Why Do Atoms Bond?
Atoms bond with one another because doing so makes them more stable. Most atoms have an incomplete outermost electron shell and seek to fill it — either by sharing, gaining, or losing electrons. The type of bond that forms depends on the nature of the atoms involved, particularly their electronegativity and electron configuration.
Ionic Bonds
An ionic bond forms when one atom transfers one or more electrons to another atom. This typically happens between a metal and a nonmetal. The atom that loses electrons becomes a positively charged cation, while the atom that gains electrons becomes a negatively charged anion. The opposite charges attract each other, holding the atoms together.
Classic example: Sodium (Na) gives one electron to chlorine (Cl) to form sodium chloride (NaCl) — table salt.
- Typically between metals and nonmetals
- Form crystalline lattice structures
- High melting and boiling points
- Conduct electricity when dissolved in water or melted
Covalent Bonds
A covalent bond forms when two atoms share electrons rather than transfer them. This is most common between nonmetal atoms. Shared electrons are attracted to the nuclei of both atoms simultaneously, holding the atoms together.
Classic example: Two hydrogen atoms share a pair of electrons to form H₂ gas. In water (H₂O), oxygen shares electrons with two hydrogen atoms.
- Single bond: One shared pair of electrons (e.g., H–H)
- Double bond: Two shared pairs (e.g., O=O in O₂)
- Triple bond: Three shared pairs (e.g., N≡N in N₂)
Covalent compounds generally have lower melting points than ionic compounds and do not conduct electricity in solution.
Metallic Bonds
In metals, atoms release their outer electrons into a shared "sea" of electrons that moves freely throughout the structure. This is called a metallic bond. The positively charged metal ions are held together by their attraction to this mobile electron sea.
- Explains why metals conduct electricity (free-moving electrons)
- Explains malleability — ions can slide past each other without the bond breaking
- Examples: iron (Fe), copper (Cu), gold (Au)
Comparing the Three Bond Types
| Property | Ionic | Covalent | Metallic |
|---|---|---|---|
| Electron behaviour | Transferred | Shared | Delocalized |
| Between | Metal + nonmetal | Nonmetals | Metals only |
| Conductivity | When dissolved/melted | Usually none | Always conducts |
| Melting point | High | Variable (often low) | Variable (often high) |
Polar vs. Nonpolar Covalent Bonds
Not all covalent bonds are equal. When two different atoms share electrons, the more electronegative atom pulls the shared electrons closer, creating a polar covalent bond with partial charges. Water (H₂O) is a classic polar molecule. When atoms of similar electronegativity share electrons (like in H₂ or O₂), the bond is nonpolar.
The Big Picture
Understanding bonding explains why substances behave as they do — why salt dissolves in water, why diamond is so hard, why copper wires conduct electricity. Bonding is chemistry's blueprint for the material world.