An atom or molecule that carries a net positive electrical charge is known as a cation.
What Defines an Ion
Atoms are typically electrically neutral because they contain an equal number of positively charged protons and negatively charged electrons. This balance results in a zero net charge for the atom in its stable state. This balance is easily disrupted, however, as electrons can be gained or lost during chemical reactions.
An ion is an atom or group of atoms that has gained or lost one or more electrons, creating an imbalance between protons and electrons. This electron transfer results in the atom carrying a net electrical charge. The charge is represented by a superscript next to the chemical symbol.
Cations: The Positively Charged Atom
The term for a positively charged ion is a cation. Cations form when a neutral atom loses one or more electrons. Since the number of protons remains unchanged, the loss of negative charge causes the total positive charge to exceed the total negative charge.
Cations are identified by a positive sign superscript, often followed by a number indicating the magnitude of the charge, such as \(\text{Na}^+\) or \(\text{Ca}^{2+}\). For instance, the sodium ion (\(\text{Na}^+\)) has lost one electron, and the calcium ion (\(\text{Ca}^{2+}\)) has lost two electrons. Most elements classified as metals, such as those in Groups 1 and 2 of the periodic table, readily form cations.
The Process of Cation Formation
Cation formation is driven by an atom’s tendency to achieve maximum stability. Atoms react chemically to attain a stable electron configuration, which often means having eight electrons in their outermost electron shell, known as the Octet Rule. This stable configuration mimics that of the chemically inert noble gases.
Atoms that have only one, two, or three electrons in their outermost shell, like the alkali and alkaline earth metals, find it energetically favorable to lose these electrons. For example, a neutral sodium atom has one valence electron. By shedding this single electron, the sodium atom exposes the next inner shell, which contains a stable set of eight electrons, thus forming the \(\text{Na}^+\) cation.
The loss of an electron requires a small input of energy, but the resulting stability of the closed electron shell configuration outweighs this initial energy cost. The resulting cation is also physically smaller than the neutral atom from which it was derived, because the remaining electrons are pulled closer to the positively charged nucleus with less electron-electron repulsion. This process is why elements like potassium (\(\text{K}^+\)) and aluminum (\(\text{Al}^{3+}\)) are routinely found as positively charged ions.
Anions: The Negatively Charged Counterpart
The opposite of a cation is an anion, which is a negatively charged ion. Anions form when a neutral atom gains one or more electrons. This addition increases the total number of negative charges, resulting in a net negative charge.
Non-metal elements, such as those found on the right side of the periodic table, typically form anions. These atoms have nearly full outer shells and only need to gain a few electrons to satisfy the Octet Rule. Examples include the chloride ion (\(\text{Cl}^-\)) and the oxide ion (\(\text{O}^{2-}\)). The attraction between positively charged cations and negatively charged anions is the fundamental force that creates ionic compounds, such as table salt (sodium chloride, \(\text{Na}^+\text{Cl}^-\)).