Question

Question: Classify each of the following solids as molecular, ionic, metallic, or covalent.

(a) Rb

(b)${\mathit{C}}_{\mathbf{5}}{\mathit{H}}_{\mathbf{12}}$

(c) B

(d)$\mathit{N}\mathit{a}\mathit{H}\mathit{P}{\mathit{O}}_{\mathbf{4}}$

Short answer

(a) Metallic bonding

(b) Molecular bonding

(c) Covalent bonding

(d) Ionic bonding

Step-by-step solution

Subpart (a) Rubidium.

Because of the small difference in electronegativity, Rubidium does not form ionic bonding. The nearly-filled subshells, do not let the metal form covalent bonds.

The arrangement of positively charged ions in the crystal lattice makes Rubidium a neutral metal as the density of the sea of mobile electrons is similar to that of positively charged ions.

Therefore, rubidium shows metallic bonding.

Subpart (b) Pentane.

The intermolecular forces that hold pentane are Van der Waals forces. This force is quite weaker than ionic, metallic, and covalent bonding because of which molecular crystals have low melting points, which makes them soft.

Therefore, pentane is also a molecular crystal, just like many organic compounds where the molecules are packed together without overlapping.

Subpart (c) Boron.

The bonding between boron atoms is covalent.

As boron valency is 3, it neither accepts nor donates electrons from its valence shell, and therefore, it bonds covalently to complete its octet. Example: $B_{12}$

All the boron atoms in this compound coordinate covalently (shares electrons), as shown above.

Subpart (d) Sodium Phosphate.

This compound is formed because of the large difference between the electronegativities of the two atoms.

${\text{Na}}^{+}-{\text{PO}}_4^{2-}-{\text{H}}^{+}$

This difference in electronegativities makes the compound ionic.

These ions are usually hard and charged spheres occupying different positions on the crystal lattice.