1.5 : 極性共有結合

Nonmetals have high ionization energies, making it difficult to transfer valence electrons from one atom to another. Hence, nonmetals tend to share valence electrons between atoms, forming covalent bonds.

The shared pair of electrons in the covalent bond is called a bonding pair. Any valence electrons that do not participate in bonding are called the lone pair, or nonbonding electrons.

According to the octet rule, when certain atoms have fewer than eight electrons in their valence shell, they react to form stable compounds by achieving the configuration of the nearest noble gas.

For example, in a molecule of ammonia, nitrogen requires three more electrons to reach its nearest noble gas configuration, or an octet, whereas, hydrogen needs one more electron to reach its nearest noble gas configuration, or a duet. Thus, the nitrogen atom forms single bonds with three hydrogen atoms.

On the other hand, the carbon dioxide and carbon monoxide molecules are held together by carbon–oxygen double and triple bonds, respectively. The formation of the double and triple bonds ensures that each of the constituent atoms achieves the nearest noble gas configuration.

When covalent bonds form between two atoms of different elements, the more electronegative atom attracts the electrons more strongly, forming a polar covalent bond. The ability of an atom to attract electrons towards itself is called electronegativity. The greater the difference in electronegativity, the more polar the bond will be.

The Pauling scale provides the electronegativity value for each element based on bond energy calculations. Nitrogen is more electronegative than carbon, which in turn is more electronegative than hydrogen.

Thus, in a covalent bond between carbon and nitrogen, the more electronegative nitrogen attracts the shared electrons towards itself, whereas, in a bond between carbon and hydrogen, the more electronegative carbon attracts the electron density away from the less electronegative hydrogen.