In contrast to ionic bonding, where electrons are transferred, covalent bonding involves the sharing of outer shell electrons. When metals bond with other metals, they form a metallic lattice held together by delocalised electrons. Understanding these bonding models is essential for explaining the properties of substances.
🔑 Key Principle
The electrostatic attraction in a covalent bond acts between the shared pair of electrons (negative) and the two nuclei (positive) of the bonded atoms. In a metallic bond, the electrostatic attraction acts between the positive metal ions and the surrounding sea of delocalised electrons.
Covalent Bonding
Non-metal atoms share electrons to achieve a stable full outer shell. A single covalent bond represents one shared pair of electrons. Multiple bonds can also form:
- Single Covalent Bond: One shared pair of electrons (e.g. \( \text{H}_2 \), \( \text{Cl}_2 \), \( \text{CH}_4 \)).
- Double Covalent Bond: Two shared pairs of electrons (e.g. \( \text{O}_2 \), \( \text{CO}_2 \)).
- Triple Covalent Bond: Three shared pairs of electrons (e.g. \( \text{N}_2 \)).
The strong electrostatic attraction between a shared pair of electrons and the nuclei of the two bonded atoms.
Coordinate (Dative Covalent) Bonding
A coordinate bond (also known as a dative covalent bond) is a type of covalent bond where both electrons in the shared pair come from the same atom. Once formed, a coordinate bond is identical in strength, length, and behavior to an ordinary covalent bond.
A covalent bond in which both electrons in the shared pair are provided by a single donor atom.
To form a coordinate bond, the donor atom must have a lone pair of electrons in its outer shell, and the acceptor atom must have an empty orbital that can accommodate the electron pair. Examples include:
- Ammonium ion (\( \text{NH}_4^+ \)): The nitrogen atom in ammonia (\( \text{NH}_3 \)) donates its lone pair of electrons to a hydrogen ion (\( \text{H}^+ \)).
- Hydronium ion (\( \text{H}_3\text{O}^+ \)): The oxygen atom in water (\( \text{H}_2\text{O} \)) donates a lone pair to a hydrogen ion (\( \text{H}^+ \)).
- Aluminium chloride dimer (\( \text{Al}_2\text{Cl}_6 \)): Two \( \text{AlCl}_3 \) molecules dimerise because a chlorine atom on each molecule donates a lone pair to the aluminium atom on the other.
In structural formulas, a coordinate bond is represented by an arrow (\( \rightarrow \)) pointing from the donor atom to the acceptor atom.
When drawing a dative covalent bond in a dot-and-cross diagram, the shared pair must be shown using two identical symbols (such as two dots) that clearly belong to the donor atom, whilst the acceptor atom supplies zero electrons to that bond.
Metallic Bonding
Metallic bonding occurs in metals and alloys. It consists of a giant, regular lattice of positive metal ions (cations) held in fixed positions, surrounded by a mobile "sea" of delocalised electrons.
The electrons are delocalised because the outer shell electrons of the metal atoms can detach easily from the nuclei and move freely throughout the structure.
The electrostatic attraction between positive metal ions and the sea of delocalised electrons throughout a giant metallic lattice.
Solution:
1. Compare valencies: Magnesium forms \( \text{Mg}^{2+} \) ions and donates 2 delocalised electrons per atom. Sodium forms \( \text{Na}^+ \) ions and donates only 1 delocalised electron per atom.
2. Compare ionic radii: Magnesium ions (\( \text{Mg}^{2+} \)) are smaller than sodium ions (\( \text{Na}^+ \)) due to a greater nuclear charge pulling on the same number of shells.
3. Explain electrostatic attraction: The higher charge (\( 2+ \) vs \( 1+ \)) and smaller size of the magnesium cation, combined with double the density of delocalised electrons, leads to much stronger electrostatic attractions in the metallic lattice of \( \text{Mg} \) compared to \( \text{Na} \).
4. Relate to melting point: More thermal energy is required to overcome these stronger metallic bonds in magnesium, resulting in a higher melting point.
Crystal Structures
You must be able to describe and compare the structure and properties of substances with different types of crystal lattice:
1. Giant Macromolecular (Covalent) Structures
These substances consist of giant networks of atoms held together by strong covalent bonds throughout the lattice. Examples include:
- Diamond: A giant tetrahedral lattice of carbon atoms. Each carbon atom forms four strong covalent bonds. Diamond is extremely hard, has a very high melting point, and does not conduct electricity (no mobile charged particles).
- Graphite: Consists of hexagonal layers of carbon atoms. Each carbon atom forms three covalent bonds, leaving one delocalised electron per carbon atom. The layers are held together by weak intermolecular van der Waals forces, allowing them to slide over each other (making it soft and slippery). Graphite conducts electricity along its layers due to the mobile delocalised electrons.
- Graphene: A single two-dimensional sheet of carbon atoms, one atom thick, arranged in a hexagonal lattice. It is incredibly strong, lightweight, and conducts electricity extremely well.
- Silicon Dioxide (\( \text{SiO}_2 \)): A giant macromolecular structure similar to diamond, where each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms. It is hard, has a high melting point, and is an insulator.
2. Molecular Structures
These substances consist of small molecules. Whilst strong covalent bonds act within the molecules (intramolecular), only weak intermolecular forces act between the molecules. Examples include:
- Iodine (\( \text{I}_2 \)): Forms a crystalline molecular lattice held together by weak London dispersion forces. It has a low melting point and easily sublimes.
- Ice (\( \text{H}_2\text{O} \)): Consists of water molecules arranged in a regular, open hexagonal framework held together by hydrogen bonds. This open structure makes ice less dense than liquid water.
Comparing Types of Structures
| Type of Structure | Particles Present | Attractive Forces | Melting Point | Electrical Conductivity |
|---|---|---|---|---|
| Giant Ionic (e.g. \( \text{NaCl} \)) | Positive & negative ions | Ionic bonds (electrostatic attractions between ions) | High | Only when molten or in solution |
| Giant Metallic (e.g. \( \text{Mg} \)) | Positive ions & delocalised electrons | Metallic bonds (electrostatic attractions) | High | Conducts in both solid and molten states |
| Giant Macromolecular (e.g. Diamond, \( \text{SiO}_2 \)) | Atoms | Covalent bonds (electrostatic attractions) | Very High | Insulator (except graphite/graphene) |
| Molecular (e.g. \( \text{I}_2 \), Ice) | Molecules | Weak intermolecular forces | Low | Insulator |
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