Compounds are substances made from two or more different elements chemically bonded in a fixed ratio. Unlike individual elements, which consist of only one type of atom, compounds are distinct from mixtures, where substances are physically combined but not chemically joined. Understanding how these atomic connections form is fundamental to grasping the nature of all matter.
Why Atoms Combine
Atoms are inherently unstable when their outermost electron layers are not complete. Atoms generally seek to achieve a more stable, lower-energy state by completing their outermost electron shell, often aiming for eight electrons in this outer layer, or two electrons for very small atoms like hydrogen. By forming bonds, atoms can attain a configuration that is energetically more favorable, much like a ball rolling downhill to reach a stable resting position.
Ionic Bonds: Electron Transfer
One way atoms achieve stability is through ionic bonding, which typically occurs between a metal atom and a nonmetal atom. In this process, one atom completely transfers one or more electrons to another atom. The metal atom, which tends to lose electrons, becomes a positively charged ion called a cation. Conversely, the nonmetal atom, which gains electrons, becomes a negatively charged ion known as an anion.
These oppositely charged ions are then strongly attracted to each other through electrostatic forces, forming the ionic bond. A common example is table salt, or sodium chloride (NaCl), where a sodium atom donates an electron to a chlorine atom.
Covalent Bonds: Electron Sharing
Another primary method for atoms to achieve stability is through covalent bonding, which typically forms between two nonmetal atoms. Instead of transferring electrons, atoms in a covalent bond achieve a full outer shell by sharing electrons. The shared electrons are simultaneously attracted to the nuclei of both atoms involved in the bond, effectively holding the atoms together.
Covalent bonds can involve the sharing of one, two, or three pairs of electrons, known as single, double, and triple bonds, respectively. Sharing more electron pairs generally results in a stronger and shorter bond between the atoms. Water (H2O) is a classic example of a covalent compound, where oxygen shares electrons with two hydrogen atoms. Carbon dioxide (CO2) also features covalent bonds, with carbon sharing electrons with two oxygen atoms.
How Bond Type Influences Properties
The type of chemical bond formed significantly dictates the physical and chemical properties of the resulting compound. Ionic compounds, characterized by strong electrostatic attractions between ions, generally have high melting and boiling points. For instance, sodium chloride melts at 801 degrees Celsius. Many ionic compounds also dissolve readily in water and conduct electricity when dissolved or melted, as their ions become free to move. In contrast, covalent compounds often exhibit lower melting and boiling points because the forces between individual molecules are typically weaker than the ionic bonds holding ions together. Water, a covalent compound, boils at 100 degrees Celsius. Their solubility in water varies widely, and most covalent compounds do not conduct electricity in any state, as they lack free-moving charged particles.