Atoms are the fundamental building blocks of all matter. They are typically electrically neutral, meaning they carry no overall electrical charge. Under certain conditions, however, atoms can acquire an electrical charge, transforming into ions. These charged atoms play a foundational role in chemical reactions and the formation of various substances.
What Are Atoms and Ions?
An atom consists of a central nucleus, which contains positively charged particles called protons and neutral particles called neutrons. Surrounding this nucleus are negatively charged particles known as electrons, which occupy specific energy levels or shells. An ion is an atom that has gained or lost one or more electrons, leading to a net electrical charge. When an atom loses electrons, its positive charge from the protons becomes greater than its negative charge from the remaining electrons, forming a positive ion. Conversely, if an atom gains electrons, the total negative charge from the electrons surpasses the positive charge of the protons, resulting in a negative ion.
How Atoms Gain Electrons
A negative ion, specifically called an anion, forms when a neutral atom acquires one or more additional electrons. The electrons involved in chemical interactions are primarily those located in the outermost shell of an atom, known as valence electrons. Some atoms exhibit a stronger attraction for additional electrons, a property referred to as electron affinity. This inherent attraction facilitates the process of an atom accepting extra electrons into its valence shell, leading to the formation of a negative ion.
The Drive for Stability
Atoms gain electrons to become negative ions primarily because they are seeking to achieve a more stable electron configuration. This stable state often involves filling their outermost electron shell, mirroring the electron arrangement found in noble gases; these elements are notably unreactive because they naturally possess a full outer shell of electrons, which is an energetically favorable state. The “octet rule” describes this tendency for main group elements to gain, lose, or share electrons until they have eight electrons in their outermost shell. Gaining electrons allows certain atoms to complete this octet, thereby reaching a lower-energy, more stable condition. For example, a chlorine atom, which needs one electron to complete its outer shell, readily gains an electron to become a stable chloride ion; similarly, an oxygen atom can gain two electrons to achieve a full outer shell, forming an oxide ion and attaining a stable configuration.