The P-Block Elements (Group 13 - Boron Family)

Group 13 Elements: The Boron Family

Group 13 elements, starting with Boron (B), include Aluminum (Al), Gallium (Ga), Indium (In), Thallium (Tl), and Nihonium (Nh). Boron is a non-metal, Aluminum is a metal, and the rest are post-transition metals.

Electronic Configuration

General Configuration: The general valence shell electronic configuration of Group 13 elements is $ns^2np^1$.

B: $[He] 2s^22p^1$
Al: $[Ne] 3s^23p^1$
Ga: $[Ar] 3d^{10} 4s^24p^1$
In: $[Kr] 4d^{10} 5s^25p^1$
Tl: $[Xe] 4f^{14} 5d^{10} 6s^26p^1$
Nh: $[Og] 7s^27p^1$

Significance: The presence of three valence electrons explains their tendency to lose these electrons to form $+3$ ions or to form covalent compounds by sharing these electrons.

Atomic Radii

Trend: Atomic radii increase down the group from B to Tl.

Reasons:

Increase in the number of electron shells.
Increased shielding effect of inner electrons.

Anomalous Behavior of Ga, In, Tl:

The atomic radii of Gallium (Ga) is slightly smaller than Aluminum (Al). This is due to the relatively poor shielding effect of the ten $3d$ electrons introduced in Gallium, which increases the effective nuclear charge experienced by the outer electrons.
After Gallium, the atomic radii increase down the group as expected.

Ionic Radii ($M^{3+}$): The ionic radii of $+3$ ions also generally increase down the group, with a similar anomaly for $Ga^{3+}$ being smaller than $Al^{3+}$.

Ionization Enthalpies

Trend: First ionization enthalpies generally decrease from B to Tl, as expected due to increasing atomic size.

Anomalies:

The first ionization enthalpy of Gallium (Ga) is higher than that of Aluminum (Al). This is due to the poor shielding effect of the intervening $3d$ electrons.
The ionization enthalpies of Indium (In) and Thallium (Tl) are significantly higher than expected. This is due to the introduction of the $4f^{14}$ and $5d^{10}$ electron shells in Thallium, which cause poor shielding and a substantial increase in effective nuclear charge.

Second and Third Ionization Enthalpies: The sum of the first three ionization enthalpies is very high, indicating that the $+3$ oxidation state is generally more stable than $+1$ oxidation state, except for Gallium, Indium, and Thallium where the $+1$ state becomes increasingly stable.

Electronegativity

Trend: Electronegativity values are relatively similar across the group, but show a slight increase from Al to Tl, and then a slight decrease for Nh. Boron has the highest electronegativity.

B: 2.04
Al: 1.61
Ga: 1.81
In: 1.78
Tl: 2.04
Nh: ~2.0 (?) (Estimated)

Anomalies: The increase in electronegativity from Al to Ga and Tl is related to the poor shielding effect of the $d$ and $f$ electrons.

Physical Properties

Appearance:

Boron: Black solid, very hard, poor conductor of heat and electricity.
Aluminum: Silvery-white metal, good conductor of heat and electricity.
Ga, In, Tl: Silvery-white metals with low melting points (Gallium melts in hand).

Hardness: Boron is extremely hard. Aluminum is moderately hard. Ga, In, Tl are relatively soft metals.

Melting and Boiling Points: Melting and boiling points decrease from B to Al, then increase for Ga and In, and finally decrease for Tl. The high melting point of Boron is due to its covalent network structure.

Density: Density generally increases down the group, with anomalies for Ga and Tl.

Chemical Properties

Oxidation State: The most common oxidation state is $+3$, due to the loss of all three valence electrons ($ns^2np^1$). However, due to the inert pair effect, the $+1$ oxidation state becomes increasingly stable for Ga, In, and especially Tl down the group.

Reactivity:

Boron is non-metallic and behaves like a typical non-metal.
Aluminum is a highly reactive metal but forms a protective oxide layer ($Al_2O_3$) that prevents further reaction.
Gallium, Indium, and Thallium are metals with progressively decreasing reactivity.

Action with Air:

Boron forms boric oxide ($B_2O_3$).
Aluminum forms a protective oxide layer ($Al_2O_3$).
Other elements form oxides ($Ga_2O_3$, $In_2O_3$, $Tl_2O_3$).

Action with Water:

Boron and Silicon are unaffected by water.
Aluminum reacts slowly with water but vigorously with steam.
$2Al(s) + 6H_2O(g) \xrightarrow{high \ T} 2Al_2O_3(s) + 3H_2(g)$
Ga, In, Tl are unaffected by water.

Action with Acids and Alkalis:

Boron does not react with acids or alkalis.
Aluminum reacts with both dilute acids and strong alkalis to liberate $H_2$ gas.

With acids: $2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)$

With alkalis: $2Al(s) + 2NaOH(aq) + 6H_2O(l) \rightarrow 2Na[Al(OH)_4](aq) + 3H_2(g)$

Ga, In, Tl react with dilute non-oxidizing acids but not with alkalis.

Formation of Halides:

Boron forms covalent halides ($BCl_3$, $BBr_3$) which are hydrolysed in water.
Aluminum forms covalent halides ($AlCl_3$, $AlBr_3$) which dimerize in the vapor phase and form hydated ionic salts in aqueous solution.
Ga, In, Tl form predominantly ionic halides ($GaCl_3$, $InCl_3$, $TlCl_3$, $TlCl$).

Formation of Oxo-salts:

Boron forms boric acid ($H_3BO_3$) and borates.
Aluminum forms aluminum hydroxide ($Al(OH)_3$) and aluminates.

Important Trends And Anomalous Properties Of Boron

Boron (B) is the first element of Group 13 and exhibits several properties that are distinct from the other members of the group, which are predominantly metallic.

Diagonal Relationship Between Boron And Silicon

Boron shows a diagonal relationship with Silicon (Si), the second element of Group 14. This similarity arises from their comparable atomic sizes and charge densities, leading to similar polarizing power.

Similarities:

Non-Metallic Character: Both Boron and Silicon are non-metals, although Silicon exhibits some metallic properties (metalloid).
Formation of Oxides: Both form acidic oxides ($B_2O_3$ and $SiO_2$) which are hard solids with high melting points.
Reactions with Water: Neither Boron nor Silicon reacts with water.
Reactions with Acids: Neither reacts with non-oxidizing acids. Both react with alkalis to liberate hydrogen gas.

$2B + 2NaOH + 6H_2O \rightarrow 2Na[B(OH)_4] + 3H_2$
$Si + 2NaOH + H_2O \rightarrow Na_2[SiO(OH)_3] + 2H_2$ (Sodium silicate or Sodium metasilicate)

Formation of Halides: Both form covalent halides that are readily hydrolyzed by water.

$BCl_3 + 3H_2O \rightarrow B(OH)_3 + 3HCl$
$SiCl_4 + 2H_2O \rightarrow SiO_2 + 4HCl$

Formation of Borides and Silicides: Both form compounds known as borides and silicides with metals.

Anomalous Properties Of Boron

Boron differs significantly from other members of Group 13 due to its unique characteristics:

Non-Metallic Nature: Boron is a non-metal, whereas all other elements in Group 13 are metals.
Small Atomic and Ionic Size: Boron has the smallest atomic and ionic ($B^{3+}$) radii in Group 13.
High Ionization Enthalpy: Boron has the highest first ionization enthalpy in Group 13.
High Electronegativity: Boron has the highest electronegativity in Group 13.
Tendency to Form Covalent Compounds: Due to its small size and high ionization enthalpy, boron predominantly forms covalent compounds. Its compounds are generally resistant to hydrolysis.
Formation of Boron Nitride ($BN$): Boron forms a covalent compound $BN$ with nitrogen, which has a structure similar to graphite or diamond and exhibits exceptional hardness.
Electronic Deficiency: Boron halides (like $BF_3$) are electron-deficient and act as Lewis acids. They readily accept a lone pair of electrons from Lewis bases.
Oxidation State: Boron almost exclusively exhibits the $+3$ oxidation state. Unlike other elements in the group, the $+1$ oxidation state is not observed for boron.
Hydrides: Boron forms a series of electron-deficient covalent hydrides called boranes, which are often complex and polymeric (e.g., diborane, $B_2H_6$). Other Group 13 elements also form hydrides, but they are typically simpler and monomeric (e.g., $AlH_3$ is polymeric, but different from boranes; $GaH_3$, $InH_3$, $TlH_3$ are less stable).

These anomalous properties of Boron are the reason why it is often placed separately at the top of Group 13.