Reason Behind the Anomalous Behaviour of Boron

Boron is a chemical element found in group 13 of the periodic table, and is essential for the growth of plants. It is also widely used in industries. It is pure crystalline in nature, and is a lustrous black semiconductor, which means it acts like a metal at extremely high temperatures and an insulator if the temperature drops.

Boron is extremely brittle and can’t be used as a tool. But it is hard enough to scratch some abrasives. These anomalous properties of boron occur due to various reasons. This article explores the reason behind the anomalous behaviour of boron.

Boron’s Properties, Use, and Occurrences

Boron was first discovered by Joseph-Louis Gay-Lussac and Louis-Jacques Thenard. These French chemists found boron in an isolated form. Later British chemist Sir Humphry Davy discovered boron oxide (B2O3) along with potassium. For many centuries, this amorphous product was only in the form of black powder. 

Boron has amorphous properties and is used as a rocket fuel igniter and in pyrotechnic flares. Due to the properties of the element, the flares have a distinctive green colour. Boron is composed of compounds such as boric (or boracic acid), boric oxide, and borax (sodium borate). These components can be found in everyday-use products like eye drops, washing powders, and antiseptics. 

It is hard to obtain boron in the pure crystalline form. This is usually procured by reducing chloride or bromide (BBr3, BCl3) with hydrogen. This reaction is performed on an electrically-heated tantalum filament. Boron is used on a limited scale to harden steel. Many non-ferrous industries use boron as a deoxidiser. At the same time,  boron is also used as a detoxifier for copper-base alloys.

Basic boron facts 

• It is the fifth element of the periodic table.

• The atomic number is 5.

• B is used to represent boron.

• Boron belongs to group 13, period 2, and block P in the periodic table. 

• This element belongs to the metalloid family.

• Electron configuration is [He]2s2²p¹

• Natural boron is made of two stable isotopes, B-10 and B-11.

• Boron’s melting point is 2350 K (2077 ºC or 3771 ºF), and its boiling point is 4000 K (4273 ºC or 7232 ºF).

• The density is 2.08g/cm3

Reasons behind the Anomalous behaviour of Boron

Firstly, the anomalous behaviour of any element refers to its unique properties as compared to another element of the same group. Boron, which is the 13th element of the periodic table, represents certain anomalous properties. It also differs from other elements in its family. The reasons behind the anomalous behaviour of boron include: 

• Boron has an exceptionally small atomic number and ionic size. 

• Boron does not have a d orbital in its valence shell. 

• This element has a high ionisation enthalpy since its size is small.

• Other elements of the family are metals, while boron is a metalloid.

• Boron is a bad conductor of electricity as opposed to other elements. 

• This element occurs in amorphous, as well as crystalline form, and it shows allotropy. 

Anomalous properties of Boron:

• As boron is a small element, it is harder compared with others. 

• It also has a high melting and boiling point. 

• Other elements in the group form ionic and covalent compounds, whereas boron forms only covalent compounds. 

• Boron oxides, along with their hydroxides are weakly acidic, which dissolves alkalis, thus forming metaborates. 

4 H3BO3 + 2 NaOH→Na2B4O7+7 H2O

• All the trihalides of group 13 elements are covalent, which helps them hydrolyze with water, whereas boron lacks a d orbital due to which its trihalides form tetrahedral species in water.

BCl3+4H2O→ [B(OH)4]–H++3HCl

• Boron is small in size due to which its trihalides are monomeric. Additionally, due to its small size, it cannot pack itself with four large halogens around it. Other elements of the boron family can dimerise and polymerise.

• The maximum covalency of B is 4 whereas other elements of the Boron family show covalency of 6.

Boron Reactions 

4 B + 3 O2 → 2 B2O3

Boron does not react with water, hydrofluoric acid, or hydrochloric acid. The element reacts with only certain elements and chemicals like sulfuric or nitric acid in heated conditions. Under these circumstances, boron will oxidise to form boric oxide. Thus, it can be concluded that Boron reacts only with oxidising acids.

B+HCl→ No reaction

2B + 3H2SO4→ 2H3BO3+ 3SO2 

Boron is chemically inert at room temperature. Due to its reaction with fluorine and oxygen, boron forms trifluoride and boric oxide. If the temperature increases, boron reacts with almost every element except noble gases and tellurium. This is one of the anomalous behaviours of boron that has been observed. 

Compounds of Boron 

Boron doesn’t exist in its pure form in the environment. Instead, its compounds exist in the form of borax (Na₂[B₄O₅(OH)₄]·8H₂O), boric acid (H3BO3), and borates. It also forms permanent covalent bonds with other elements. However, it also differentiates itself with one less valence electron in its orbital. Boron has +3 oxidation states, which help the element to form other oxidation states. It is usually found in covalent form.

How does boron affect the environment? 

Boron cannot be artificially created or produced, it only exists in natural form. It can be released into the environment through water, air, or soil. Humans can also add this element by manufacturing glass, melting copper, increasing agricultural fertilisers, or through the combustion of coal. Boron self-accumulates, but can be consumed through plants that absorb minerals from the soil. 

Conclusion 

Boron shows many anomalous properties compared with other elements of the same group. This is mainly owing to its small size and absence of d orbitals in its valence shell. Thus, it follows with the trend that almost all elements of the second period show anomalous properties with its group. Boron also shows a diagonal relationship.