Latest about Covalent and Molecular Solid 2022
The crystalline solids in which neutral atoms are held together by covalent bonds are called covalent crystals.
These are also known as atomic solids.
The atoms may be of the same or different elements.
Types of covalent solids:
Covalent crystals are of two types
(1) Giant Covalent Solids: The crystal in which atoms are joined together to form big molecules. Examples: Diamond, SiC or AIN, etc.
(H) Layered Covalent Solids: The crystal in which atoms are joined together to form separate layers. Examples: graphite, cadmium iodide and BN, etc.
Properties of Covalent Solids
1. These crystals have a three-dimensional network of atoms joined together by covalent bonds.
2. Since covalent bonds are directional bonds, therefore, considerable free space is present in these crystals and their packing is looser than ionic crystals. Covalent crystals have an open structure.
3. These are hard and a lot of energy is required to break them
4. These have high melting points and boiling points.
5. These have low volatility due to strong covalent bonds.
6. In these, no free ions are present, hence these are generally bad conductors of electricity. However, graphite parallel layers have free electrons. Hence it is a good conductor of electricity parallel to its layer.
However, graphite is not a conductor perpendicular to its layers.
7. These are insoluble in polar solvents like water but soluble in non-polar solvents like benzene, CCI, etc. Covalent crystals having big molecules like a diamond, SiC, etc are insoluble in all solvents. It is because they cannot develop forces with solvent molecules due to their big size.
8. These solids show very slow reactions.
Structure of Diamond
Diamond is an allotropic form of carbon. There are four valence electrons in carbon.
The four orbitals (one 2s and three 2p) undergo sp3 hybridization to give four sp3 hybrid orbitals.
Thus, each carbon has four sp3 hybrid orbitals, which are directed towards the comers of a regular tetrahedron. This is the unit cell of the diamond. Many unit cells undergo sp3 – sp3 overlapping to form a huge three-dimensional structure of diamond. Sp3-sp3 overlapping form covalent bonds.
Each carbon is linked to four other carbon atoms through covalent bonds. The structure of diamond continuous and carbon-carbon overlapping form a big structure. The whole crystal looks like a huge carbon molecule. Thus, diamond is also called the polymer of carbon. It is a macromolecule.
In diamond all the bond angles are 109.5˚” and the bond lengths are 154 pm. The overall structure of the diamond is a face-centered cubic lattice.
The solids in which molecules of a substance are held together by weak Intermolecular forces are molecular crystals.
Two types of intermolecular forces are present in molecular solids
a) Dipole-dipole interaction b) van der Waal’s forces
These forces are much weaker than ionic or covalent bonds.
Polar Molecular Crystals: Ice, Sugar
Non-Polar Molecular Crystals: iodine, Sulphur, P, CO, etc
In solidified noble gases, non-polar atoms are present.
Properties of Molecular Solids
1. These have a regular arrangement of atoms in molecules. The position of atoms can be determined by X – rays being analyzed.
2. These are soft and easily compressible due to the presence of weak intermolecular forces.
3. These have low melting points and boiling points due to the presence of weak intermolecular forces.
4. These are mostly volatile due to the presence of weak intermolecular forces.
5. These are bad conductors of electricity, due to the absence of ions or free electrons.
6. These have large empty spaces, in a crystal hence these have low densities.
7. Sometimes, these are transparent to light.
8. Polar molecular crystals are soluble in polar solvents, while non-polar molecular crystals are soluble in non-polar solvents.
9. Generally, polar molecular solids have higher melting points and boiling points than nonpolar ones.
Structure of I₂ Molecular Solid
In solid form, I₂ molecules have a layered structure.
In solid I₂, each 1-1 bond distance is 271.5 pm. This distance is longer than 1-1 distance in gaseous iodine, which is 266.6 pm.
This structure has no free electrons so I₂ is a poor conductor of electricity.