Atoms combine to form molecules and compounds through chemical bonds. This occurs because atoms seek a lower, more stable energy state, usually by filling their outermost electron shells. Most atoms aim to have eight electrons in this outer shell, a tendency known as the octet rule. Atoms interact either by transferring or sharing electrons, which determines the bond type and the resulting compound’s properties.
Defining the Ionic Bond
An ionic bond forms through the complete transfer of one or more valence electrons from one atom to another. This transfer typically occurs between a metal atom, which easily loses electrons (low electronegativity), and a nonmetal atom, which strongly attracts electrons (high electronegativity). A difference in electronegativity greater than 1.7 often results in a bond that is predominantly ionic.
When a metal atom loses an electron, it becomes a positively charged cation. Conversely, when a nonmetal atom gains the electron, it acquires a negative charge and is called an anion. The ionic bond is the powerful electrostatic attraction between these newly formed, oppositely charged ions.
How Sodium and Chlorine Interact
Sodium chloride (NaCl), commonly known as table salt, is a classic example of an ionic compound. A neutral sodium (Na) atom has one valence electron, while a neutral chlorine (Cl) atom has seven. To achieve a stable configuration, the sodium atom readily gives up its single valence electron.
By losing this electron, the sodium atom becomes a positively charged sodium ion (\(\text{Na}^{+}\)), similar to the noble gas neon. The chlorine atom accepts the electron from sodium, transforming into a negatively charged chloride ion (\(\text{Cl}^{-}\)), similar to the noble gas argon. The resulting \(\text{Na}^{+}\) and \(\text{Cl}^{-}\) ions are strongly attracted to each other by the electrostatic force, which constitutes the ionic bond. This bond is highly stable because both ions have satisfied the octet rule and reached a lower energy state.
Covalent Bonds: The Key Difference
In contrast to ionic bonding, a covalent bond involves the sharing of electron pairs between atoms. Covalent bonds typically form between two nonmetal atoms that have similar tendencies to attract electrons. The atoms share their valence electrons to complete their respective outer shells.
If the sharing of electrons is equal, a nonpolar covalent bond forms, such as in diatomic oxygen (\(\text{O}_{2}\)). Unequal sharing occurs if one atom has a greater pull on the electrons, leading to a polar covalent bond. This creates a slight negative charge on the more attractive atom and a slight positive charge on the other, forming a dipole.
Physical Properties of Ionic Compounds
The strong electrostatic forces within ionic bonds dictate the physical properties of ionic compounds like sodium chloride. These compounds do not form individual molecules but arrange themselves into a highly ordered, three-dimensional crystal lattice. This structure maximizes the attractive forces between the alternating positive and negative ions.
Because breaking the numerous attractions in the crystal lattice requires a massive amount of energy, ionic compounds have high melting and boiling points. For instance, sodium chloride melts at about \(800^{\circ}\text{C}\). Ionic solids are hard but brittle; striking the crystal shifts layers, causing repulsion and shattering when like-charged ions align.
Ionic compounds do not conduct electricity in the solid state because the ions are fixed in the lattice. However, when dissolved in water or melted, the ions become mobile. This mobility allows the charged particles to move and carry an electric current.