Atoms bond to achieve a more stable electron configuration, typically resembling the electron arrangement of noble gases. Covalent bonds are a fundamental type of chemical bond, characterized by the sharing of electrons between atoms.
How Covalent Bonds Form
Covalent bonds form when atoms share their valence electrons, allowing each atom to achieve a stable electron configuration. Most atoms strive for eight electrons in their outermost shell, known as the octet rule. Hydrogen is an exception, seeking only two electrons to complete its outer shell, following the duet rule.
This sharing creates a strong attractive force holding atoms together within a molecule. Atoms can share one, two, or three pairs of electrons, forming single, double, or triple covalent bonds. For example, a hydrogen molecule (H₂) has a single bond, an oxygen molecule (O₂) has a double bond, and a nitrogen molecule (N₂) has a triple bond.
Elements That Form Covalent Bonds
Covalent bonds primarily form between nonmetal elements like carbon, oxygen, nitrogen, hydrogen, and halogens. These elements have similar electronegativity values, their tendency to attract electrons. Since neither atom strongly “takes” electrons, sharing is the energetically favorable option for stability.
Metalloids like boron, silicon, and germanium also form covalent bonds. These elements possess properties intermediate between metals and nonmetals, and their intermediate electronegativity allows them to share electrons in certain compounds. Silicon dioxide (SiO₂), for instance, is a covalent compound of silicon (a metalloid) and oxygen (a nonmetal). Most nonmetals form one to four covalent bonds for stability.
Common Covalent Compounds and Their Characteristics
Many familiar substances are covalent compounds, also known as molecular compounds. Water (H₂O) forms from covalent bonds between hydrogen and oxygen atoms. Carbon dioxide (CO₂) and methane (CH₄) are other common examples. Organic molecules, including sugars, proteins, nucleic acids, and fats, are all large covalent compounds.
Covalent compounds generally exhibit different physical properties compared to ionic compounds. They typically possess lower melting and boiling points. This is because the forces holding individual molecules together are weaker than the strong bonds within the molecules themselves. When a covalent compound melts or boils, these weaker intermolecular forces are overcome, rather than the strong covalent bonds between atoms.
Covalent compounds are also generally poor conductors of electricity. This is due to the absence of free-moving charged particles like ions or delocalized electrons. While some polar covalent substances can ionize in water and conduct electricity, their solubility in water varies depending on their molecular structure.