Question

Explain how anions form.

Short answer

Anions form when atoms gain electrons, resulting in a net negative charge.

Step-by-step solution

Understanding Atoms

Atoms are composed of protons, neutrons, and electrons. Protons, which have a positive charge, are located in the nucleus along with neutrons, which have no charge. Electrons, which carry a negative charge, orbit the nucleus in electron shells.

Reviewing the Concept of Ions

An ion is an atom or a molecule that carries an electric charge due to the loss or gain of one or more electrons. The type of ion formed depends on electron gain or loss: gaining electrons results in a negative charge (anion) and losing electrons results in a positive charge (cation).

Identifying the Need for Electron Gain

Atoms tend to form ions to achieve a more stable electron configuration, often mimicking the nearest noble gas in terms of electron arrangement. Atoms usually gain, lose, or share electrons in a process that follows the octet rule, aiming for eight electrons in the outer shell.

Formation of Anions

An anion forms when an atom gains one or more electrons. This gain increases the number of electrons compared to protons, resulting in a net negative charge. For example, a chlorine atom that gains one electron becomes a chloride ion (Cl⁻).

Recognizing Common Anions

Common anions include halide ions such as fluoride (F⁻), chloride (Cl⁻), bromide (Br⁻), and iodide (I⁻). These elements are in Group 17 (halogens) and typically gain one electron to form anions. Other elements can form anions by gaining more than one electron.

Key concepts

Electron Gain

When atoms gain electrons, they undergo a process that leads to the formation of negatively charged particles known as anions. Electrons, existing as negatively charged subatomic particles, orbit the nucleus of an atom. The gain of electrons by an atom usually occurs because it strives for a more stable electronic arrangement.
Anions form when an atom ends up having more electrons than protons, resulting in an overall negative electric charge. This usually happens with non-metals, which have a higher affinity for electrons compared to metals.
For example, a chlorine atom, which originally has 17 electrons, may gain an extra electron to achieve a filled outer shell. This results in a negatively charged chloride ion (Cl⁻).

Charge of Ion

The charge of an ion is determined by the difference between the number of protons and electrons in an atom. Protons have a positive charge, while electrons have a negative charge. When an atom gains electrons, the number of electrons becomes greater than the number of protons, causing the ion to have a negative charge.
This negative charge is what defines an anion. For instance, if a neutral atom gains one electron, its charge becomes -1. The magnitude of the charge depends on how many electrons are gained.

• An atom that gains two electrons will have a 2- charge.
• Atoms that gain three electrons will have a 3- charge, and so forth.
Understanding the charge of ions helps in predicting the chemical behavior of atoms, such as their ability to bond with other ions to create compounds.

Octet Rule

The octet rule is an important principle in chemistry that guides the formation of ions, including anions. It states that atoms tend to adjust their number of electrons so to have eight electrons in their valence shell, the outermost shell of its electron configuration.
The octet rule suggests that an atom is most stable when surrounded by eight electrons. Such a configuration often resembles that of the nearest noble gas, which naturally possesses a full outer shell, making these elements extremely stable.
When an atom such as a halogen (e.g., chlorine or fluorine) gains electrons, it achieves this stable configuration, resembling that of noble gases like argon or neon. By fulfilling the octet rule, atoms achieve stability, reducing their potential energy, which is a driving force behind ion formation.

Noble Gas Configuration

The concept of noble gas configuration is central to understanding why atoms gain or lose electrons to form ions. Noble gases have fully filled valence electron shells, which makes them particularly stable and unreactive.
When atoms form anions, they do so to closely mimic this configuration. For example, when a chlorine atom gains one electron, it achieves the same electron configuration as the noble gas argon. This is referred to as achieving a noble gas configuration.

• A nitrogen atom, initially having 5 electrons in its outer shell, will gain 3 electrons to attain a stable electronic configuration like that of neon.
• Achieving a noble gas configuration confers stability and makes further chemical reactions less likely.

This pursuit of electron configurations similar to noble gases is fundamental in explaining why atoms undergo electron gain to become stable and form anions.