Molecular Bonds (Covalent Bonds)
* Formation: Atoms share electrons to achieve a stable electron configuration (usually a full outer shell).
* Electronegativity: Atoms involved have similar electronegativities (attraction for electrons). The difference in electronegativity is small, usually less than 1.7.
* Bonding Type: Sharing of electrons.
* Resulting Compounds: Usually form molecules (discrete units), often gases or liquids at room temperature.
* Properties:
* Lower melting and boiling points: Because the bonds are weaker, less energy is needed to break them.
* Typically non-conductive: Electrons are localized within the molecule, not free to move.
* Can be polar or non-polar: Depends on the symmetry of the molecule and the electronegativity difference between the atoms.
Examples:
* Water (H₂O) - The hydrogen and oxygen atoms share electrons.
* Methane (CH₄) - Carbon and hydrogen atoms share electrons.
Ionic Bonds
* Formation: One atom (typically a metal) loses one or more electrons, becoming a positively charged ion (cation). Another atom (typically a non-metal) gains these electrons, becoming a negatively charged ion (anion). Opposite charges attract, forming the bond.
* Electronegativity: Atoms involved have significantly different electronegativities. The difference in electronegativity is typically greater than 1.7.
* Bonding Type: Electrostatic attraction between oppositely charged ions.
* Resulting Compounds: Form ionic compounds (salts), often crystalline solids at room temperature.
* Properties:
* High melting and boiling points: Strong electrostatic forces require a lot of energy to break.
* Conductive when dissolved or melted: Free ions can carry electrical current.
* Often brittle: The rigid structure can easily fracture.
Examples:
* Sodium chloride (NaCl) - Sodium loses an electron to become Na⁺, while chlorine gains an electron to become Cl⁻.
* Magnesium oxide (MgO) - Magnesium loses two electrons to become Mg²⁺, and oxygen gains two electrons to become O²⁻.
Key Point: The classification of a bond as purely ionic or covalent is often a simplification. Many bonds have characteristics of both. The term "polar covalent" is used when there is an uneven sharing of electrons, leading to a slightly positive and slightly negative end of the molecule.
