Anomalous Properties of Boron
The group-13 elements present in the modern periodic table are much better known as the members of the Boron family. The members of the boron family exhibit a wide range of both physical and chemical properties. The electronic configuration of the elements of the boron family can be given by ns2 np1.
The members of this family include Boron (B), Gallium(Ga), Aluminium (Al), Thallium (Tl), Indium (In) and including a radioactive synthetic element, Nihonium (Nh), which was formerly known as ununtrium.
Properties of Boron Family
The chemical and physical properties of the boron family members are found to follow a specific trend. Also, the properties of boron vary from the other members of the group because of the absence of the d orbital and its smaller size. These deviations in the boron properties lead to the classification of boron’s anomalous properties.
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Trends in Properties of Members of the Boron Family
Let us look at the trends in properties of the boron family members listed as follows:
The boron family members react with halogens to produce bromides, iodides, and tri-chlorides. All these halides are covalent in nature and hydrolyzed in water.
The compounds of these elements, such as octahedral [M(H2O)6]3+ (where M denotes a member of the boron family) and tetrahedral [M(OH)4]–, exists in an aqueous medium.
These trihalides are strong Lewis acids due to the deficiency of electrons.
The metallic character of the boron increases down the group while we move from boron to thallium.
First, the electronegativity of the elements decreases down the group from B to Al and, after that, increases marginally due to the discrepancies existing in the atomic size of the elements.
Anomalous Properties of Boron
Because of the unavailability of d-electrons and their smaller size, boron is found to exhibit properties that are in contrast to the other elements associated with the boron family. These properties are referred to as anomalous properties of boron. A few of these anomalous properties can be listed as follows:
Except for boron, the compounds of the elements of the boron family such as octahedral [M(H2O)6]3+ (where M denotes the member of boron family), and tetrahedral [M(OH)4]– exists in an aqueous medium.
Due to the absence of d orbitals, the maximum covalency of boron is 4.
While the rest of the family are post-transition metals, boron is given as a metalloid.
Hydroxides and boron oxides are of an acidic nature, while, on the other hand, the other elements in the boron family form hydroxides and oxides of an amphoteric nature.
Characteristics of Boron Family
The boron group is notable for its trends in the electron configuration and a few of its characteristics of the elements. Boron varies from the other group members in its refractivity, reluctance, and hardness to participate in metallic bonding. One of the examples of a trend in reactivity is given as the tendency of boron to form reactive compounds with hydrogen.
While located in the p-block, the party is notorious for the octet violation rule of boron and (to a lesser extent) aluminium by its members. These elements can place only six electrons (in 3 molecular orbitals) onto the valence shell. All the members of this group are characterized as trivalent.
Chemical Reactivity
Hydrides
Most of the elements found in the boron group show increasing reactivity as the elements get heavier in the atomic mass and higher in the atomic number. Boron, which is the first element in the group, is normally unreactive with several elements except at high temperatures, though it is capable of producing several compounds with hydrogen, at times called boranes. The simplest borane is either B2H6 or diborane. B10H14 is another example.
Oxides
All the boron-group elements are much known to produce a trivalent oxide, involving two atoms of the element, which is covalently bonded with three oxygen atoms. These elements exhibit an increasing pH trend (from acidic to basic).
Toxicity
All the elements in the boron group can be said to be toxic, given a high enough dose. A few of them are only toxic to animals, some only to plants, and a few to both.
An example of boron toxicity: It has been noticed to harm barley in concentrations exceeding 20 mm. The boron toxicity symptoms are numerous in plants. As per the research, they include decreased shoot and root growth, reduced cell division, inhibition of photosynthesis, decreased production of leaf chlorophyll, reduced proton extrusion from roots, lowering of stomata conductance, and the deposition of suborgin and lignin.
Aluminium does not give a prominent toxicity hazard in smaller quantities, but it is slightly toxic in very large doses. Gallium is not considered to be toxic, although it may contain some minor effects. Indium is not toxic and can be handled with approximately similar precautions as gallium, but a few of its compounds are slightly to moderately toxic.
FAQs on Boron Family
1. What is the Boron family and which elements does it include?
The Boron family refers to the elements in Group 13 of the modern periodic table. These elements are characterised by having three valence electrons. The members of this family are: Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), Thallium (Tl), and the synthetic, radioactive element Nihonium (Nh).
2. What is the general valence shell electronic configuration for the Boron family?
The general valence shell electronic configuration for the elements of the Boron family is ns²np¹. This configuration indicates that each element in this group has three electrons in its outermost shell, which determines many of their common chemical properties, such as the tendency to form a +3 oxidation state.
3. Why does boron show anomalous properties compared to other elements in its family?
Boron exhibits properties that are significantly different from other Group 13 elements. This anomalous behaviour is primarily due to:
- Extremely small atomic size: Boron is the smallest element in the group.
- High ionisation enthalpy: It requires a large amount of energy to remove its valence electrons.
- High electronegativity: It has a greater tendency to attract electrons compared to other members.
- Absence of d-orbitals: Unlike Al, Ga, In, and Tl, boron does not have d-orbitals in its valence shell, limiting its maximum covalency to 4.
4. What are the common oxidation states exhibited by the Boron family elements?
The Boron family elements primarily exhibit a +3 oxidation state by using all three valence electrons. However, as we move down the group, the stability of the +1 oxidation state increases and becomes more predominant for heavier elements like Thallium. This trend is explained by the 'inert pair effect'.
5. How does the inert pair effect explain the stability of the +1 oxidation state in the lower members of the Boron family?
The inert pair effect is the reluctance of the two s-orbital electrons in the valence shell (the ns² pair) to participate in chemical bonding. In heavier elements like Indium and Thallium, the intervening d- and f-orbitals offer poor shielding of the nuclear charge. This makes the s-electrons more tightly held by the nucleus and less available for bonding. As a result, only the single p-electron is lost, leading to a stable +1 oxidation state.
6. Why do the trihalides of Boron family elements act as strong Lewis acids?
The trihalides of the Boron family elements, such as Boron trifluoride (BF₃) or Aluminium chloride (AlCl₃), act as strong Lewis acids because the central atom has an incomplete octet. With only six electrons in its valence shell after forming three covalent bonds, the central atom has a vacant p-orbital and can readily accept a pair of electrons from a Lewis base to complete its octet.
7. What is unique about the structure of diborane (B₂H₆), and why can't it be explained by simple covalent bonding rules?
Diborane (B₂H₆) is an electron-deficient molecule. It has 12 valence electrons, but a simple ethane-like structure would require 14 electrons to form all the necessary two-centre, two-electron bonds. Its unique structure is explained by the presence of two 3-centre-2-electron (3c-2e) bonds, often called 'banana bonds'. In these bonds, a pair of electrons binds three atoms (B-H-B) together, allowing the molecule to achieve stability despite its electron deficiency.
8. How does the nature of oxides change down the group in the Boron family?
The chemical nature of the oxides of Group 13 elements shows a clear trend down the group:
- Boron oxide (B₂O₃) is purely acidic.
- Aluminium (Al₂O₃) and Gallium (Ga₂O₃) oxides are amphoteric, meaning they react with both acids and bases.
- Indium (In₂O₃) and Thallium (Tl₂O₃) oxides are predominantly basic in nature.
9. What are some important real-world applications of Boron and Aluminium?
Boron and Aluminium have numerous important applications. For example:
- Boron: It is used to make heat-resistant borosilicate glass (like Pyrex), as a cleaning agent in the form of borax, and in the nuclear industry as a neutron absorber in control rods.
- Aluminium: It is widely used for making lightweight alloys for aircraft and vehicles, manufacturing kitchen utensils due to its good thermal conductivity, and in packaging materials like foils and cans because of its resistance to corrosion.
10. Why are the elements of the Boron family sometimes called 'Icosagens'?
The name 'Icosagens' is sometimes used for the Boron family because the crystalline form of elemental boron is composed of complex structural units based on the icosahedron. An icosahedron is a polyhedron with 20 faces. This unique and stable structure, formed from B₁₂ units, is a defining characteristic of elemental boron and gives the group this alternative name.
