Covalent Bonding (AQA A-Level Chemistry): Revision Notes
1.4.3 Covalent Bonding
Covalent bonding involves the sharing of electrons between atoms to achieve a full outer electron shell, giving the atoms greater stability. Covalent bonds typically form between non-metal atoms.
What is Covalent Bonding?
A covalent bond is the electrostatic attraction between a shared pair of electrons and the nuclei of the two atoms involved in the bond. The atoms involved share electrons to fill their outermost electron shells.
- Single covalent bond: A single shared pair of electrons between two atoms. For example, in a hydrogen molecule (H₂), each hydrogen atom shares one electron, forming a single covalent bond.
- Multiple covalent bonds: When more than one pair of electrons is shared between atoms. For example:
- Double bond: Two pairs of electrons are shared (e.g., O₂, where two oxygen atoms share two pairs of electrons).
- Triple bond: Three pairs of electrons are shared (e.g., N₂, where two nitrogen atoms share three pairs of electrons).
Representing Covalent Bonds
Covalent bonds can be represented using lines to show the shared pairs of electrons:
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A single line (−) represents a single covalent bond.
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A double line (＝) represents a double bond.
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A triple line (≡****) represents a triple bond. For example:
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Hydrogen (H₂): This represents a single covalent bond between two hydrogen atoms.
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Oxygen (O₂): This shows a double bond between two oxygen atoms.
Properties of Covalent Compounds
- Low melting and boiling points: Covalent molecules generally have lower melting and boiling points compared to ionic compounds because they have weak intermolecular forces (e.g., van der Waals forces or hydrogen bonds).
- Non-conductive: Most covalent compounds do not conduct electricity, as they do not have free-moving charged particles (ions or electrons).
- Solubility: Covalent compounds may or may not dissolve in water, depending on their polarity. Polar covalent molecules (e.g., water) tend to dissolve in polar solvents, while non-polar molecules (e.g., methane) dissolve in non-polar solvents like oil.
Key Examples of Covalent Bonding
Water (H₂O)
- Water molecules have a bent structure due to two single covalent bonds between hydrogen and oxygen.
Carbon Dioxide (CO₂)
- Carbon dioxide consists of two double covalent bonds between carbon and oxygen.
Methane (CH₄)
- Methane has four single covalent bonds between carbon and hydrogen.
Summary
- Covalent bonds form when atoms share electrons, creating an electrostatic attraction between the shared electron pair and the nuclei of the bonded atoms.
- Single, double, and triple bonds involve different numbers of shared electron pairs, influencing the bond strength and molecular structure.
- Covalent compounds exhibit unique properties, such as lower melting points, non-conductivity, and variable solubility, depending on the nature of their bonding.