Covalent Bonding

What is Covalent Bonding?

In this type of bond, a pair of electrons is shared between two atoms and each of the bonded atoms contributes one electron to the shared pair.

Molecules: Minute Size in Covalent Bonding

Molecules formed by covalent bonding are extremely small, with sizes ranging from fractions of a nanometer to several nanometers.

• Water ($H₂O$): Approx. 0.275 nm in diameter, formed by sharing electrons between hydrogen and oxygen.
• Oxygen ($O₂$): A simple diatomic molecule, around 0.12 nm, formed by a double covalent bond between two oxygen atoms.
• Glucose ($C₆H₁₂O₆$): Larger molecule (~0.9 nm) made from covalent bonds between carbon, hydrogen, and oxygen atoms.

Pure Covalent Bonds

A pure covalent bond is a type of bond where two atoms share electrons equally due to having identical or very similar electronegativities, resulting in no partial charges.

Double Covalent Bonds

A double covalent bond is a bond where two atoms share two pairs of electrons, resulting in a stronger and shorter bond compared to a single covalent bond.

Triple Covalent Bonds

A triple covalent bond is a bond where two atoms share three pairs of electrons, making it the strongest and shortest type of covalent bond.

Characteristics of Covalent Substances

Covalent substances have distinct properties due to the nature of covalent bonds, where atoms share electrons.

• Low Melting and Boiling Points: Covalent substances usually have lower melting and boiling points because the intermolecular forces (e.g., van der Waals or dipole-dipole) between molecules are relatively weak.
• Non-Conductive: Most covalent substances do not conduct electricity because they lack free-moving charged particles (ions or electrons).
• Solubility: Covalent substances are generally insoluble in water (if non-polar) but soluble in non-polar solvents (e.g., oils, benzene). Polar covalent substances, however, may dissolve in water.
• Soft or Brittle Solids: Covalent compounds tend to be soft or brittle because the weak intermolecular forces cannot hold the molecules in a rigid structure.

Sigma and Pi Bonding

Sigma (σ) Bonding

• Formation: A sigma bond forms when two atomic orbitals overlap head-on (along the axis between the nuclei) allowing electrons to be shared between the two atoms.
• Strength: Sigma bonds are stronger than pi bonds due to greater overlap of orbitals.
• Location: Found in all single covalent bonds. In double and triple bonds, one of the bonds is always a sigma bond.

Pi (π) Bonding

• Formation: A pi bond forms when two atomic orbitals overlap sideways, above and below the axis of the bond.
• Strength: Pi bonds are weaker than Sigma bonds because the sideways overlap is less extensive.
• Location: Pi bonds occur in double and triple bonds. In a double bond, there is one sigma bond and one pi bond; in a triple bond, there is one sigma bond and two pi bonds.

Key Differences

• Overlap: Sigma bonds involve head-on overlap, while pi bonds involve sideways overlap.
• Strength: Sigma bonds are stronger than pi bonds.
• Occurrence: Sigma bonds are present in all single, double, and triple bonds; pi bonds are only found in multiple bonds (double or triple).

Polar Covalent Bonding

A polar covalent bond is a bond where two atoms share electrons unequally due to a difference in electronegativities, causing a partial positive charge on one atom and a partial negative charge on the other.

Polar Materials

• Water ($H₂O$): Water is polar because oxygen is more electronegative than hydrogen, causing unequal electron sharing and a dipole.
• Hydrochloric Acid ($HCl$): $HCl$ is polar as chlorine's higher electronegativity causes an uneven distribution of electrons, creating partial charges.

Non-Polar Covalent Bonding

A non-polar covalent bond is a bond where two atoms share electrons equally, because they have identical or very similar electronegativities, resulting in no charge separation or dipole.

Non-Polar Materials

• Oxygen ($O₂$): Oxygen is non-polar as two identical atoms equally share electrons, resulting in no charge separation.
• Oils (e.g., vegetable oil): Oils are non-polar because their long hydrocarbon chains share electrons equally, preventing dipole formation.

Polarity Test for Liquids (Use of Charged Plastic Rod)

To test the polarity of a liquid, a charged plastic rod is used to observe how the liquid reacts:

• Polar liquids (e.g., water) are attracted to the charged rod because they have dipoles, where one part of the molecule is slightly positive, and the other is slightly negative.
• Non-polar liquids (e.g., oil) will not be attracted to the charged rod, as they do not have dipoles and have no interaction with the electric charge. This test helps distinguish between polar and non-polar substances based on their behaviour with electric charges.

Testing Solubility in Different Solvents of Ionic and Covalent Substances

The solubility of substances depends on whether they are ionic or covalent, and the type of solvent:

• Ionic substances (e.g., $NaCl$) dissolve well in polar solvents like water, as the solvent molecules can separate and surround the ions.
• Covalent substances:
  • Polar covalent compounds (e.g., sugar) dissolve in polar solvents like water due to attraction between the polar molecules.
  • Non-polar covalent compounds (e.g., oil) dissolve in non-polar solvents like hexane, as "like dissolves like." Testing solubility helps identify whether a substance is ionic or covalent and the nature of its polarity.