Group 13 Elements

Group 13 Elements are a group of elements in the periodic table consisting of boron (B), aluminum (Al), gallium (Ga), indium (In) and thallium (Tl).

The elements of Group 13 are located in the p-block of the periodic table and are referred to as the boron family. The periodic table is divided into four blocks based on the valence electron; s, p, d and f. If the valence electron is found in the p subshell, it is placed in the p-block and so on.

The Group 13 Elements are:

Boron

Aluminum

Gallium

Indium

**Thallium**

The general electronic configuration for the group 13 elements is ns^2 np^1.

Oxidation States and Inert Pair Effect

Covalent Character of Group 13 Elements

Anomalous Behaviour of Boron

Chemical Properties of Group 13 Elements

Physical Properties of Group 13 Elements

Compounds of Group 13 Elements

Oxidation States and Inert Pair Effect

The general oxidation states exhibited by the group 13 elements in the group are +3 and +1. As we go down the group of Boron family, the tendency to form +1 ion increases. This is due to the inert pair effect.

To elucidate, consider B³⁺ and B⁺. It is shown experimentally that B³⁺ is more stable than B⁺. Now consider, Tl³⁺ and Tl⁺. It was seen that Tl⁺ is more stable than Tl³⁺.

The inert pair effect can be used to explain why the s-orbital does not participate in chemical bonding. This is because the intervening electrons are not adequately shielded.

Simply put, for elements like Indium and Thallium, the d and f orbitals are filled with electrons. The shielding ability of d and f orbitals is very poor, so the nuclear charge seeps through and attracts the s orbital closer to the nucleus. This makes the s orbital reluctant to bond, so only the p electrons are involved in bonding.

Covalent Character of Group 13 Elements

There are three main factors that lead to the formation of covalent compounds by group 13 elements:

Electronegativity of the elements
Small size of the elements
Low ionization energies of the elements

The smaller the cation, the greater the covalence, and Fajan’s rule may be applied.

The high ionisation enthalpies (IE1+IE2+IE3) of their elements make it difficult to form ionic compounds.

The relatively higher electronegativities of these elements mean that the formation of compounds would not lead to a large difference in electronegativities.

⇒ Also Read:

Boron

Aluminium

Gallium

Indium

Thallium

Causes of Anomalous Behaviour of Boron

Boron behaves differently from the other elements in Group 13 due to the following reasons:

It has a very tiny size

It has an extremely high ionization enthalpy.

It has high electronegativity due to its small size

The absence of d-orbital in the valence shell

⇒ Check: List of all periodic table elements

Chemical Properties of Group 13 Elements

Reactivity of Group 13 Elements towards Oxygen

All elements of group 13 form trioxides, M2O3, when reacted at high temperatures.

4M(s) + 3O2 (g) → 2M2O3(s)

Tl besides forming Tl_2O_3 also forms Tl_2O.

The reactivity of Group 13 elements towards oxygen increases down the group.

Finely divided amorphous boron reacts with oxygen on heating to form B2O3, whereas Boron in its crystalline form is unreactive towards oxygen.

Aluminium is thermodynamically expected to react with air, but it is stable due to the formation of Al2O3 as a protective layer on its surface, making it inert.

Reactivity of Group 13 Elements towards Acids and Alkalis

Boron does not react with non-oxidising acids such as HCl, however, at higher temperatures it does react with strong oxidizing acids, such as a mixture of hot concentrated H2SO4 and HNO3, to produce boric acid.

B(s) + 3HNO3 (aq) → H3BO3 (aq) + 3NO2 (g)

Boron resists the action of alkalis (NaOH and KOH) up to 773 K, above which they form borates.

2B(s) + 6KOH(s) → 2K3BO3(s) + 3H2(g)

All elements in group 13 of the periodic table react with both non-oxidizing and oxidizing acids, releasing hydrogen gas.

Note: The action of concentrated HNO3 forms a protective layer of oxide on Aluminium and Gallium, rendering them passive.

Aluminium and Gallium can also react with alkalis, releasing hydrogen gas.

2Al(s) + 2NaOH (aq) + 6H2O (l) → 2NaAl(OH)_4 + H2(g)

Reactivity of Group 13 Elements towards Halogens

They react with halogens at high temperatures to form trihalides MX$_3$.

2M(s) + 3X₂(g) → 2MX₃(s) (where X = F, Cl, Br, I)

Tl however, only forms TlF3 and TlCl3.

Note: TL also forms mono-halides.

Reactivity of Group 13 Elements towards Water and Metals

Reactivity with Water:

Boron does not react with water or steam; however, at very high temperatures, it reacts with steam.

2B + 3H₂O → B₂O₃ + 3H₂

If the oxide layer is absent, Aluminium decomposes cold water to yield hydrogen gas. Gallium and Indium do not react with water unless oxygen gas is present. Thallium forms TlOH in moist air.

4Tl + 2H2O + O2 → 4TlOH

Reactivity of Metals:

Only Boron combines with metals to form borides, while the other elements of Group 13 display a more non-metallic character. This illustrates Boron’s distinct nature.

3Mg + 2B → Mg3B2

Complex Forming Tendency

Group 13 elements have a greater tendency to form complexes than s-block elements, due to their smaller size and increased polarising power.

Boron can form many complexes like BF₄–. It has an sp³ hybridized orbitals and tetrahedral geometry. Other elements also form complex compounds like Li[AlH₄], [GaCl₆]^3-.

Physical Properties of Group 13 Elements

Atomic and Ionic Radii

The atomic radii of group 13 elements are smaller than their corresponding group 2 elements. This is because the effective nuclear charge increases, which makes the size of the atom smaller.

The atomic and ionic radii increase down the group due to the addition of a new shell. There is a deviation, however, on moving from Aluminium (143 pm) to Gallium (135 pm). This arises due to the poor shielding of the intervening d-orbitals in Gallium, making the size smaller than Aluminium.

Boron < Aluminium < Gallium < Indium < Thallium

Ionization Energy

From Boron to Aluminium, the Ionization Enthalpy increases as expected; however, from Aluminium to Gallium, the Ionization Enthalpy increases only slightly. Furthermore, the first Ionization Enthalpy of Thallium is greater than that of Aluminium, which is unexpected given the general trend of decreasing Ionization Enthalpy down the group.

Explanation: This trend is observed because the d and f orbitals have poor shielding. Gallium is smaller than Aluminium because the d shielding is poor, thus resulting in Aluminium having a lower IE1 than Gallium. Similarly, Thallium has intervening f orbitals which have poor shielding, causing Thallium to have a higher IE1.

| Element | IE1 (KJmol^-1) |

| B | 801 |

| Al | 577 |

| Gallium | 579 |

| In | 558 |

| TL | 589 |

Electronegativity

The electronegativity first decreases from B to Al, then it increases slightly from Aluminium to Tl. This can be attributed to the poor shielding of the intervening d and f orbitals.

Electropositivity

The trend expected should be the exact opposite of electronegativity. The metallic character first increases from B to Al, then it decreases slightly from Aluminium to Tl.

The high Ionization Enthalpy of Group 13 is the primary reason for Aluminium being the most metallic. Additionally, the larger the size of an ion, the lower its Ionization Enthalpy. This can be further clarified by looking at the standard reduction potentials.

Element	B	Al	Ga	In	Tl
Symbol	B	Al	Ga	In	Tl

| M3+(aq) | -0.87 V | M(s) | -1.66 V | M2+(aq) | -0.56 V | M3+(aq) | -0.34 V | M4+(aq) | +1.26 V |

This shows that Aluminium is the most metallic and that Tl³⁺ isn’t that stable, as the potential is positive, making Gibb’s free energy positive(∆G = -nFE).

Density

Group 13 elements have higher densities than those of group 2 because they have smaller sizes and therefore smaller volumes. The density increases from Boron (B) to Thallium (Tl).

Acid-Base Characteristics

The acidic character of oxides of group 13 elements decreases down the group and the basic character increases.

Note: Both Al and Gallium exhibit amphoterism.

H3BO3 is a monobasic acid which acts as a Lewis acid when dissolved in water. This is because water acts as a Lewis base, resulting in the release of a proton. More information about boric acid.

B (OH)_3 + H_2O \leftrightharpoons [B (OH)_4]^- + H^+

Compounds of Group 13 Elements

Oxides

Group 13 elements form sesquioxides, with the formula MO3/2 or M2O3, where ‘sesqui’ means one and a half.

B2O3 is formed by heating amorphous boron in air.

4B + 3O2 → 2B2O3

Boron suboxide (BO$_2$) is formed when B$_2$O$_3$ is heated with boron at 1050°C.

B + B_2O_3 → (BO)_2

The nitrates or hydroxides of other elements can be decomposed thermally to produce their oxides.

2Al(OH)3 → Al2O3 + 3H2O

Halides

Boron forms trihalides with Fluorine, Chlorine, and Iodine. All of the trihalides formed are planar molecules and sp2 hybridized.

Since all the elements of group 13 possess only 6 electrons in their valance shell, they act as Lewis acids, due to their tendency to accept lone pairs of electrons.

The order of Lewis acid character exhibited by the trihalides is:

BB r3>BC l3>BF 3

Conclusion: We can conclude that BF3 is the most acidic as F is the most electronegative.

The trend is due to pπ-pπ back bonding. The lone pair on F is donated to the empty p-orbital of B, making it less electropositive and thus reducing its acidic character.

The overlap of B and F is maximum as their sizes are compatible. Boron cannot form effective back bonding with Cl or Br as they are bigger than B. The halides of Al, Gallium, In and Tl are largely covalent.

Borates

Borates are compounds of group 13 that contain distinct [BO3]3- units. Each unit is sp2 hybridized, and they are classified according to the way the individual units are connected.

Orthoborates: They contain discrete BO₃^3- units. For example, Mg₃(BO₃)₂.

Pyroborates: Two units of BO₃^3- are linked via a common oxygen atom. The formula is B₂O₅^4-. For example, Mg₂B₂O₅.

Metaborates: They have a structure where each unit shares two oxygen atoms, and are either in the form of chains or cyclic. The general formula is (BO₂)nⁿ.

Sheet Borate: The two-dimensional network of borates where all three oxygen atoms are connected to each other.

Boron Hydrides

Boranes are binary compounds formed by boron and hydrogen, with the simplest one being B2H6. These compounds are classified into three major types.

Closo-boranes (BnH2n+2)

Nido-boranes (BnH2n+4)

Arachno-boranes (BnH$_{2n+6}$)

Diborane (B_2H_6)

It can be prepared by reacting BCl3 with hydrogen gas over a Cu-Al catalyst at 450°C.

2BCl3 + 6H2 → 6HCl + B2H6

On heating Diborane, either alone or in the presence of hydrogen, higher boranes are produced.

Structure of Diborane

The total number of valence electrons present in diborane = 3 × 2 + 1 × 6 = 12 electrons.

The number of valence electrons in ethane (C₂H₆) = 4 × 2 + 1 × 6 = 14 electrons.

Therefore, we can conclude that diborane is electron deficient, making it an unstable molecule.

We were able to determine the structure of diborane through data obtained from electron diffraction studies.

It has two types of hydrogen atoms: terminal and bridged. The four terminal B-H bonds have the same bond length and are normal covalent bonds.

The two bridged hydrogen atoms have an H-B-H bond which is much longer than the terminal B-H bond. This H-B-H bond is unusual as it involves only one electron from each hydrogen atom, giving a total of four electrons. Thus, each H-B-H bond has two electrons delocalized over three centres, resulting in a three-centred two-electron bond.

Borazine

Borazine (B₃N₃H₆) is a species of Boron-Nitrogen that carries only one substituent on each atom and exists as trimers.

Heating diborane and ammonia in a 1:2 molar ratio at -120°C produces ionic species, which, when heated, yields borazine.

2BH6 + 2NH3 → [B2H6(NH3)2]+[NH4]– → B3N3H6 + 6H2

Similarities Between Borazine and Benzene

Borazine is isoelectric with benzene, i.e. both molecules have a total of 42 electrons. This is because benzene has 6 × 6 + 1 × 6 = 42 electrons, and borazine has 3 × 5 + 3 × 7 + 6 × 1 = 42 electrons.
Borazine is isosteric with benzene, meaning that they have the same number of atoms.

3. Borazine has a cyclic structure composed of alternating boron and nitrogen atoms.

4. Both the N and B atoms are sp² hybridized.

Properties of Borazine

The B-N bond is polar, unlike the covalent C-C bond, which makes it more susceptible to addition reactions.