When two atoms are brought together, they can link to form a molecule. Any bond that forms between two atoms is called a primary bond. There are three types of primary bond: covalent, ionic and metallic.
Covalent Bonds
The strongest primary bond.
Forms when atoms (usually non-metals) share their electrons so each electron shell achieves an inert gas structure.
Forms when orbitals overlap and each atom attains a more stable electron configuration.
Electrons will spend most of their times in the region where the orbitals overlap, making covalent bonds highly directional.
Covalent Solids
Covalent solids (network solids) are extended 3D or layered frameworks where atoms are linked by directional covalent bonds rather than existing as discrete molecules; classic examples include diamond and SiO₂ (3D tetrahedral networks). Since breaking the solid requires breaking covalent bonds, they have very high melting points and are hard and brittle.
Example:
Polymers
Ionic Bonds
Ionic bonding is the electrostatic attraction between oppositely charged ions (cations and anions).
It forms when one atom (usually a metal) transfers one or more electrons to another atom (usually a non-metal).
The metal becomes a positively charged ion (e.g., Na → Na⁺ + e⁻); the non-metal becomes a negatively charged ion (e.g., Cl + e⁻ → Cl⁻).
Opposite charges attract strongly; ions pack into a giant 3D lattice (not discrete molecules).
Strength of the bond depends on charge and distance: larger charges and smaller ionic radii → stronger attraction (Coulomb’s law).
Lattice energy (energy released when gaseous ions form the crystal) measures how strong the ionic lattice is.
Unlike covalent bonds, ionic bonding is not directional.
Ionic Solids
Ionic solids are crystals made of positively charged cations and negatively charged anions held together by strong, nondirectional electrostatic attraction, so the ions pack into extended lattices determined largely by ion sizes and charges.
Their stability is captured by lattice enthalpy: higher charges and smaller ions give stronger attraction, leading to high melting/boiling points and hardness. They’re brittle because a small shear can align like charges across a slip plane, causing repulsion and a clean fracture
Example:
Metallic Bonds
Positive metal ions in a lattice held by a “sea” of delocalized valence electrons (nondirectional attraction).
Structured into packed lattices wit no discrete molecules meaning atoms share electrons across the whole solid.
Conductivity: mobile electrons give high electrical and thermal conductivity.
Malleable and ductile because planes of ions can slip while the electron sea maintains cohesion.
Thermal stability: generally high melting/boiling points from strong cohesive energy.
Metallic bonding is also not directional.