An atom or molecule that possesses a net electrical charge is called an ion. This charge imbalance occurs when the number of electrons (negative charge) does not equal the number of protons (positive charge) within the particle. When an ion has more electrons than protons, it results in a net negative charge. This negatively charged particle is specifically known as an anion.
How Anions Form
Anions are formed when a neutral atom or molecule gains one or more electrons. A neutral atom naturally has an equal number of protons and electrons, resulting in a zero net charge. The process of forming an anion involves adding negative charge to this neutral state.
Atoms seek a stable configuration, often achieved by filling their outermost electron shell, a concept related to the octet rule. Non-metal atoms, typically found on the right side of the periodic table, have a strong tendency to attract and gain electrons to complete this shell. The gain of an electron shifts the balance, as the negative charge now outweighs the positive charge from the protons. For instance, a chlorine atom gains one electron to become the chloride anion, written as \(\text{Cl}^{-}\).
Anions Versus Cations
Ions are divided into two types based on their charge: anions and cations. A cation is a positively charged ion formed when an atom or molecule loses one or more electrons. The loss of an electron leaves the cation with more positive protons than electrons, creating a net positive charge.
This difference in charge is predictable based on an element’s position on the periodic table. Non-metal elements, like chlorine and oxygen, form anions because they are highly electronegative and pull electrons toward themselves. Conversely, metal elements, such as sodium and calcium, typically form cations because they readily lose their outer electrons. These oppositely charged ions are attracted to each other by electrostatic forces, often coming together to form electrically neutral ionic compounds, like sodium chloride (table salt).
Anions in Everyday Life
Anions are present in countless substances and biological functions. The chloride anion (\(\text{Cl}^{-}\)), for example, is a component of table salt and is necessary for maintaining fluid balance and nerve signaling within the human body. The phosphate anion (\(\text{PO}_4^{3-}\)), a polyatomic anion composed of multiple atoms, is an integral part of DNA structure and the energy-carrying molecule adenosine triphosphate (ATP).
Environmental and industrial processes also rely on anions. The sulfate anion (\(\text{SO}_4^{2-}\)), often found in fertilizers, is also a component of acid rain. The oxide anion (\(\text{O}^{2-}\)), formed from oxygen, is the most common anion on Earth and is a constituent of many minerals and rust. Anions like bicarbonate (\(\text{HCO}_3^{-}\)), found in baking soda, play a fundamental role in regulating the pH of blood and other bodily fluids.