An ion is an atom or molecule that carries a net electric charge due to the gain or loss of one or more electrons. Positively charged ions, known as cations, are formed when an atom loses electrons, causing the number of protons to exceed the number of electrons. Positive ions overwhelmingly tend to be metals.
The Goal of Ion Formation
The underlying drive for nearly all chemical interaction, including ion formation, is the pursuit of atomic stability. Atoms are more stable when their outermost electron shell, the valence shell, is completely filled with electrons. This state mimics the electron configuration of noble gases, which are largely unreactive due to their full outer shells.
To achieve this stable configuration, atoms either lose or gain valence electrons during a chemical reaction. The path an atom takes is determined by which action requires less energy. This energy consideration dictates the fundamental difference between the behavior of metals and nonmetals.
Losing electrons results in a net positive charge (a cation) because the atom retains the same number of protons but has fewer electrons. Conversely, gaining electrons results in an anion, an ion with a net negative charge. The ease with which elements achieve full-shell stability determines their ionic fate.
The Chemistry of Positive Ions (Cations)
Metals are predisposed to forming positive ions due to specific characteristics of their atomic structure. They are typically found on the left side of the periodic table and possess a small number of valence electrons, commonly one, two, or three. For these atoms, it is energetically much easier to shed these few outer electrons than to gain the many electrons required to fill the shell completely.
This tendency to lose electrons is quantified by low ionization energy. Ionization energy is the energy required to remove an electron from an atom. In metals, the outermost electrons are held loosely because they are shielded from the positive nucleus. When a metal atom loses its valence electron(s), the electron shell below becomes the new outermost shell, which is already full.
For example, a sodium atom (\(\text{Na}\)), an alkali metal, has only one valence electron. Losing this single electron transforms the atom into a sodium ion (\(\text{Na}^+\)). This achieves a stable electron configuration identical to the noble gas neon.
Similarly, magnesium (\(\text{Mg}\)), an alkaline earth metal, has two valence electrons. The loss of both electrons yields a magnesium ion (\(\text{Mg}^{2+}\)). This stable state is why metals are described as electropositive, meaning they readily give up electrons in chemical reactions.
The Contrast: Nonmetals and Negative Ions (Anions)
While metals tend to lose electrons, nonmetals exhibit the opposite behavior, generally forming negative ions (anions). Nonmetals are located on the right side of the periodic table and typically have five, six, or seven valence electrons. For these atoms, achieving a full outer shell requires gaining only a few electrons rather than losing many.
Nonmetals have a high electron affinity, which is the measure of the energy change when an electron is added to a neutral atom. This high affinity means they have a strong attraction for outside electrons to complete their valence shell. Halogens, such as chlorine (\(\text{Cl}\)), illustrate this tendency, needing to gain just one electron to achieve stability.
When a chlorine atom gains a single electron, it forms a chloride ion (\(\text{Cl}^-\)), which carries a negative charge. The transfer of electrons from a metal to a nonmetal is the mechanism that forms ionic compounds, such as sodium chloride (\(\text{NaCl}\)).