What is an Anion?

An anion is a negatively charged ion that is formed when an atom gains one or more electrons. This occurs when an atom's outer electron shell is not filled, and it becomes stable by gaining electrons to achieve a full outer shell.

Formation of Anions

Anions are formed when atoms gain electrons to achieve a stable electron configuration. For example, chlorine (Cl) has 17 electrons and requires one more electron to achieve a stable configuration. When a chlorine atom gains an electron, it forms a chloride ion (Cl-) with a negative charge.

Naming Anions

Anions are named by adding the suffix "-ide" to the root of the element's name. For example, when oxygen gains two electrons to form the O2- ion, it is named oxide.

Properties of Anions

Anions are typically larger than their parent atoms because they have gained electrons, resulting in increased electron-electron repulsion and a larger atomic radius. Anions are attracted to the positively charged cations in ionic compounds, forming ionic bonds.

Examples of Anions

Common examples of anions include chloride (Cl-), oxide (O2-), nitride (N3-), and sulfide (S2-).

Practice Questions

What is an anion and how is it formed?
Provide an example of an anion and explain its formation.
How are anions named?
Explain the properties of anions.
Identify the anions in the following compounds: NaCl, CaO, and Al2S3.

◂Physics Worksheets and Study Guides High School. Thermodynamics

The resources above cover the following skills:

PHYSICAL SCIENCE (NGSS)

Matter and Its Interactions

Students who demonstrate understanding can:

Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.

Energy

Students who demonstrate understanding can:

Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.

Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system (second law of thermodynamics).