Behaviour of Boron

Anomalous Behaviour of Boron :

Behaviour of Boron it typically different from other elements. Boron differs from the other elements of the group significantly. The following aspects are those in which boron differs from other elements. This may be because the atom is small and to the difference in the penultimate shell configuration ; its first ionization potential is high.

  • Boron is a non – metal. Al is an amphoteric metal Ga, In and Tl are metals.
  • It always forms covalent compounds while other may form ionic compounds.
  • Boron show diagonal relationship with Si. Such a relationship is not shown by other elements.
  • Boron does not displace hydrogen from acids while other elements, being metals, displace hydrogen from acids under suitable conditions.
  • \text{B}_{2} \text {O}_{3} is an acidic oxide like \text{SiO}_{2}. The trioxides of other elements are either amphoteric or basic in nature.
  • Ex : \text{A}l_{2} \text {O}_{3} \Rightarrow amphoteric ; TIOH is a strong base.
  • \text {B(OH)}_{3} or \text {H}_{3} \text {BO}_{3} is an acid whilst the hydroxides of other elements are either amphoteric or basic in nature.
  • Simple borates and silicates can polymerize readily forming poly – acids while others do not form such polymers.
  • Boron has covalency maximum of 4 only. But another exhibit a covalency maximum of 6.
  • Boron forms covalent hydrides which are stable. It is difficult to have stable hydrogen compounds.
  • Boron never appears as a cation whereas the other elements have a good number of compounds in which the elements form trivalent cations.

Because of these reasons behaviour of Boron is different from other elements.

The properties of boron :

Atomic Symbol                :                B
Atomic Number              :                5
Element Category           :                Metalloid
Density                              :                2.08g/cm3
Melting Point                   :                3769 F (2076 C)
Boiling Point                    :                7101 F (3927 C)
Moh’s Hardness              :                ~9.5

Chemical properties of Boron :

Action of air of \text {O}_{2} : Amorphous Boron, on heating in air \left (\text {O}_{2}, \text {N}_{2} \;\text {etc.} \right ) to about 700^{\circ} \text {C}, form \text {B}_{2} \text {O}_{3} and BN.

\text {4B + 3O}_{2} \longrightarrow \text {2B}_{2} \text {O}_{3} \;(\text {Boron trioxide})

\text {2B + N}_{2} \longrightarrow \text {2BN} \; (\text {Boron nitride})

With non – metals : On heating amorphous Boron will react with \text {N}_{2}, \text {O}_{2}, \text {S}, \text{F}_{2}, \text {C}l_{2}, \text {and} \; \text {Br}_{2}

directly forming binary compounds.

\text {E}x : \text {4B + 3O}_{2} \longrightarrow \text {2B}_{2} \text {O}_{3}

Boron trioxide (or) Boric anhydride

Boron is a very powerful reducing agent for \text {CO}_{2} \; \text {or SiO}_{2}.

\text {3SiO}_{2} + \text {4B} \longrightarrow \text {2B}_{2} \text {O}_{3} + \text {3Si}

It reduces \text {HNO}_{3} \; \text {to NO.}

\text {B + 3HNO}_{3} \longrightarrow \text {H}_{3} \text {BO}_{3} + \text {3NO}

Orthoboric acid

When fused with alkalies, ‘B’ liberates \text {H}_{2}.

\text {2B + 6NaOH} \longrightarrow \text {2Na}_{3}\text {BO}_{3} + \text {3H}_{2} \uparrow