Covalent bonds Knowledge-Based Study

Study notes

1. What is a Covalent Bond?

   A covalent bond is a chemical bond formed when two nonmetal atoms share one or more pairs of valence electrons. Unlike ionic bonding, where electrons are transferred from a metal to a nonmetal to form ions, covalent bonding involves the mutual sharing of electrons so that each participating atom achieves a stable, noble-gas electron configuration (usually an octet, or a duet in the case of hydrogen).

   Key characteristics of covalent bonds include:

   • They primarily form between nonmetal atoms with similar or moderately different electronegativities.
   • The shared electrons are attracted simultaneously to the positive nuclei of both bonded atoms, creating an electrostatic force of attraction that holds the molecule together.
   • The bonded atoms form discrete, neutral entities called molecules.

   Bond Order and Multiplicity

   Depending on the number of electron pairs shared between two atoms, covalent bonds are categorized into three primary types:

   • Single Covalent Bond: One pair of electrons (two electrons total) is shared between two atoms. For example, two hydrogen atoms share one pair of electrons to form a single bond.
   • Double Covalent Bond: Two pairs of electrons (four electrons total) are shared between two atoms. For example, two oxygen atoms share two pairs of electrons to fulfill their octets.
   • Triple Covalent Bond: Three pairs of electrons (six electrons total) are shared between two atoms. For example, two nitrogen atoms share three pairs of electrons.

   Relationship Between Bond Strength and Length: As bond multiplicity (bond order) increases from single to double to triple, the number of shared electrons increases. This pulls the atomic nuclei closer together, resulting in a shorter bond length and a greater bond dissociation energy (stronger bond).

   Polar vs. Nonpolar Covalent Bonds

   Electrons in a covalent bond are not always shared equally. The distribution of electron density depends on the electronegativity of the bonded atoms, which is an atom's ability to attract shared electrons in a bond.

   • Nonpolar Covalent Bonds: Formed between two atoms of identical or very close electronegativity. The bonding electrons are shared equally, resulting in a symmetrical distribution of electrical charge. Diatomic elemental molecules like homonuclear halogens are classic examples.
   • Polar Covalent Bonds: Formed between atoms with a moderate difference in electronegativity. The more electronegative atom attracts the shared electron pair more strongly, developing a partial negative charge (δ-), while the less electronegative atom acquires a partial positive charge (δ+). This separation of charge produces a bond dipole.

   Key Takeaways

   Understanding covalent bonds requires remembering three core principles:

   1. Formation: Covalent bonds occur via the sharing of valence electrons between nonmetals to achieve stable electron configurations.
   2. Multiplicity: Atoms can share one, two, or three electron pairs. Greater sharing leads to shorter and stronger bonds (Triple > Double > Single).
   3. Polarity: Unequal sharing caused by differences in electronegativity creates polar covalent bonds, whereas equal sharing produces nonpolar covalent bonds.