An ion is an atom or molecule that carries a net electrical charge due to an unequal number of electrons and protons. Both gaining and losing electrons occur, depending on the element involved, which transforms an electrically neutral atom into a charged ion. This charge imbalance drives chemical bonding, particularly in the formation of ionic compounds like common table salt.
Atomic Neutrality: The Starting Point
Before an atom becomes an ion, it exists in a neutral state with no net electrical charge. This neutrality is maintained because the number of positively charged protons in the nucleus precisely equals the number of negatively charged electrons orbiting it. The number of protons (the atomic number) determines the element’s identity and remains constant during chemical reactions. Therefore, any change in the net charge must result from the movement of electrons (gaining or losing them) to achieve a more stable electron configuration, often resembling a noble gas.
Cations: Forming Positive Ions by Losing Electrons
Cations are ions that possess a net positive charge, forming when a neutral atom loses one or more electrons. Since electrons are negatively charged, their removal decreases the total negative charge, allowing the fixed number of positive protons in the nucleus to dominate. For instance, a neutral sodium atom (\(\text{Na}\)) has 11 protons and 11 electrons. When it ionizes, it typically sheds its single outermost electron. The resulting sodium ion (\(\text{Na}^+\)) still contains 11 protons but only 10 electrons, leading to a net charge of positive one.
The formation of a cation is represented by notation such as \(\text{Na} \rightarrow \text{Na}^+ + \text{e}^-\), where \(\text{e}^-\) symbolizes the lost electron. Metals, such as those in Groups 1 and 2 of the periodic table, are the elements most likely to form cations. For example, magnesium (\(\text{Mg}\)) readily loses two electrons to form a \(\text{Mg}^{2+}\) cation, achieving a stable outer electron shell. Because electrons are removed, the resulting cation is physically smaller than its parent neutral atom.
Anions: Forming Negative Ions by Gaining Electrons
Anions are ions characterized by a net negative charge, resulting from a neutral atom gaining one or more electrons. This addition causes the total electron count to exceed the fixed number of protons in the nucleus. Non-metals, generally located on the right side of the periodic table, tend to form anions. These elements have a high attraction for electrons and are typically closer to achieving a stable, eight-electron configuration in their outermost shell.
Consider chlorine (\(\text{Cl}\)), which needs only one additional electron to complete its outer shell. When a neutral chlorine atom gains an electron, the process is written as \(\text{Cl} + \text{e}^- \rightarrow \text{Cl}^-\). The resulting chloride ion (\(\text{Cl}^-\)) carries a charge of negative one because it possesses one more electron than protons. Similarly, an oxygen atom (\(\text{O}\)) needs to gain two electrons to reach stability, forming the oxide ion (\(\text{O}^{2-}\)), which has a negative two charge. Gaining electrons causes the electron cloud to expand, making the anion physically larger than its original neutral atom.