THE p-BLOCK ELEMENTS

          1. Introduction 

In p-block elements the last electron enters the outermost p orbital. As we know that the number of p orbitals is three  and, therefore, the maximum number of electrons that can be accommodated in a set of p orbitals is six. Consequently there are six groups of p–block elements in the periodic table numbering from 13 to 18. Boron, carbon, nitrogen, oxygen, fluorine and helium head the groups. Their valence shell electronic configuration is n s²

np¹-6(except for He).

The inner core of the electronic configuration may, however, differ. The difference in inner core of elements greatly influences their physical properties (such as atomic and ionic radii, ionisation enthalpy, etc.) as well as chemical 

properties. Consequently, a lot of variation in properties of elements in a group of p-block is observed. The maximum oxidation state shown by a p-block element is equal to the total number of valence electrons (i.e., the sum of the s- and p-electrons). Clearly, the number of possible oxidation states increases towards the right of the periodic table. In addition to this so called group oxidation state, p-block elements may show other oxidation states which normally, 

but not necessarily, differ from the total number of valence electron by unit of two. The important oxidation states exhibited by p-block elements. In  boron, carbon and nitrogen families the group oxidation state is the most stable state for the lighter elements in the group. However, the oxidation state two unit less than the 

group oxidation state becomes progressively more stable for the heavier elements in each group. The occurrence of oxidation states two unit less than the group oxidation states are sometime attributed to the ‘inert pair effect’.

The relative stabilities of these two oxidation

states – group oxidation state and two unit less

than the group oxidation state – may vary from

group to group and will be discussed at

appropriate places. It is interesting to note that the non-metals and metalloids exist only in the p-block of the periodic table. The non-metallic character of elements decreases down the group. In fact the heaviest element in each p-block group is the most metallic in nature. This change from non- metallic to metallic character brings diversity in the chemistry of these elements depending on the group to which they belong. 

In general, non-metals have higher ionisation

enthalpy and higher electronegativities than

the metals. Hence, in contrast to metals which

readily form cation, non-metals readily form

anions. The compounds formed by highly

reactive non-metals with highly reactive metals

are generally ionic because of large differences

in their electronegativities. On the other hand,

compounds formed between non-metals

themselves are largely covalent in character

because of small differences in their electro negativities. The change of non-metallic to metallic character can be best illustrated by 

the nature of oxides they form. The non-metal

oxides are acidic or neutral whereas metal

oxides are basic in nature.

The first member of p-block differs from the

remaining members of their corresponding

group in two major respects. First is the size

and all other properties which depend on size.

Thus, the lightest p-block elements show the

same kind of differences as the lightest s-block

elements, lithium and beryllium. The second

important difference, which applies only to the

p-block elements, arises from the effect of d-

orbitals in the valence shell of heavier elements

(starting from the third period onwards) and

their lack in second period elements. The

second period elements of p-groups starting

from boron are restricted to a maximum

covalence of four (using 2s and three 2p

orbitals). In contrast, the third period elements

of p-groups with the electronic configuration

3s² 3pn have the vacant 3d orbitals lying between the 3p and the 4s levels of energy. 

Using these d-orbitals the third period elements can expand their covalence above  four. For example, while boron forms only [BF4] – , aluminium gives [Al F 6] 3– ion. The presence of these d-orbitals influences the chemistry of the heavier elements in a number of other ways. The combined effect of size and availability of d orbitals considerably influences the ability of these elements to form π bonds. The first member of a group differs from the heavier members in its ability to form p π - p π multiple bonds to itself ( e.g., C=C, C≡C, N≡N) and to other second row elements (e.g., C=O, C=N, C≡N, N=O). This type of π - bonding is not particularly strong for the heavier p-block elements. The heavier elements do form π bonds but this involves d orbitals (d π – p π or d π –d π ). As the d orbitals are of higher energy than the p orbitals, they contribute less to the overall stability of molecules than does p π - p π bonding of the second row elements. However, the coordination number in species of heavier elements may be higher than for the first element in the same oxidation state. For example, in +5 oxidation state both N and P form oxo anions : N O 3 – (three-coordination with π – bond involving one nitrogen p-orbital) and 3 P O 4 − (four coordination involving s, p and d orbitals contributing to the π – bond). In this unit we will study the chemistry of group 13 and 14 elements of the periodic table.

 2. GROUP 13 ELEMENTS: THE BORON  FAMILY

This group elements show a wide variation in

properties. Boron is a typical non-metal, aluminium is a metal but shows many chemical similarities to boron, and gallium, indium, thallium and n i h onium are almost exclusively metallic in character. Boron is a fairly rare element, mainly occurs as o r t h o boric acid, (H3 BO3), borax, Na 2B4O7·10H2O, and k er nite, Na 2B4 O 7·4H2O. In India borax occurs in P u g a Valley (La d a k h) and Sambhar Lake (Rajasthan). The abundance of boron in earth crust is less than 0.0001% by mass. There are two isotopic forms of boron 10B (19%) and 11B (81%). Aluminium is the most abundant metal and 

the third most abundant element in the earth’s

crust (8.3% by mass) after oxygen (45.5%) and

Si (27.7%). Bauxite, Al 2 O 3. 2H2O and cryolite,

Na 3AlF6 are the important minerals of aluminium. In India it is found as mica in 

Madhya Pradesh, Karnataka, Orissa and

Jammu. Gallium, indium and thallium are less

abundant elements in nature. N i h onium has

symbol N h, atomic number 113, atomic mass

286 g /mole and electronic configuration [Rn]

5f¹⁴ 6d¹0 7s² 7p². So far it has been prepared 

in small amount and half life of its most stable

isotope is 20 seconds. Due to these reasons its

chemistry has not been established. N i h onium is a synthetically prepared radioactive element. Here atomic, physical and chemical properties of elements of this group leaving n i h onium are discussed below. 

1. Electronic Configuration

The outer electronic configuration of these

elements is n s 2 np ¹ . A close look at the 

electronic configuration suggests that while

boron and aluminium have noble gas

core, gallium and indium have noble gas plus

10 d-electrons, and thallium has noble gas

plus 14 f- electrons plus 10 d-electron cores.

Thus, the electronic structures of these

elements are more complex than for the first

two groups of elements discussed in unit 10.

This difference in electronic structures affects

the other properties and consequently the

chemistry of all the elements of this group.

2. Atomic Radii

On moving down the group, for each successive

member one extra shell of electrons is added

and, therefore, atomic radius is expected to

increase. However, a deviation can be seen.

Atomic radius of G a is less than that of Al. This

can be understood from the variation in the

inner core of the electronic configuration. The

presence of additional 10 d-electrons offer

only poor screening effect (Unit 2) for the outer

electrons from the increased nuclear charge in

gallium. Consequently, the atomic radius of

gallium (135 pm) is less than that of aluminium (143 pm). 

3. Ionization Enthalpy 

The ionisation enthalpy values as expected

from the general trends do not decrease

smoothly down the group. The decrease from

B to Al is associated with increase in size. The

observed discontinuity in the ionisation enthalpy values between Al and G a, and between In and T l are due to inability of d- and f-electrons ,which have low screening effect, to compensate the increase in nuclear charge. The order of ionisation enthalpy, as expected, is ∆i H1<∆i H2<∆i H3. The sum of the first three ionisation enthalpy for each of the elements is very high. Effect of this will be apparent when you study their chemical properties.

 

4. Electronegativity

Down the group, electronegativity first

decreases from B to Al and then increases

marginally. This is because of the thallium discrepancies in atomic size of the elements. 

5. Physical Properties

Boron is non-metallic in nature. It is extremely

hard and black coloured solid. It exists in many

allotropic forms. Due to very strong crystalline

lattice, boron has unusually high melting point.

Rest of the members are soft metals with low

melting point and high electrical conductivity.

It is worthwhile to note that gallium with

unusually low melting point (303K), could

exist in liquid state during summer. Its high

boiling point (2676 K) makes it a useful material for measuring high temperatures. Density of the elements increases down the group from boron to thallium.

3. IMPORTANT TRENDS AND
ANOMALOUS PROPERTIES OF
BORON

Certain important trends can be observed

in the chemical behaviour of group 13 elements. The tri-chlorides, bromides and iodides of all these elements being covalent in nature are hydrolysed in water. Species like tetrahedral [M(OH)4] – and octahedral [M(H2O)6] 3+, except in boron, exist in aqueous medium. The monomeric tri halides, being electron deficient, are strong Lewis acids. Boron trifluoride easily reacts with Lewis bases such as N H3 to complete octet around boron. F B :NH F B NH 3 + →← 3 3 3

It is due to the absence of d orbitals that the maximum covalence of B is 4. Since the d orbitals are available with Al and other elements, the maximum covalence can be expected beyond 4. Most of the other metal halides (e.g., Al Cl 3) are dim er i sed through halogen bridging (e.g., Al 2Cl6). The metal species completes its octet by accepting electrons from halogen in these halogen bridged molecules.

4. USES OF BORON AND ALUMINIUM  AND THEIR COMPOUNDS

Boron being extremely hard refractory solid of

high melting point, low density and very low

electrical conductivity, finds many

applications. Boron fibres are used in making

bullet-proof vest and light composite material

for aircraft. The boron-10 (10B) isotope has high

ability to absorb neutrons and, therefore,

metal borides are used in nuclear industry as

protective shields and control rods. The main

industrial application of borax and boric acid

is in the manufacture of heat resistant glasses

(e.g., Pyrex), glass-wool and fibreglass. Borax

is also used as a flux for soldering metals, for

heat, scratch and stain resistant glazed coating

to earthenwares and as constituent of medicinal soaps. An aqueous solution of o r t ho boric acid is generally used as a mild antiseptic. Aluminium is a bright silvery-white metal, with high tensile strength. It has a high electrical and thermal conductivity. On a weight-to-weight basis, the electrical conductivity of aluminium is twice that of  copper. Aluminium is used extensively in 

industry and everyday life. It forms alloys with

Cu, M n, Mg, Si and Z n. Aluminium and its

alloys can be given shapes of pipe, tubes,

rods, wires, plates or foils and, therefore, find

uses in packing, utensil making, construction, aeroplane and transportation  industry. The use of aluminium and its  compounds for domestic purposes is now  reduced considerably because of their toxic  nature.

5. GROUP 14 ELEMENTS: THE CARBON  FAMILY

Carbon, silicon, germanium, tin lead and

f l e r o vi um are the members of group 14. Carbon is the seventeenth most abundant element by mass in the earth’s crust. It is widely 

distributed in nature in free as well as in the

combined state. In elemental state it is available

as coal, graphite and diamond; however, in

combined state it is present as metal

carbonates, hydrocarbons and carbon dioxide

gas (0.03%) in air. One can emphatically say

that carbon is the most versatile element in the

world. Its combination with other elements

such as dihydrogen, di oxygen, chlorine and

sulphur provides an astonishing array of

materials ranging from living tissues to drugs

and plastics. Organic chemistry is devoted to

carbon containing compounds. It is an

essential constituent of all living organisms.

Naturally occurring carbon contains two stable

isotopes:12C and 13C. In addition to these, third

isotope, 14C is also present. It is a radioactive

isotope with half-life 5770 years and used for

radiocarbon dating. Silicon is the second

(27.7 % by mass) most abundant element on

the earth’s crust and is present in nature in

the form of silica and silicates. Silicon is a very

important component of ceramics, glass and

cement. Germanium exists only in traces. Tin

occurs mainly as cassiterite, S n O2 and lead as

galena, P b S. F l er o v i u m is synthetically prepared radioactive element Ultra pure form of germanium and silicon  are used to make transistors and  semiconductor devices.