Types of chemical bonds — ionic, covalent, metallic, hydrogen, van der Waals

Question

Classify chemical bonds into their major types. For each type, explain the mechanism of bond formation, give examples, and state typical properties of substances held by that bond.

Bond Type Selection Flowchart

flowchart TD
    A["Two atoms interacting"] --> B["Large electronegativity difference?"]
    B -->|"Yes (> 1.7)"| C["Ionic Bond — electron transfer"]
    B -->|"No"| D["Both are metals?"]
    D -->|Yes| E["Metallic Bond — electron sea"]
    D -->|No| F["Share electrons?"]
    F -->|"Yes (non-metals)"| G["Covalent Bond"]
    G --> H["Equal sharing? Non-polar covalent"]
    G --> I["Unequal sharing? Polar covalent"]
    A --> J["Between molecules (not atoms)"]
    J --> K["H bonded to F, O, or N?"]
    K -->|Yes| L["Hydrogen Bond"]
    K -->|No| M["van der Waals Forces"]

Solution — Step by Step

Formed when one atom transfers electrons to another. Typically occurs between a metal (low ionisation energy) and a non-metal (high electron affinity). The resulting ions are held together by electrostatic attraction.

Example: NaCl — Na loses one electron to become $\text{Na}^+$ , Cl gains it to become $\text{Cl}^-$ .

Properties of ionic compounds: High melting/boiling points, conduct electricity when molten or dissolved, hard and brittle, form crystal lattices.

Rule of thumb: Electronegativity difference greater than 1.7 usually gives ionic bonding.

Formed when two atoms share electron pairs. Occurs between non-metals. If sharing is equal, it is non-polar covalent; if unequal, it is polar covalent.

Examples:

Non-polar: $\text{H}_2$ , $\text{Cl}_2$ , $\text{N}_2$ (identical atoms, equal sharing)
Polar: $\text{HCl}$ , $\text{H}_2\text{O}$ (Cl and O are more electronegative, pulling shared electrons)

Properties: Lower melting points than ionic, poor conductors, can exist as gases/liquids/soft solids. Directional bonds — responsible for molecular shapes.

Metal atoms release their valence electrons into a shared “sea” of delocalised electrons. The positive metal ion cores are held together by their attraction to this electron sea.

Examples: All metals — Fe, Cu, Al, Au.

Properties: Good electrical and thermal conductivity (free electrons carry charge and heat), malleable and ductile (layers of ions can slide without breaking the bond), lustrous (free electrons absorb and re-emit light).

A type of strong dipole-dipole interaction. Occurs when H is bonded to a highly electronegative atom (F, O, or N), creating a strong partial positive charge on H that attracts a lone pair on a nearby F, O, or N atom.

Examples: Water ( $\text{H}_2\text{O}$ ), HF, $\text{NH}_3$ , DNA base pairing.

Effect: Raises boiling point dramatically. Water boils at 100 degrees C while $\text{H}_2\text{S}$ (no H-bonding) boils at -60 degrees C, despite $\text{H}_2\text{S}$ being heavier.

Present in all molecules, but are the only attractive forces in non-polar molecules. Two types:

London dispersion forces — temporary dipoles due to instantaneous electron fluctuations. Strength increases with molecular size and surface area.
Dipole-dipole forces — between polar molecules.

Examples: Noble gases (He, Ne, Ar) are held in liquid state only by London forces. $\text{I}_2$ is solid at room temperature because its large electron cloud creates strong London forces.

Why This Works

The type of bond depends on the nature of the atoms involved and their electronegativity difference. The strength hierarchy is: ionic $\approx$ covalent $\approx$ metallic (all strong primary bonds) $\gg$ hydrogen bond $\gg$ van der Waals forces. This explains why NaCl melts at 801 degrees C (ionic) while ice melts at 0 degrees C (hydrogen bonds between covalent water molecules).

For JEE/NEET, remember that hydrogen bonding explains anomalous properties of water (high bp, high specific heat, ice floats). It also explains why HF has a higher boiling point than HCl despite being lighter. These are extremely common questions across all exams.

Quick Comparison

Bond Type	Between	Strength	Conductivity	Example
Ionic	Metal + Non-metal	Strong	When molten/dissolved	NaCl
Covalent	Non-metal + Non-metal	Strong	Usually poor	$\text{H}_2\text{O}$
Metallic	Metal + Metal	Strong	Excellent	Cu, Fe
Hydrogen	H-F/O/N molecules	Moderate	N/A	Water, DNA
van der Waals	All molecules	Weak	N/A	Noble gases

Common Mistake

Students often say “hydrogen bond is a type of chemical bond.” It is NOT a chemical bond in the traditional sense — it is an intermolecular force. A hydrogen bond (5-30 kJ/mol) is about 10-20 times weaker than a covalent bond (150-400 kJ/mol). In exams, if the question asks about “types of chemical bonds,” list ionic, covalent, and metallic. Hydrogen bonds and van der Waals forces are intermolecular forces, not chemical bonds.