THE p- BLOCK ELEMENTS BORON FAMILY OF ELEMENTS

What are p – block elements?

p – block elements are the elements in which the last electron enters “np” orbital which include six groups of elements i.e. group 13 to 18

Boron family Carbon family Nitrogen family (Pnictogens) Oxygen family (Chalcogens) Halogens Nobel gases

• As metals, non-metals and metalloids exist only in p-block of periodic table

• The heaviest element in each p – block group is mostly metallic in nature

• Non-metallic character of elements decreases down the group

• Non metals have higher ionization enthalpy and electronegativity than the metals. Hence metals form cations, and non metals form anions

• i.e. Al3+, Bi3+, Sn2+

• Cl-, S2-, O2-

• Compounds formed between non metals are largely

covalent in nature, while the compounds formed by highly reactive non-metal and metal have large difference in electronegativities

• Cl2, S8, O2

• i.e. AlCl3, BiCl3, SnCl2

General electronic configuration and oxidation states

Group 13

(3A) 14

(4A) 15

(5A) 16

(6A) 17

(7A) 18

(8A)

General electronic

configuration ns2np1 ns2np2

ns2np3

ns2np4

ns2np5

ns2np6

First element of the group

B C N O F He

Oxidation states

+3 +1

+4 +2 -4

+5 +3 -3

+6 +4 +2 -2

+7 +5 +3 -1 +1

+8 +6 +4

Group 3A

BORON FAMILY (nS2nP1)

KEY ATOMIC PROPERTIES, PHYSICAL PROPERTIES, AND REACTIONS

• Boron heads family, but other elements in group 3A exhibit

diverse properties

• Boron & Aluminum, especially Aluminum, are much more

abundant than the others, but still quite rare

• Group 3A elements include “p” orbitals for first time

• Physical Properties are influenced by type of bonding

• The atomic radii increases from boron to thallium.

• The abrupt increase in atomic radius of Al is due to greater screening effect in Al than B

• The first ionization energies of these elements are less than corresponding value of s-block elements. This is because p-electrons are less penetrating and more shielded than s-electrons.

• Down the group electronegativity first decreases

from B to Al and then increases till thallium. This is because of the difference in atomic size of elements.

Influence of Transition Elements on Group 3A(13)

Zeff increases for the larger 3A elements due to poor shielding by d and f electrons.

The larger 3A elements have smaller atomic radii and larger ionization energies than electronegativities than expected.

These properties influence the physical and chemical behavior of these elements.

Features of Group 3A(13) Elements

Larger Group 3A elements exhibit multiple oxidation states. They may lose either the np electron only, or both the np and ns electrons.

The lower oxidation state becomes increasingly prominent down the group, since the ns2 electrons form an inert pair.

Oxides of the element in the lower oxidation state are more basic than oxides in the higher oxidation state. In2O is more basic than In2O3.

The elements of boron family have ns2np1 electron configuration

which means that they have 3 valence electrons available for bond formation. By losing these electrons they are accepted to show +3 or +1 oxidation state in their compounds.

ALUMINIUM and OTHER GROUP ELEMENTS

CHEMISTRY

Aluminium can easy lose electrons and it is able to form ion Al3+. In the other elements, due to poor shielding effect of d and f orbitals, the nucleous holds the outer most s electrons tightly. Thus only p electrons may be available for bonding. In indium, gallium and thallium, both +1 and +3 oxidation states are seen The compound in +1 state are more ionic than those in +3 state.

• Boron is a network covalent metalloid - Black, hard, very high

melting point

• A network solid or covalent network solid is a chemical

compound in which the atoms are bonded by covalent bonds in

a continuous network

• In a network solid there are no individual molecules and the

entire crystal may be considered a macromolecule

• Boron (metalloid) is much less reactive than the others

members of the 3A group because it forms covalent bonds

Highlights of Boron Chemistry

Highlights of Boron Chemistry

All boron compounds are covalent, and B forms a variety of network covalent compounds with other elements.

Boron is often electron-deficient in compounds, and acts effectively as a Lewis acid since it can accept an e- pair.

BF3(g) + :NH3(g) → F3B–NH3(g)

Boron forms bridge bonds, in which one pair of electrons is shared between three atoms. This type of bonding is common between B and H.

• Other group members are metals – shiny, relatively soft with low melting points

• Aluminum is more ionic; its low density and 3 valence electrons make it a good electrical conductor

• On moving down the group the metallic character increases initially from B to Al but decreases from Al to Tl

1. The elements react sluggishly, if at all, with water: 2. When strongly heated in pure O2, all members form oxides: Oxide acidity decreases down the group: B2O3 (weakly acidic) > Al2O3 > Ga2O3 > In2O3 > Tl2O (strongly basic) for Tl, the +1 oxide is more basic than the +3 oxide.

2 Ga + 6 H2O(Hot) 2 Ga(OH)3 + 3 H2

2 Tl + 2 H2O(steam) 2 TlOH + H2

4 M + 3 O2 2 M2O3 M = B, Al, Ga, In

4 Tl + O2 2 Tl2O

Important reactions of Boron group elements

3. All members reduce halogens (X2) : BX3 are volatile covalent molecules. Trihalides of Al, Ga, and In are (mostly) ionic solids but occur as covalent dimers in the gas phase; in this way, the 3A atoms attains a filled outer level. 4. Acid treatment of Al2O3 is important in water purification: 2 Al2O3 (s) + 3 H2SO4 (l) Al2(SO4)3 (s) + 4 H2O(l) In water, Al2(SO4)3 and CaO form a colloid that aids in removing suspended particles.

2 M + 3X2 2 MX3 (M = B, Al, Ga, In)

2 Tl + X2 2 TlX

Important reactions of Boron group elements

The dimeric structure of gaseous aluminum chloride.

Compounds of 3A elements have more covalent character than similar 2A compounds. Aluminum has the physical properties of a metal, but its halides exist as covalent dimers.

5. The overall reaction in the production of aluminum metal is a redox process: This electrochemical process is carried out in the presence of cryolite (Na3AlF6), which lowers the melting point of the reactant mixture and takes part in the change. 6. A displacement reaction produces gallium arsenide, GaAs:

2 Al2O3 (s) + 3 C(s) 4 Al(s) + 3 CO2 (g)

(CH3)3Ga(g) + AsH3 (g) _________________

Important reactions of Boron group elements

• Aluminium forms a very thin oxide layer on its surface, that prevent it from the reaction with acids and bases at RT.

• It dissolves in mineral acids and aqueous alkalies at high temperatures and thus show amphoteric character.

Highlights of Aluminium Chemistry

2Al + 6HCl 2 AlCl3 + 3H2

2Al + 6NaOH + 6H2O 2Na3[Al(OH)6] + 3H2

Highlights of Aluminium Chemistry

Aluminium oxide and aluminium hydroxide also show amphoteric properties!

Al2O3 + 6HCl 2AlCl3 + 3H2O

Al2O3 + 6NaOH + 3H2O 2Na3[Al(OH)6]

Al(OH)3 + 3HCl AlCl3 + 3H2O

Al(OH)3 + 3NaOH Na3[Al(OH)6]

The same properties Ga

has

Beryllium and Aluminum

• There is a diagonal relationship between Be and Al.

• Both form oxoanions in strong base:

– beryllate, Be(OH)42- and aluminate, Al(OH)4

-.

• Both have bridge bonds in their hydrides and chlorides.

• Their oxides are

– coatings that are impervious to water,

– amphoteric, and

– extremely hard with high melting points.

• Some Al and all Be compounds have significant covalent character.

1. Boron oxide, B2O3. Used in the production of borosilicate glass. 2. Borax, Na2B4O7 ·10H2O. Major mineral source of boron compounds and B2O3. Used as a fireproof insulation material and as a washing powder (20-Mule Team Borax). 3. Boric acid, H3BO3 [ or B(OH)3]. Used as external disinfectant, eyewash, and insecticide. 4. Diborane, B2H6. A powerful reductant for possible use as a rocket fuel. Used to synthesize higher boranes, compounds that led to new theories of chemical bonding. 5. Aluminum sulfate (alum), Al2(SO4)3 18H2O. Used in water purification, tanning leather, and as an antiperspirant.

.

.

Important compounds

of Boron group elements

6. Aluminum oxide, Al2O3. Major compound in natural source (bauxite) of Al metal. Used as abrasive in sandpaper, sanding and cutting tools, and toothpaste. Large crystals with metal ion impurities often of gemstone quality. Inert support for chromatography. In fibrous forms, woven into heat-resistant fabrics; also used to strengthen ceramics and metals. 7. Tl2Ba2Ca2Cu3O10. Becomes a high-temperature superconductor at 125 K, which is readily attained with liquid Nitrogen. (77K)

Important compounds

of Boron group elements

BORAX

It is the most important compound of boron. It is a white crystalline solid of formula Na2B4O7 ·10H2O. Borax dissolves in water to give an alkaline solution.

Na2B4O7 ·10H2O + 7H2O 2NaOH + 4H3BO3

On heating, borax first loses water molecules and swells up. On further heating it turns into a transparent liquid, which solidifies into glass like material known as borax bead.

Na2B4O7 ·10H2O Na2B4O7 2NaBO2 + B2O3

Orthoboric acid

Sodium metaborate

ORTHOBORIC ACID

Orthoboric acid, H3BO3, is a white crystalline solid, with soapy touch. It is sparingly soluble in water but highly soluble in hot water. It can be prepared by acidifying an aqueous solution of borax.

Na2B4O7 ·10H2O + 2HCl + 5H2O 2NaCl + 4H3BO3

It has a layer structure in which planar BO3 units are joined by hydrogen bonds. Boric acid is a weak monobasic acid. On heating above 370 K orthoboric acid forms metaboric acid, HBO2 which on further heating yields boric oxide, B2O3

H3BO3 HBO2 B2O3

DIBORANE, B2H6

The simplest boron hydride known, is diborane. It is prepared on an industrial scale by treating boron trifluoride with sodium hydride

2BF2 + 6NaH B2H6 + 6NaF

• Diborane is a colourless, highly toxic gas with a b.p. of 180 K.

• Diborane catches fire spontaneously upon exposure to air. It burns in oxygen releasing an enormous amount of energy.

• Reaction of ammonia with diborane gives initially B2H6·NH3, further heating gives borazine, B3N3H6 known as “inorganic benzene”.

The two types of covalent bonding in diborane.

Normal covalent bond formed by overlap of sp3 hybrid orbital from B with s orbital from H.

Hydride bridge bond , or three-center, two-electron bond.

The structure of diborane is shown below. The four terminal hydrogen atoms and two boron atoms lie in one plane. Above and below this plane, there are two bridging hydrogen atoms. The four terminal B-H bonds are regular two center-two electron bonds while the two bridge (B-H-B) bonds are different and can be described in terms of three-center two electron bond.

Application of boron and aluminium

and their compounds

Boron has low dencity and very low electrical conductivity, finds many applications.

• Boron fibers are used in making bullet-proof vest and light

material for aircraft.

• The boron-10 isotope has high ability to absorb neutrons and their f metal borides are used in nuclear industry as protective shields and control rods.

• The main industrial application of borqax and boric acid is in the manufacture of heat resistant glasses like glass-wool and fiberglass.

• Borax is used as a constituent of medicinal soaps. An aqueous solution of orthoboric acid is generally used as a mild antiseptic.

• Aluminium is a bright silvery white metal. It has a high electrical and thermal conductivity.

• It forms alloys with Cu, Mn, Mg, Si, and Zn. 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, aerolplane and transportation industry.

• The use of aluminium and its compounds for domestic purposes is now reduced considerably because of their toxic nature..