The Nature of Covalent Bonding 8.2

Page 217

The Octet Rule in Covalent Bonding • • • In forming covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases.

A pair of hydrogen atoms share 2 electrons giving the electron configuration of helium.

Combinations of atoms of the nonmetallic elements in Groups 4A, 5A, 6A, and 7A are likely to form covalent bonds.

Single Covalent Bonds • Two atoms held together by sharing a pair of electrons are said to be joined by a single covalent bond.

Single Covalent Bonds • • • The pair of shared electrons forming the covalent bond is often represented as a dash.

A structural formula represents the covalent bonds by dashes and shows the arrangement of covalently bonded atoms.

The molecular formula indicates only the number of hydrogen atoms in each molecule

Single Bonds of the Halogens • • Halogens form single covalent bonds in their diatomic molecules.

These atoms have seven valence electrons, needing only one additional electron to obtain the configuration of a noble gas.

Single Covalent Bonds • A pair of valence electrons that is not shared between atoms is called an unshared pair, also know as, a lone pair or a nonbonding pair.

Double and Triple Covalent Bonds • • Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing two pairs or three pairs of electrons.

Single, double, and triple covalent bonds also exist between unlike atoms.

Coordinate Covalent Bonds • • • In a coordinate covalent bond, the shared electron pair comes from one of the bonding atoms.

One atom contributes both bonding electrons.

Once formed, a coordinate covalent bond is like any other covalent bond.

Polyatomic Ions • • • Tightly bound group of atoms that has a positive or negative charge and behaves as a unit. Most polyatomic cations and anions contain both covalent and coordinate covalent bonds.

Therefore, compounds containing polyatomic ions include both ionic and covalent bonding.

Polyatomic Ion

Bond Dissociation Energies • • • The energy required to break the bond between two covalently bonded atoms.

Usually expressed as the energy needed to break one mole of bonds.

A large bond dissociation energy corresponds to a strong covalent bond.

Exceptions to the Octet Rule • The octet rule cannot be satisfied in molecules whose total number of valence electrons is an odd number. There are also molecules in which an atom has fewer, or more, than a complete octet of valence electrons.

– ClO 2 – NO – Read page 228-229