Written By Umesh_K
Last Modified 25-01-2023

Important Trends and Anomalous Properties of Boron

Group \({\text{III}}\,{\text{A}}\) or \({13^{{\text{th}}}}\) group of the periodic table comprises of six elements, namely Boron \(\left({\text{B}} \right),\) Aluminium \(\left({{\text{Al}}} \right),\) Gallium \(\left({{\text{Ga}}} \right),\) Indium \(\left({{\text{In}}} \right),\) Thalium \(\left({{\text{Tl}}}\right),\) Nihonium \(\left({{\text{Nh}}} \right)\)

The last differentiating electron of this series enters the ‘\({\text{p}}\)’ orbitals to form the ‘\({\text{p}}\)’ block elements. The outermost electronic configuration of the elements is given by \({\text{n}}{{\text{s}}^2},{\text{n}}{{\text{p}}^1}.\) Hence, they are the first group in the ‘\({\text{p}}\)’ block elements.

While they have a lot of similarities as a group, the first two elements, \({\text{B}}\) and \({\text{Al,}}\) are distinct from the other four. Boron is a non-metal, while \({\text{Al,}}\) is a metal with qualities comparable to boron, but the other four members have stronger metallic properties. Boron also exhibits unusual behaviour and has features that are distinct from those of aluminium, which are been discussed in this article.

Physical Properties of Boron Family

The members of the boron family are less metallic than alkali \(\left({{\text{IA}}} \right)\) and alkaline Earth \(\left({{\text{IIA}}} \right)\) metals. Boron is considered a semi-metal and is much closer to non-metals in its character. The rest of the elements such as aluminium and others, exhibit metallic character.

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Trends of Boron Family

The physical properties of the elements are as follows:

1. Occurrence in Nature

Aluminium is the most abundant metal and is considered the third most abundant element after oxygen and silicon by mass on the Earth’s crust. Aluminium occurs in nature as bauxite, its oxide ore. Boron, on the other hand, is a fairly rare element and occurs in concentrated deposits in the form of borax, tourmaline, and kernite crystals.

Galium is twice as abundant as boron, and the other three elements, \({\text{Ga}},{\text{In}},\) and \({\text{Tl}}\) are available in nature as sulphides.

2. Electronic Configuration

The outer electronic configuration of the \(13{\text{th}}\) group elements is \({\text{n}}{{\text{s}}^2},{\text{n}}{{\text{p}}^1}.\)

The difference in the electronic configuration affects the properties of the elements.

1. Boron and aluminium have \({\text{s}}\) and \({\text{p}}\) electrons only
2. Gallium and Indium have additional \(10\,{\text{d}}\) electrons
3. Thallium has additional \(10\,{\text{d}}\) electrons and \(14\,{\text{f}}\) electrons

The electronic configurations of individual elements are as given in the table:

Element	Atomic Number	Electronic Configuration
Boron	\(5\)	\(1{{\text{s}}^2},2{{\text{s}}^2},2{{\text{p}}^1}\)
Aluminium	\(13\)	\(1{{\text{s}}^2},2{{\text{s}}^2},2{{\text{p}}^6},3{{\text{s}}^2},3{{\text{p}}^1}\)
Gallium	\(31\)	\(1{{\text{s}}^2},2{{\text{s}}^2},2{{\text{p}}^6},3{{\text{s}}^2},3{{\text{p}}^6},3{{\text{d}}^{10}},4{{\text{s}}^2},4{{\text{p}}^1}\)
Indium	\(49\)	\(1{{\text{s}}^2},2{{\text{s}}^2},2{{\text{p}}^6},3{{\text{s}}^2},3{{\text{p}}^6},3{{\text{d}}^{10}},4{{\text{s}}^2},4{{\text{p}}^6},4{{\text{d}}^{10}},5{{\text{s}}^2},5{{\text{p}}^1}\)
Thallium	\(81\)	\(1{{\text{s}}^2},2{{\text{s}}^2},2{{\text{p}}^6},3{{\text{s}}^2},3{{\text{p}}^6},3{{\text{d}}^{10}},4{{\text{s}}^2},4{{\text{p}}^6},4{{\text{d}}^{10}},5{{\text{s}}^2},5{{\text{p}}^6},4{{\text{f}}^{14}},5{{\text{d}}^{10}},6{{\text{s}}^2},6{{\text{p}}^1}\)
Nihonium	\(113\)	\(1{{\text{s}}^2},2{{\text{s}}^2},2{{\text{p}}^6},3{{\text{s}}^2},3{{\text{p}}^6},3{{\text{d}}^{10}},4{{\text{s}}^2},4{{\text{p}}^6},4{{\text{d}}^{10}},5{{\text{s}}^2},5{{\text{p}}^6},4{{\text{f}}^{14}},5{{\text{d}}^{10}},6{{\text{s}}^2},6{{\text{p}}^6},5{{\text{f}}^{14}},6{{\text{d}}^{10}},7{{\text{s}}^2},7{{\text{p}}^1}\)

3. Atomic and Ionic Radii

The atomic and ionic radii are as follows:

Elements	\({\text{B}}\)	\({\text{Al}}\)	\({\text{Ga}}\)	\({\text{In}}\)	\({\text{Tl}}\)
Atomic Radii \(\left({{\text{pm}}} \right)\)	\(88\)	\(143\)	\(135\)	\(167\)	\(170\)
Ionic Radii \({{\text{M}}^{3 + }}\left({{\text{pm}}} \right)\)	\(27\)	\(53.5\)	\(62\)	\(80\)	\(88.5\)

1. One extra shell is added to each successive member as we move down the group from Boron to Thallium.
2. A deviation is seen in atomic radii of Gallium (less than aluminium) due to differences in inner core electronic configuration.
3. \(10\,{\text{d}}\) electrons in \({\text{Ga}}\) offer less screening effect for its outer electrons from its increased nuclear charge.

4. Ionization Enthalpy

1. The \({13^{{\text{th}}}}\) group of elements, on moving down the group from Boron to Thallium, shows an irregular trend in ionization enthalpy.
2.Ionisation enthalpy decreases from Boron to Aluminium due to an increase in size.
3. The first ionization enthalpies of group \(13\left({{\text{IIIA}}} \right)\) elements are as shown:

Elements	Boron	Aluminium	Gallium	Indium	Thallium
First Ionization enthalpy	\(801\)	\(577\)	\(579\)	\(558\)	\(589\)

5. Electronegative Character

Due to the discrepancies in the atomic radii of the elements, the electronegativity decreases from Boron to Aluminium and then marginally increases for the other three elements of the group.

6. Melting and Boiling Points

1. Even in the melting points, the trend is not regular.
2. The melting point of the elements decreases when going down from Boron to Gallium. However, it increases in Indium and Thallium.
3. It is due to the giant, polymeric structure of boron in both solid and liquid states, while \({\text{Al}},{\text{In}}\) and \({\text{TI}}\) have closely-packed metallic structures. \({\text{Ga}}\) exists as \({\text{G}}{{\text{a}}_2}\) molecules and has a low melting point.
4. Boiling points of the elements show a regular trend and decrease from Boron to Thallium.

7. Oxidation States of Elements

The expected oxidation states of elements (according to the \({\text{n}}{{\text{s}}^2},{\text{n}}{{\text{p}}^1}\) configuration) are \( + 3\) and \( + 1.\) While boron shows \( + 3\) oxidation states in all of its compounds, the other four elements show both \( + 1\) and \( + 3\) oxidation states.

However, it is seen that:

1. The stability of the \( + 3\) oxidation state decreases from \({\text{Al}}\) to \({\text{TI}}\)
2. The stability of the \( + 1\) oxidation state increases as we move down the group.

Anomalous Behavior of Boron

Boron behaves markedly differently from other members of group \(13\left({{\text{III}} – {\text{A}}} \right).\)

Examples of anomalous or abnormal behavior of boron are shown in the following properties:

1. The trichlorides, bromides, iodides of all elements except boron are covalent, hydrolyzed in water, and form tetrahedral and octahedral species in an aqueous medium:
\({\left[{{\text{M}}{{\left({{\text{OH}}}\right)}_4}} \right]^ – },\) and \({\left[{{\text{M}}{{\left({{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}.\)
2. Boron trifluoride, being electron-deficient, reacts with Lewis bases like NH3 to complete the outer shell octet.
\({{\text{F}}_3}{\text{B}} +{\text{N}}{{\text{H}}_3}: \to {{\text{F}}_3}{\text{B}} \leftarrow {\text{N}}{{\text{H}}_3}\)
3. Due to the absence of \({\text{d}}\) orbitals in boron, the maximum covalency of boron is \(4.\) But since \({\text{d}}\) orbitals are available in the other \(4\) elements of the group, their maximum covalency is more than \(4.\)
4. Other metal halides (Eg: \({\text{AlC}}{{\text{l}}_3}\)) are dimerized by halogen bridging \(\left( {{\rm{A}}{{\rm{l}}_2}{\rm{C}}{{\rm{l}}_6}} \right).\) The octet of the metal species gets completed by electrons obtained from halogens in the bridging.
5. Since boron has small atomic radii, it has a greater nuclear attraction on its outermost electrons. So, it has high ionization energy, and therefore, exhibits a non-metallic character.
6. Only boron exhibit allotropy amongst the \({13^{{\text{th}}}}\) group elements.
7. Boron exhibits only a \( + 3\) oxidation state, unlike others which can exhibit both \( + 1\) and \( + 3\) oxidation states.
8. The oxide of Boron, \({{\text{B}}_2}{{\text{O}}_3},\) is acidic, while the oxides of other elements are either amphoteric or basic.
9. With metals, boron forms borides, while the other \(4\) elements in the group do not react but form alloys with metals.

Important Compounds of Boron

Boron forms several important compounds with other elements. Some examples include:

S.NO	Name of the Compound	Formula
1.	Boron Trioxide	\({{\text{B}}_2}{{\text{O}}_3}\)
2.	Borax	\({\text{N}}{{\text{a}}_2}{{\text{B}}_4}{{\text{O}}_7}.10{{\text{H}}_2}{\text{O}}\)
3.	Orthoboric Acid	\({{\text{H}}_3}{\text{B}}{{\text{O}}_3}\)
4.	Boron Nitride	\({\text{BN}}\)
5.	Boron Halide	\({\text{B}}{{\text{X}}_3}\left({{\text{X}} = {\text{Cl}},{\text{Br}},{\text{I}}} \right)\)
6.	Diborane	\({{\text{B}}_2}{{\text{H}}_6}\)

Summary

The group \({\text{IIIA}}\) or \({13^{{\text{th}}}}\) group elements from the first series of \({\text{p}}\) block elements, displaying discrepancies in the atomic radii and other properties as we move down the group. Boron, being small, shows anomalous behavior and shows distinct properties when compared to other elements in the group.

FAQ on Important Trends and Anomalous Properties of Boron

Q.1. What is an important property of boron?
Ans: Boron combines with electropositive metals to form Borides.
\(3{\text{Mg}} + {{\text{B}}_2} \to {\text{M}}{{\text{g}}_3}{{\text{B}}_2}\)

Q.2. What is the anomalous behavior of boron?
Ans: Boron has smaller atomic radii and a maximum covalency of four. Hence, it behaves differently from the other elements in its group and has distinctive properties, which accounts for its anomalous behavior. Example: Unlike the other four metals, \({\text{Al}},{\text{In}},{\text{Ga}}\) and \({\text{TI}},\) Boron do not form dimeric halides but only form monomeric halide \(\left({{\text{B}}{{\text{X}}_3}} \right).\)

Q.3. What are some important compounds of boron?
Ans: Some of the important compounds of boron include: Borax, Boron Nitride \(\left({{\text{BN}}} \right),\) Boric acid \(\left({{{\text{H}}_3}{\text{B}}{{\text{O}}_3}} \right),\) Boron trioxide \(\left({{{\text{B}}_2}{{\text{O}}_3}} \right),\) etc.

Q.4. Why is the maximum covalency of boron 4?
Ans: The outer shell of the boron, which is available for bonding, contains a total of \(4\) orbitals (one \({\text{s}}\)-orbital and three \({\text{p}}\)-orbitals). Hence, the maximum covalency of boron is only \(4.\)

Q.5. Why is boron carbide so hard?
Ans: Boron carbide is extremely hard and used in nuclear reactors. The hardness is due to its unique structure and the rhombohedral lattice present in it.

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