Chemistry > p Block Elements > 1.0 Group $13$ – The Boron Family
p Block Elements
1.0 Group $13$ – The Boron Family
2.0 Boron
3.0 Compounds of boron
4.0 Compounds of Aluminium
5.0 Group $14$ – The Carbon family
6.0 Allotropes of Carbon
7.0 Compounds of Carbon
8.0 Properties of Silicon
9.0 Group $15$-The Nitrogen Family
10.0 Oxides of nitrogen
10.1 Nitrogen Oxide $N_2O$ or Laughing gas (Neutral)
10.2 Nitric Oxide $NO$ (Neutral)
10.3 Nitrogen trioxide $N_2O_3$
10.4 Nitrogen dioxide or Di-nitrogen tetroxide $NO_2$ or $N_2O$
10.5 Nitrogen pentaoxide $N_2O_5$
11.0 Oxyacids of Nitrogen
11.1 Nitric acid $HNO_3$
11.2 Oxidation of Metalloid and Inorganic compounds by Nitric acid
11.3 Action of Metals & Proteins
12.0 Phosphorus
13.0 Oxygen
14.0 Sulphur
1.2 Chemical Properties
10.2 Nitric Oxide $NO$ (Neutral)
10.3 Nitrogen trioxide $N_2O_3$
10.4 Nitrogen dioxide or Di-nitrogen tetroxide $NO_2$ or $N_2O$
10.5 Nitrogen pentaoxide $N_2O_5$
11.2 Oxidation of Metalloid and Inorganic compounds by Nitric acid
11.3 Action of Metals & Proteins
Reaction with air: Boron is unreactive in crystalline form. Aluminium forms a very thin oxide layer on the surface which protects the metal from further attack. Amorphous boron and aluminium metal on heating in air form $B_2O_3$ and $Al_2O_3$ respectively. With dinitrogen at high temperature they form nitrides.
$$\begin{equation} \begin{aligned} 2E\left( s \right) + 3{O_2}\left( g \right)\mathop \to \limits^\Delta 2{E_2}{O_3}\left( s \right) \\ 2E\left( s \right) + {N_2}\left( g \right)\mathop \to \limits^\Delta 2EN\left( s \right) \\\end{aligned} \end{equation} $$
Reaction with acid and alkalies: Boron is not affected by non-oxidising acids like $HCl$ and dil. $H_2SO_4$ and alkalies even at moderate temperature while other elements dissolve forming trivalent salts and liberates dihydrogen respectively.
$$\begin{equation} \begin{aligned} 2Al\left( s \right) + 6HCl\left( {aq} \right) \to 2A{l^{3 + }}\left( {aq} \right) + 6C{l^ - }\left( {aq} \right) + 3{H_2}\left( g \right) \\ 2Al\left( s \right) + 2NaOH\left( {aq} \right) + 6{H_2}O\left( l \right) \to 2N{a^ + }{[Al{(OH)_4}]^ - }\left( {aq} \right)\left[ {{\text{Sodiumtetra hydroxoaluminate}}\left( {{\text{III}}} \right)} \right] + 3{H_2}\left( g \right) \\\end{aligned} \end{equation} $$
Reaction with Halogens: These elements reacts with halogens to form trihalides except $TlI_3$. The trihalides of all the elements of this group act as lewis acids.
- $B>Al>Ga>In$: electron accepting tendency
- $BF_3$ and $BCl_3$: gases at room temperature
- $BBr_3$: volatile liquid
- $BI_3$: solid
- Lewis Acidity: $BF_3< BCl_3< BBr_3< BI_3$
Because electron density around boron in $BF_3$ increases due to $p\pi - p\pi $ back bonding. The tendency of back donation decreases to $BCl_3$ and $BBr_3$ because the energy difference between fully filled orbital of halogen ($Cl-3p$, $Br-4p$) and vacant orbital of $B-2p$ increases.
- Trihalides of $Al$ exist in dimeric form both in vapour state and in non-polar solvent.
- ${\left[ {Al{F_6}} \right]^{3 - }}$, ${\left[ {GaC{l_6}} \right]^{3 - }}$, ${\left[ {InC{l_6}} \right]^{3 - }}$ are formed by $Al$, $Ga$ and $In$ due to availability of vacant $d$-orbital in central atoms.
Hydrides:
These elements do not react directly with hydrogen.
Boron forms covalent hydrides, $B_nH_{n+4}$ and $B_nH_{n+6}$ called boranes.
$Al$: polymeric hydride – ${\left[ {Al{H_3}} \right]_n}$: decomposes on heating.
$InH_3$ and $TiH_3$: extremely unstable.
$B$, $Al$ and $Ga$ have the tendency to form complex anionic hydrides like $Li\left[ {Al{H_4}} \right]$, $Li\left[ {B{H_4}} \right]$, $Li\left[ {Ga{H_4}} \right]$ etc. due to the presence of nascent $p$-orbital in their outermost shell due to which they readily accepts electron pair from the hydride ion ($H^-$) as $$A{H_3} + {H^ - } \to {[A{H_4}]^ - }\quad \left( {A = Al,Ga{\text{ }}and{\text{ }}B} \right)$$
Oxides:
All members of the group forms oxides of $M_2O_3$ type.
As we move down the group, the oxides change from acidic to amphoteric and then basic character as shown below with the types of oxides formed.
| $$B{\left( {OH} \right)_3}$$ | $$Al{\left( {OH} \right)_3}$$ | $$Ga{\left( {OH} \right)_3}$$ | $$In{(OH)_3}$$ | $$Ti{\left( {OH} \right)_3}$$ |
| $${B_2}{O_3}$$ | $$A{l_2}{O_3}$$ | $$G{a_2}{O_3}$$ | $$I{n_2}{O_3}$$ | $$T{i_2}{O_3}$$ |
| Acidic | Amphoteric | Amphoteric | Basic | Strong basic |
| Basic character increases as we move down the group | ||||
Explanation: Acidic character of $B_2O_3$
As the size of boron is small, there is a very high positive charge density which attracts the electrons of water molecule and results in making the $O-H$ bond weaker.
While in case of $A{l^{3 + }}$ and $G{a^{3 + }}$, ions are relatively larger as compared to boron. Their tendency to break the $O-H$ bond becomes less and which results in decreasing acidic character and the oxides become amphoteric in nature.
On further moving down the group, $I{n^{3 + }}$ and $T{i^{3 + }}$ ions are larger and interaction with $H_2O$ is negligible which results in basic nature of the oxides.
Hydroxides:
All elements form hydroxides of type $M(OH)_3$.
The basic nature of hydroxides increases as we move down the group which can be explained in the similar way as explained in case of oxides.
