Unit V: p- Block Elements (Boron group)
Group 13 (boron group): General Characteristics, extraction of boron, Anomalous behaviour
of Boron, Diagonal relationship of boron with silicon, reaction of B with other elements,
water, air, acids, alkali, metals and non-metals. Preparation, Properties and structure of
diborane. Structure of borazine, structure and medicinal properties of boric acid,
borohydrides- Hydroboration- Ultramarine. Anomalous behaviour of Aluminium, Inert pair
effect of Thallium.
Boron family
Extraction of Boron
The group 13 elements in the modern periodic table are better known as the members of
Boron family. The members of this family exhibit a wide range of physical and chemical
properties. The electronic configuration of these elements is given by ns2 np1.
The members of this family are:
Boron (B), Aluminium (Al), Gallium(Ga), Indium (In), Thallium (Tl) and a radioactive
synthetic element, Nihonium (Nh) formerly known as ununtrium.
The chemical and physical properties of the members of boron family are found to follow a
particular trend. Properties of boron differ from other members of the group due to its smaller
size and absence of the d orbital. These deviations in properties of boron lead to the
classification of anomalous properties of boron.
, Trends in properties of members of boron family:
1. The members of the boron family react with halogens to form tri-chlorides, bromides,
and iodides. These halides are covalent in nature and are hydrolyzed in water.
2. The compounds of these elements like tetrahedral [M(OH)4]– and octahedral
[M(H2O)6]3+ (where M denotes a member of the boron family), exists in an aqueous
medium.
3. Because of the deficiency of electrons, these trihalides are strong Lewis acids.
4. Metallic character increases down the group as we move from boron to thallium.
5. Electronegativity of the elements first decreases down the group from B to Al and
then increases marginally because of the discrepancies in the atomic size of the
elements.
Anomalous properties of boron:
Due to its smaller size and unavailability of d-electrons boron is found to exhibit properties
which are in contrast to the other elements of the boron family. These properties are known
as anomalous properties of boron. Some of these anomalous properties are discussed below.
1. Except for boron, the compounds of the elements of the boron family like tetrahedral
[M(OH)4]– and octahedral [M(H2O)6]3+ (where M denotes the member of boron
family) exists in an aqueous medium.
2. The maximum covalence of boron is 4 due to the absence of d orbitals.
3. Boron is a metalloid while the rest of the family are post transition metals.
4. The boron oxides and hydroxides are acidic in nature, whereas the other elements in
the family form oxides and hydroxides which are amphoteric in nature.
5. Like Li and Be, Boron – the first member of group 13 also shows anomalous
behaviour due to extremely low size and high nuclear charge/size ratio, high
electronegativity and non-availability of d electrons. The main point of differences
are,
6. Boron is a typical non- metal whereas other members are metals.
7. Boron is a bad conductor of electricity whereas other metals are good conductors.
8. Boron shows allotropy and exists in two forms – crystalline and amorphous.
Aluminium is a soft metal and does not exist in different forms.
9. Like other non-metals, the melting point and boiling point of boron are much higher
than those of other elements of group 13.
10. Boron forms only covalent compounds whereas aluminium and other elements of
group 13 form even some ionic compounds.
11. The hydroxides and oxides of boron are acidic in nature whereas those of others are
amphoteric and basic.
12. The trihalides of boron (BX3) exist as monomers On the other hand, aluminium
halides exist as dimers (Al2X6).
13. The hydrides of boron i.e. boranes are quite stable while those of aluminium are
unstable.
14. Dilute acids have no action on boron others liberate H2 from them.
15. Borates are more stable than aluminates.
16. Boron exhibit maximum covalency of four e.g., BH–4 ion while other members
exhibit a maximum covalency of six e.g., [Al(OH)6]3-.
17. Boron does not decompose steam while other members do so.
Group 13 (boron group): General Characteristics, extraction of boron, Anomalous behaviour
of Boron, Diagonal relationship of boron with silicon, reaction of B with other elements,
water, air, acids, alkali, metals and non-metals. Preparation, Properties and structure of
diborane. Structure of borazine, structure and medicinal properties of boric acid,
borohydrides- Hydroboration- Ultramarine. Anomalous behaviour of Aluminium, Inert pair
effect of Thallium.
Boron family
Extraction of Boron
The group 13 elements in the modern periodic table are better known as the members of
Boron family. The members of this family exhibit a wide range of physical and chemical
properties. The electronic configuration of these elements is given by ns2 np1.
The members of this family are:
Boron (B), Aluminium (Al), Gallium(Ga), Indium (In), Thallium (Tl) and a radioactive
synthetic element, Nihonium (Nh) formerly known as ununtrium.
The chemical and physical properties of the members of boron family are found to follow a
particular trend. Properties of boron differ from other members of the group due to its smaller
size and absence of the d orbital. These deviations in properties of boron lead to the
classification of anomalous properties of boron.
, Trends in properties of members of boron family:
1. The members of the boron family react with halogens to form tri-chlorides, bromides,
and iodides. These halides are covalent in nature and are hydrolyzed in water.
2. The compounds of these elements like tetrahedral [M(OH)4]– and octahedral
[M(H2O)6]3+ (where M denotes a member of the boron family), exists in an aqueous
medium.
3. Because of the deficiency of electrons, these trihalides are strong Lewis acids.
4. Metallic character increases down the group as we move from boron to thallium.
5. Electronegativity of the elements first decreases down the group from B to Al and
then increases marginally because of the discrepancies in the atomic size of the
elements.
Anomalous properties of boron:
Due to its smaller size and unavailability of d-electrons boron is found to exhibit properties
which are in contrast to the other elements of the boron family. These properties are known
as anomalous properties of boron. Some of these anomalous properties are discussed below.
1. Except for boron, the compounds of the elements of the boron family like tetrahedral
[M(OH)4]– and octahedral [M(H2O)6]3+ (where M denotes the member of boron
family) exists in an aqueous medium.
2. The maximum covalence of boron is 4 due to the absence of d orbitals.
3. Boron is a metalloid while the rest of the family are post transition metals.
4. The boron oxides and hydroxides are acidic in nature, whereas the other elements in
the family form oxides and hydroxides which are amphoteric in nature.
5. Like Li and Be, Boron – the first member of group 13 also shows anomalous
behaviour due to extremely low size and high nuclear charge/size ratio, high
electronegativity and non-availability of d electrons. The main point of differences
are,
6. Boron is a typical non- metal whereas other members are metals.
7. Boron is a bad conductor of electricity whereas other metals are good conductors.
8. Boron shows allotropy and exists in two forms – crystalline and amorphous.
Aluminium is a soft metal and does not exist in different forms.
9. Like other non-metals, the melting point and boiling point of boron are much higher
than those of other elements of group 13.
10. Boron forms only covalent compounds whereas aluminium and other elements of
group 13 form even some ionic compounds.
11. The hydroxides and oxides of boron are acidic in nature whereas those of others are
amphoteric and basic.
12. The trihalides of boron (BX3) exist as monomers On the other hand, aluminium
halides exist as dimers (Al2X6).
13. The hydrides of boron i.e. boranes are quite stable while those of aluminium are
unstable.
14. Dilute acids have no action on boron others liberate H2 from them.
15. Borates are more stable than aluminates.
16. Boron exhibit maximum covalency of four e.g., BH–4 ion while other members
exhibit a maximum covalency of six e.g., [Al(OH)6]3-.
17. Boron does not decompose steam while other members do so.
