Group 13 Elements: The Boron Family
The semi-metal boron (B) and the metals aluminium (Al), gallium (Ga), indium (In), and thallium (T) are all members of the boron family, which is found in group 13 of the periodic table (Tl). With the valence electron configuration ns2np1, aluminium, gallium, indium, and thallium have three electrons in their outermost shell (a complete s orbital and one electron in the p orbital). The boron family elements have oxidation states of +3 or +1. Except for heavier elements like Tl, which prefer the +1 oxidation state due to its stability, the +3 oxidation states are preferred; this is known as the inert pair effect.
Group 13 Elements: The Boron Family
The first group in the p-block is the periodic table’s group 13 elements. The boron family refers to all of the elements in group 13. The periodic table is divided into four sections: s, p, d, and f. This segregation is based on the valence electron; if the valence electron is on the p subshell, it enters the p-block, and so on. Boron, Aluminium, Gallium, Indium, and Thallium are Group 13 elements.
The general electronic configuration for the group 13 elements is ns2 np1
Oxidation States and Inert Pair Effect
The group 13 elements have the following general oxidation states: +3, and +1. The tendency to form +1 ion increases as we move down the group as explained by the inert pair effect which is the absence of the s-orbital during chemical bonding as a result of poor shielding of the intervening electrons. Electrons fill the d and f orbitals of elements such as Indium and Thallium. Because the d and f orbitals have poor shielding abilities, the nuclear charge that seeps through attracts the s orbital closer to the nucleus. As a result, the s orbital is reluctant to bond, and only the p electrons are involved in bonding.
Covalent Character of Group 13 Elements
There are three reasons why group 13 elements form covalent compounds.
- Fajan’s rule can be used. The greater the covalence, the smaller the cation.
- They have extremely high ionization enthalpies, making the formation of ionic compounds difficult.
- Since they have higher electronegativities, the formation of compounds would not result in a higher electronegativity difference.
Reason Behind the Anomalous Behaviour of Boron
- Boron behaves differently than the other elements in group 13 for the following reasons.
- It is very small in size.
- It has an extremely high ionization enthalpy.
- Because of its small size, it has a high electronegativity. The valence shell’s lack of d-orbital.
Chemical Properties of Group 13 Elements
- Reactivity of Group 13 towards Oxygen
At high temperatures, all of the elements in Group 13 react to form trioxides, M2O3.
4M(s) + O2 (g) → 2M2O3(s)
Tl, in addition to producing Tl2O3, can also produce Tl2O. The reactivity of group 13 elements to oxygen increases as one moves down the group. Boron, in its crystalline form, is unreactive to oxygen. When heated, finely divided amorphous boron reacts with oxygen to form B2O3. Aluminium should react with air thermodynamically, but it is stable. This is because Al2O3 forms a protective layer on the metal’s surface, rendering it inert.
- Reactivity of Group 13 towards Acids and Alkalis
Boron does not react with non-oxidizing acids such as HCl, but at higher temperatures, it reacts with strong oxidizing acids such as a hot concentrated mixture of H2SO4 and HNO3 to produce boric acid.
B(s) + 3HNO3 (aq) → H3BO3 (aq) + 3NO2 (g)
Boron is resistant to alkali (NaOH and KOH) up to 773 K, after which it forms borates.
2B(s) + 6KOH(s) → 2K3BO3(s) + 3H2(g)
The remaining elements in group 13 react with both non-oxidizing and oxidizing acids, releasing hydrogen gas.
- Reactivity of Group 13 towards Halogens
At high temperatures, they react with halogens to form trihalides MX3. Tl, on the other hand, only produces TlF3 and TlCl3.
2M(s) + 3X2 (g) → 2MX3
- Reactivity towards Water
Boron does not react with water or steam, but it does react with steam at very high temperatures.
2B + 3H2O → B2O3 + 3H2
In the absence of an oxide layer, aluminium decomposes cold water to produce hydrogen gas. Unless there is an oxygen gas present, gallium and indium do not react with water. In moist air, thallium produces TlOH.
4Tl + 2H2O + O2 → 4TlOH
- Reactivity towards Metals
Borides are formed when boron combines with metals. The remaining elements in Group 13 are wary of combining with metals. Boron’s nonmetallic nature is portrayed in this way.
3Mg + 2B → Mg3B2
Given a high enough dose, all elements in the boron group can be considered toxic. Some are only toxic to animals, while others are only toxic to plants, and still, others are toxic to both. It has been observed, for example, to harm barley at concentrations greater than 20 mm. Plants exhibit a wide range of boron toxicity symptoms. According to the study, they include decreased shoot and root growth, decreased cell division, photosynthesis inhibition, decreased production of leaf chlorophyll, decreased proton extrusion from roots, decreased stomatal conductance, and the deposition of suberin and lignin.
Aluminium does not pose a significant toxicity risk in small doses, but it is slightly toxic in very large doses. Gallium is not considered toxic, though it may have some minor side effects. Although indium is not toxic and can be handled with similar precautions as gallium, a few of its compounds are mild to moderately toxic.
Physical Properties of Group 13 Elements
- Atomic and Ionic Radii
The atomic radii of group 13 elements are smaller than those of group 2 elements. This is due to an increase in the effective nuclear charge, which causes the atom’s size to shrink. Because of the addition of a new shell, the atomic and ionic radii decrease down the group. However, there is a difference when switching from Aluminium (143 pm) to Gallium (135 pm). This is due to Gallium’s poor shielding of the intervening d-orbitals, which results in a smaller size than Aluminium.
Boron< Aluminium > Gallium < Indium< Thallium
- Ionization Energy
The values of Ionization Enthalpy do not decrease smoothly down the group. The Ionization Enthalpy increases as expected from Boron to Aluminium. However, the Ionization Enthalpy increases slightly from Aluminium to Gallium. Thallium has a higher first ionisation enthalpy than Aluminium. This pattern is caused by the poor shielding of the d and f orbitals.
The electronegativity decreases from B to Al, then slightly increases from Aluminium to Tl. This is due to the ineffective shielding of the intervening d and f orbitals.
The expected trend should be the inverse of electronegativity. From B to Al, the metallic character increases slightly, then decreases slightly from Al to Tl. This is due to group 13’s extremely high Ionization Enthalpy. In addition, the larger the ion, the lower its Ionization Enthalpy. As a result, aluminium is the most metallic. This can be explained further using the standard reduction potentials.
- Acid-Base Characteristics
The acidic character of group 13 elements decreases as they move down the group, while the basic character increases.
Question 1: What is Fajan’s rule?
Fajans’ rule determines whether a chemical bond is covalent or ionic.
Question 2: Why aluminium has low density than boron?
Density rises as one progresses from boron to thallium. Boron and aluminium, on the other hand, have relatively low values. This is because they have lower atomic masses than gallium, indium, and thallium.
Question 3: How are the properties of aluminium similar to boron?
Boron and aluminium are both members of the same elemental group (13th group). Elements with similar chemical properties are grouped together in the modern periodic table. Boron and aluminium both have three valence electrons and exhibit three valency.
Question 4: Give one application of the boron family.
Boron can be found in a variety of industrial applications, and new ones are constantly being discovered. Fiberglass is a widely used material. The market for borosilicate glass has also grown rapidly; one of the family’s distinguishing characteristics is greater resistance to thermal expansion when compared to regular glass.
Ceramics are another commercially expanding application for boron and its derivatives. Many boron compounds, particularly the oxides, have valuable and unique properties that have led to their substitution for less useful materials. Boron’s insulating properties can be found in vases, pots, ceramic pan-handles, and plates.
Question 5: What is Borax?
Borax is both a refined compound and a mineral with numerous applications. This mineral exists in the colourless form of soft white crystals that can occasionally be tinged with yellow, green, or brown.