1.4 : 化学結合

When atoms approach each other, their nuclei repel each other, as do the electrons. Concurrently, the electrons from each atom are attracted to each other's nucleus and arrange in such a way to maximize the attractive forces between the atoms while minimizing the repulsive forces.

If this results in a net attractive force, the potential energy of the system is lowered. This interaction is called a chemical bond.

However, not every combination of atoms can form stable chemical bonds. An atom’s chemical behavior is primarily determined by what forms bonds and what type of bonds they are. Understanding the bonding between atoms that make a compound helps to comprehend and predict their molecular behavior.

Bond energy is the energy required to break a bond in one mole of a gaseous compound. It depends on the type of bonded atoms and the number of electrons involved. Bond strength increases with bond energy.

The type of bonded atoms and number of electrons involved also influence the bond length, which is the average distance between the nuclei of two bonded atoms and is inversely proportional to the bond multiplicity and bond strength.

The most common bonding interactions fall on a spectrum ranging from ionic bonds, which involve electrostatic attractions between oppositely charged ions, to covalent bonds, which share valence electrons between atoms.

Ionic bonding is the primary interaction found in ionic compounds. The constituent cations and anions are held together by electrostatic forces of attraction. Often, the ion formation is preceded by the transfer of valence electrons between participating atoms.

The atom that loses electrons becomes the cation, while the atom that accepts electrons becomes the anion. The transfer of electrons typically gives each ion the nearest noble gas configuration, which is the configuration reached with the smallest change in the number of electrons, making the ions energetically more stable.

In covalent bonding, the shared electrons interact with the bonding atoms’ nuclei and lower the potential energy. Double and triple bonds share two and three pairs of electrons between atoms, respectively. As this increases the number of electrons subject to additional attractive forces from the other nucleus, bond strength increases with bond multiplicity.