Group13 Elements: The Boron Family

Introduction

The right side of the periodic table having group number 13, 14, 15, 16, 17 and 18 are known as p – block elements. These elements have 3, 4, 5, 6, 7 and 8 electrons in their outer most shell, respectively. The last electron of these groups' elements occupies the position in p – sub shell that is why these elements are called as p – block elements. Their general electronic configuration is ns²np^1-6.

Some important properties of p-block

(1) Electron affinity

Electron affinity increases from left to right along the period amongst the p – block elements and it decreases from top to bottom. But group 15 is having exceptionally low values of electron affinity and is due to extra stability of exactly half filled orbitals in their valence shell. Similarly, elements of group 18 (noble gases) have zero affinities due to presence of complete octet which provides them stability.

(2) Metallic Character

The metallic character is governed by

(i) Size of atoms and

(ii) Ionization energy.

The elements having bigger size and low ionization energy has a greater metallic character. After combining both the above mentioned factors we observe that the elements with above two properties are located in left corner of p – block and strong non – metallic elements are located at right corner and a diagonal strip of elements separates thus two, having in between properties are called as metalloids.

(3) Oxidation state

The p-block elements show variety of oxidation states both positive and negative. Some of the

p-block elements show different oxidation state due to inert – pair effect, where their lower oxidation state is more predominant.

(4) Diagonal relationship

On moving diagonally across the periodic table the element shows certain similarities

Elements of 2nd period differ from their own group elements in some of the properties. This is due to the following reason:

(a) Small size

(b) Absence of vacant d-orbital

(c) High IP

Some important characteristics of p – block in tabular form

Boron Family

Group 13 elements are boron (B), aluminium (Al), Gallium (Ga), Indium (In) and thallium (Tl).

Boron is the only non – metal in this group others are metal. Non – metallic character of boron is due to its small size and high ionization energy. The general valence shell electronic configuration of these elements is ns²np¹.

General Introduction of Boron family

Boron

The name boron comes form the Arabic and Persian words for borax, its principal ore. It was first isolated in 1808 by Gay – Lussac and Thenard and independently by Sir Hymphry Davy.

The pure element is shiny and black. It is very hard and in extremely pure form is nearly as hard as diamond, but too much brittle for practical use. At high temperatures it is a good conductor but at room temperature and below is an insulator.

Aluminium

Aluminium ranks third on the list of the ten most abundant elements in the earth's crust, while its oxide is fourth among the ten most common compounds in the crust. It is the most abundant metal on the planet. Its name is taken from the Latin alumen for alum. It is soft, light weight and silvery, its existence was proposed by Lavoisier in 1787, it was named by Davy in 1807 and finally isolated by Orsted in 1825.

In its purest form the metal is bluish – white and very ductile. It is an excellent conductor of heat and electricity and finds use in some wiring. When pure it is too soft for construction purposes but addition of small amounts of silicon and iron hardened it significantly. Aluminium is the most abundant element in earth crust among this family

Gallium

Gallium is one of the elements originally predicted by Mendeleev in 1871 as aluminium, indicating that it should have the properties similar to aluminium. The actual metal was isolated and named by Paul – Emile Lecog de Boisbaudran in 1875.

At room temperature gallium is soft as lead and can be cut with a knife. Its melting point is abnormally low and it will begin to melt in the palm of a warm hand. Gallium is from one of the small numbers of metals that expands on freezing.

Indium

The element indium (Latin indicum, for the colour indigo) was discovered in 1863 by Reich and Ritcher. It is a rare metal, with an abundance similar to that of silver. It is generally found in deposits with zinc and refineries which produce this more common metal often sell indium as well.

The pure metal is so soft that you can "wipe" it onto other material in much the same way as lead. It is corrosion resistant.

Thallium

Sir William Crookes discovered thallium in 1861, positively identifying it by a green line in its spectrum (hence the name, which is from the Greek, thallos for "green twig")

Thallium compounds are quite toxic and some have been used as rat poisons. A few compounds are used in glasses for special infra – red lenses.

General Trends in Physical properties

1. Density

Generally increases down the group but aluminium has an exceptionally low density.

2. Melting point and Boiling point

B to Ga decrease then Ga to Tl increases Ga has lowest M.P (29.8°C) and therefore liquid at room temperature.

3. Atomic radii and Ionic radii

On moving from B to Tl the size increases due to addition of new energy shells at each step down the group but Ga is smaller than Al.

4. Ionization Energy

Generally IE decreases down the group but Ga has higher IE than Al exceptionally due to smaller in size as compared to Al.

5. Metallic character

Electropositive character increases down the group hence metallic character also increase down the group but aluminium is having high metallic character than Gallium due to low IE than Ga.

6. Oxidation state

The elements of B group have three valence electron i.e .two in s – subshell. The most oxidation state should be +3 but due to small size of boron it can not lose its valence electrons to B³⁺ ion, and combines with other atoms through covalent bonds. Except boron, other elements also exhibit +1 oxidation state and down the group +1 state becomes more stable.

7. Reducing property

Down the group the reducing property decreases. Al > Ga > In > Tl

Group trends in chemical properties

1. Hydrides

None of the element from 13 groups reacts directly to hydrogen. However a number of hydrides of these elements have been prepared by indirect methods. Boron hydrides are called boranes.

Two types of boranes:

(a) B_nH_n+4, called nidoboranes

(b) B_nH_n+6, called arachnoboranes

The simplest borone is diborane B_­2H₆

$2B{F}_3\left(g\right)+6LiH\left(s\right)\stackrel{450K}{\to }{B}_2{H}_6\left(g\right)+6LiF\left(s\right)$

Other elements of this group forms only a few stable hydrides of NH₃ types. AlH₃ is colour less polymeric solid of formula (AlH₃)_x and contains Al --- H --- Al bridges. A complex hydride of aluminium is a very good reducing agent and used as a regent in lab is Li[AlH₄], Lithium aluminium hydrides. It is a white crystalline solid.

$H_{3}P{O}_4$

Gallium also form Li[GaH₄]

2. Oxides and hydroxides

The 13 group elements forms oxides and hydroxides of composition M₂O₃ and M(OH)₃ respectively. As we move down the group the acidic character in oxides and hydroxides decreases and basic character increases due to decrease in strength of M ¾ O bond due to which basicity increases.

B₂O₃ > Al₂O₃ > Ga₂O₃ > In₂O₃ > Tl₂O₃

B(OH)₃ > Al(OH)₃ > Ga(OH)₃ > In(OH)₃ > Tl(OH)₃

Trend in acidity

Basic Character increases

Oxides and hydroxide of Al and Ga shows amphoteric nature.

2Al(OH)₃(s) + 3H₂SO₄(aq) $\to$ Al₂ (SO₄)₃(aq) + 6H₂O $\left(\ell \right)$

Al(OH)₃(s) + NaOH(aq) $\to$ Na[Al(OH)₄](aq)

Preparation of oxides of B and Al

(i) $4B+3O_2\stackrel{\Delta }{\to }2B_2{O}_3$

B₂O₃ is a good dehydrating agent and reacts with water to form orthoboric acid (H₃BO₃).

B₂O₃ is an acidic oxide dissolves in alkalies to give borates as well as metaborates.

$B_2{O}_3+6NaOH\stackrel{\Delta }{\to }\underset{sodiumborate}{2N{a}_{2}B{O}_3}+3H_{2}O$

$H_{3}P{O}_2$

(ii) $H_{2}S$

It is amphoteric in nature.

Crystalline alumina, Al₂O₃, exists in many forms, one of these called corundum, is very hard and is used as an abrasive.

3. Halides

Boron and aluminium combine with halogens to form trihalides having the general formula MX₃.

$2B+3X_2\to 2B{X}_3\left(X=Cl,Br\right)$

$2Al+3X_2\to 2Al{X}_3\left(X=Cl,I\right)$

Halides of boron are covalent, boron trihalides exist as monomer having planar triangular geometry. BX₃ acts as lewis acid.

Among boron trihalides the order for strong lewis acid is

BF₃ < BCl₃ < BBr₃