B.Sc FY Inorganic Chemistry P Block Elements(Group 13 Elements)

P – Block elements

The elements in which last electron enters in the P – Orbital are known as P- Block elements.

Elements	At. No.	Electronic Configuration
B	5	1S² 2S² 2P¹
C	6	1s² 2S² 2P²
N	7	1S² 2S² 2P³

- In long form of the periodic table, P- Block consist of elements of group 13, 14, 15, 16, 17 & 18.

- The general electronic configuration of P- Block elements is ns²np^1-6.

- Their (n – 1) S, P & d orbitals are completely filled and only the outer most orbital of these element is incomplete.

- Their properties are dependent on the presence of P – elements.

Group 13 Elements

- Group 13 consist five elements Boron (B), Aluminium (Al), Gallium (Ga), Indium(In) & Thallium(Tl).

- Aluminium is the most abundant among these elements

Electronic Configuration

Elements	Atomic Number	Electronic configuration
B	5	[He] 2s² 2P¹
Al	13	[Ne] 3S² 3P¹
Ga	31	[Ar] 3d¹⁰ 4S² 4P¹
In	49	[Kr] 4d¹⁰ 5S² 5P¹
Tl	81	[Xe] 4f¹⁴ 5d¹⁰ 6S² 6P¹

- The general outer electronic configuration of group 13 is ns²np¹.

General properties of group 13 elements

1:- Atomic Radius:-

- The atomic radius increases from Boron to Thallium due to screening effect.

B	Al	Ga	In	Tl
0.82	1.18	1.26	1.44	1.48

2. Ionization Potential:-

- I. P. increases with increase atomic radius.

B	Al	GA	In	Tl
800	577	579	558	589

- I. P. is the highest of Boron. It decreases sharply from B to Al because size of atom (Al) increases.

- In case of Ga, these are ten d electrons in its inner shell. Since d electrons shield the nuclear charge less effectively than the S & P electrons. The outer electron is held strongly by the nucleus.

- I. P. increases from Ga to In because size of In increases.

- I. P. increases from In to Tl, the last element 14f electron in its inner shell. Since the shielding effect of f electrons is much smaller than d electrons.

- Outer electron in Tl is held very strongly & hence there is considerable increase in its I. P.

3. Oxidation State:-

- According to electronic configuration of valence shell ns²np¹ of these elements.

- Boron shows +3 oxidation state. The rest of the elements show +1 as well as +3 oxidation state.

- The +1 oxidation state become more & more stable as we move down the group from B to Tl.

- Thus Tl(I) compounds are more stable than that of Tl(III). This is due to fact that the two S electrons tend to remains paired and do not participated in compound formation (Inert pair effect).

B	Al	Ga	In	Tl
+3	+3	+1, +3	+1, +3	+1

4. Electropositive (Metallic) Character:-

- Electropositive character (Metallic) character increases from B to Al. B is semi-metal.

- It is closer to non-metals than to metals in its properties. It is high ionization energy and is also a poor conductor of electricity.

- Aluminium and rest of the elements are typically metallic and have more or less electron-positive character.

- Electropositive character increases from B to Tl.

5. Tendency to form ionic compounds:-

- Tendency to form ionic compounds is depending upon electropositive character. Electropositive character increases from B to Tl with increase tendency to form ionic compounds.

6. Tendency to form Covalent Compounds:-

- According to Fajan’s rule, smaller the size of the cation, the greater is the tendency for covalent bonding. Boron forms covalent compounds and others form electrovalent compounds.

7. Reducing Character:-

- The reducing character increases from B to Tl because ionization potential decreases.

8. Electronegativity:-

- Boron is the most electronegative & Aluminium is most electropositive. Electronegativity of group 13 is as follows:-

B ˃ Tl ≥ Ga ˃ In ≥ Al

Diagonal Relationship

The relationship in which the first elements of a group shows similarities with the second element in the next higher group in the next period.

Diagonal relationship between B & Si.

1. Non Metallic Character:-

Both Boron and Silicon have nonmetallic character, both of them have high melting points, high ionization energy and are bad conductor of electricity.

2. Density and Boiling Points:-

Both have nearly equal density,

(B = 3.309 cm^-3& Si = 3.529 cm^-3)

Both boil at temperature close to each other i.e. 2550 ⁰C & 2680 ⁰C.

3. Occurrence:-

Neither Boron nor Silicon occurs free in nature. Both occurs in combined state.

4. Stability of Cation:-

Both Boron and Silicon do not form stable cation normally.

5. Tendency to form Covalent Compounds:-

Most of the compound of Boron and Silicon are covalent in nature.

6. Oxide:-

Boron and Silicon form stable oxides B₂O₃ and SiO₂. These oxides are weakly acidic.

7. Formation of Borates and Silicates:-

The oxides B₂O₃ and SiO₂dissolve in strong alkalies forming Meta borates and Meta Silicates.

8. Formation of Esters:-

Orthoboric Acid and Orthosilic acid form esters when treated with alcohol in the presence of H₂SO₄.

9. Hydrolysis of Halides:-

Halides of Boron and Silicon are hydrolysed to form Orthoboric acid &Orthosilic acid.

Hydrides of Group 13 Elements

Group 13 elements do not react directly with hydrogen but a number of interesting hydrides of this group have been prepared indirectly.

Boron does not form simple monomeric species like BH₃ but a large number of polymeric compounds are known.

Boron hydrides are generally called boranes by analogy with alkanes. Boron hydrides are covalent in nature. The simplest and most important Boron hydride is ‘Diborane(B₂H₆)’. Boranes are electron deficient compounds.

Boron Hydrides:-

These hydrides can be divided into two series depending upon their general formula B_mH_m+4and B_mH_m+6.

B_mH_m+4	B_mH_m+6
B₂H₆ – Diborane	B₄H₁₀ – Tetraborane(10)
B₅H₉ – Pentaborane(9)	B₅H₁₁ – Pentaborane(11)
B₆H₁₀ – Hexaborane(10)	B₆H₁₂ – Hexaborane(12)
B₈H₁₂ – Octaborane(12)	B₁₀H₁₆ – Decaborane(16)

The boranes of B_MH_M+6 series are comparatively less stable.

Structure of diborane(B₂H₆)

- B₂H₆is electron deficient compound

- Four hydrogen atoms, two on the left and two on the right, known as terminal hydrogen are in one plane. Other two hydrogen atoms are present one above the plane and other below the plane called as Bridged hydrogen. The Bridged hydrogens are perpendicular to terminal hydrogen.

- Valence orbitals of each Boron atoms in B₂H₆ are sp³hybridised. It contains 4 BH (2C-2e) terminal bond and 2 BHB (BC-2e) bridged bond.

- Total number of valence electron of 2 Boron & 6 Hydrogen is 12.

- 8 electrons are used up in four 2C- 2e terminal bonds, 4 electrons are used in 3C – 2e bridged bond.

Structure of B₄H₁₀

Aluminium forms a polymeric hydride of formula (AlH₃)_n. gallium hydride GaH₃ is less stable while indium hydride(InH₃) and thallium hydride(TLH₃) are extremely unstable

Boron, Aluminium and Gallium form complex anionic hydrides like Li[BH₄], lithium aluminiumhydrade Li[AlH₄] and Lithium gallium hydride Li[GaH₄]. The formation of these anionic complexes is due to the presence of a vacant P orbital in their outermost shells which readily accepts electron pair from the hydride ion (H^-)

XH₃ + H^- → [XH₄]

Where X = Al, Ga & B

XH₃ is electron acceptor and H^- is electron donor. The alkali metal tetrahydrdoborates are ionic in character Li[BH₄] react violently with water liberating hydrogen.

Li[BH₄] + 2H₂O → LiBO₂ + 4H₂

Na[BH₄] reacts slowly with water and K[BH₄] is stable towards water.

Be[BH₄], Al[BH₄] and many other transition metal tetrahydridoborates have considerable covalent character.

The structure of Be[BH₄]₂ and Al[BH₄]₃ are as shown below:

Lithium Aluminium hydride Li[AlH₄] is more important as reducing agent.

Tetrahydridoborates and tetrahydridialuminate are strong reducing agents.

Oxides of Group 13 elements

All the elements of this group form oxides of the general formula M₂O₃ in which the elements are in +3 oxidation state

Ex. B₂O₃, Al₂O₃, Ga₂O₃, In₂O₃ and Tl₂O₃.

1. Oxides of Boron:-

The principle oxide of Boron is brown B₂O₃. It is called diborane trioxide and more commonly boric oxides. It is obtained from boric acid.

2H3BO3 →

It is highly stable substance but hydrolysis in water yielding boric acid H₃BO₃.

The trivalent boron ion B³⁺ being very small, has high positive charge density. Therefore when placed in water, it tends to pull off electrons from water molecule towards itself strongly to rupture the O-H bond. This results in the release of H⁺ ion. Hence oxides of boron are acidic in nature.

2. Oxide of Aluminium:-

The oxide of Aluminium Al2O3 is commonly known as alumina. It is obtained on heating aluminium in oxygen. It also occurs in nature in various form. The two common form of Al₂O₃ are αAl₂O₃ and ᵞ Al₂O₃.

α Al₂O₃is called corundum and ᵞ Al₂O₃called as activated alumina.

α Al₂O₃ is stable at high temperature.

It is very hard and it is used as an abrasive.

ᵞ Al₂O₃ is used in column chromatography. Aluminium oxides are amphoteric in nature. They dissolve both in acids as well as in alkalies.

3. Oxides of others:-

Gallium forms gallium oxide Ga₂O₃, which is also amphoteric. It is formed by direct combination of gallium with oxygen at moderate heat.

Indium oxide In₂O₃ is obtained on heating nitrates or sulphates of the metal. It is a yellow solid and is distinctly basic in aqueous medium.

Thallium forms two oxides in which the oxidation state of the metal is +1 & +3.

Thallium (I) oxide Tl₂O, is more stable than thallium (III) oxide Tl₂O₃. It is brown solid & shows basic character.