$ is acidic, $\mathrm{Al_2O_3}$ amphoteric (higher electronegativity of boron); $\mathrm{BCl_3}$ is a monomer, $\mathrm{AlCl_3}$ dimerises (boron too small to hold four Cl).
Aluminium is large enough to hold six fluoride ions and has extra orbitals available; boron has only four valence orbitals (2s, 2p) and is too small, so its maximum is $\mathrm{[BF_4]^-}$ .
$\mathrm{Cl_3B{\leftarrow}N(CH_3)_3}$ : the lone pair on nitrogen forms a dative bond to boron, which becomes tetrahedral with an octet.
$\mathrm{B(OH)_3 + 2H_2O \rightleftharpoons [B(OH)_4]^- + H_3O^+}$ .
$M = 2(22.99) + 4(10.81) + 7(16.00) + 10(18.02) = 381.4\ \mathrm{g\,mol^{-1}}$ ; % B $= \frac{43.24}{381.4} \times 100 = 11.3\%$ .
Any two: both are metalloid semiconductors; both oxides are acidic and form glasses; both chlorides are completely hydrolysed by water; both form many volatile hydrides; both form very hard carbides.
(a) aluminium: low density and good conductivity, so less mass per unit of current; (b) indium tin oxide: transparent and conducting; (c) aluminium hydroxide: insoluble weak base that neutralises acid; (d) borosilicate glass (from boron oxide): low thermal expansion, so it resists cracking when heated.
Partly true at best. Aluminium is a reactive metal; pots are protected only by the oxide film. Acidic or salty foods cooked or stored for a long time can attack the film and dissolve some aluminium, so the pots are safe for normal cooking, but the reason is the oxide layer, not a lack of reactivity.

Quick check

Boron forms [BF₄]⁻ but never [BF₆]³⁻, although aluminium forms [AlF₆]³⁻. Why?

Key points

Boron is anomalous because its atom is small, relatively electronegative and has no d orbitals: it is a metalloid, forms only covalent compounds and at most four bonds, and its oxide is acidic.
$\mathrm{BF_3}$ , $\mathrm{BCl_3}$ and $\mathrm{B(OH)_3}$ are electron-deficient Lewis acids; boric acid is a weak, monobasic Lewis acid (p $K_{a}$ 9.24).
Boron resembles silicon (diagonal relationship): semiconducting metalloids, acidic glass-forming oxides, hydrolysed chlorides, volatile hydrides, hard carbides.
Uses follow properties: aluminium (light, conducting, protected by oxide), $\mathrm{Al_2O_3}$ (hard), alum ( $\mathrm{Al(OH)_3}$ floc), $\mathrm{Al(OH)_3}$ (antacid), borosilicate glass (low expansion), GaN (LEDs), ITO (touchscreens).

1Glass that does not crack

2How boron differs from the rest of Group 13

3Boric acid: an acid that does not give up a proton

4Boron and silicon: a diagonal pair

5Uses of Group 13 elements and compounds

Glass that does not crack

How boron differs from the rest of Group 13

Boric acid: an acid that does not give up a proton

Boron and silicon: a diagonal pair

Uses of Group 13 elements and compounds