Atoms of nonmetals become stable by contributing and sharing electrons. This is called a covalent bond, and the particles formed in this way are molecules.
Basic Knowledge
To obtain a noble-gas electron arrangement, atoms contribute unpaired electrons and form shared electron pairs. This is a covalent bond.
Examples:
・H2: H・ + ・H → H:H (one shared electron pair, a single bond)
・H2O: The O atom has two unpaired electrons, and each H atom has one. Two O-H single bonds.
・CO2: O=C=O (two double bonds).
・N2: N≡N (a triple bond). Extremely stable and the main component of air.
Representations of covalent bonds:
・Electron-dot formula (Lewis formula): Valence electrons are written as dots (:).
・Structural formula: Shared electron pairs are written as lines (―) (H-H, O=C=O).
・Molecular formula: The types and numbers of constituent atoms (H2, H2O, CO2).
One shared electron pair = single bond (―), two pairs = double bond (=), three pairs = triple bond (≡).
Covalent bond（A bond in which atoms contribute unpaired electrons and share an electron pair）
Shared and nonshared electron pairs（Electron pairs that are shared or not shared. The latter are lone pairs）
Single, double, and triple bonds（One, two, or three shared electron pairs）
Electron-dot formula (Lewis formula)（A notation that writes valence electrons as dots）
Structural formula（A notation that represents shared electron pairs with lines. H-O-H）
Coordinate bond（A special covalent bond in which one atom provides the electron pair. NH3+H+→NH4+）
Further Study (Background and Significance)
Let us organize the electron-dot formulas and structures of major molecules.
・H2: H-H
・Cl2: Cl-Cl (three lone pairs on each atom)
・HCl: H-Cl
・H2O: H-O-H, bent shape (two lone pairs on O)
・NH3: trigonal pyramidal shape (one lone pair on N)
・CH4: tetrahedral shape (four valence electrons)
・CO2: O=C=O, linear shape
・N2: N≡N, extremely stable
A coordinate bond is a bond in which one atom provides both electrons. Once formed, it cannot be distinguished from an ordinary covalent bond. For example, a lone pair on N in NH3 is given to H+ to form NH4+; H2O+H+→H3O+ (oxonium ion) is another example.
Coordinate bonds are also important in complex-ion formation, including colorful reactions such as Cu2++4NH3→[Cu(NH3)4]2+ (tetraamminecopper complex ion, deep blue). Covalent bonds are also the main subject of organic chemistry, and research in life science and medicine begins with understanding them.
- Covalent bond = nonmetal atoms share electron pairs
- Shared electron pairs are shown with lines; lone pairs with dots
- Single, double (=), and triple (≡) bonds
- H2O is bent, NH3 trigonal pyramidal, CH4 tetrahedral
- CO2 is linear; N2 is stable with a triple bond
- Coordinate bond: one atom provides the electron pair (NH4+, H3O+)
- Complex ion [Cu(NH3)4]2+ also contains coordinate bonds
Points to Note (Common Confusions)
① Distinguish shared electron pairs (between atoms) from lone pairs (belonging to one atom). ② Distinguish the notation of an electron-dot formula (valence electrons and dots) from a structural formula (shared electron pairs and lines). ③ Once formed, a coordinate bond cannot be distinguished from an ordinary covalent bond. ④ Number of bonds and number of shared electron pairs: a single bond=1 pair (2 electrons), a double bond=2 pairs (4 electrons), and a triple bond=3 pairs (6 electrons).
Practice
- Write the structural formula of an H2O molecule. Also show the lone pairs.
- What type of bond exists between the atoms in an N2 molecule?
- Explain what type of bonds form NH4+.