Introduction:
Anions are negatively charged ions that are formed when an atom or a group of atoms gain one or more electrons. This process occurs through the gaining of electrons by an atom or a group of atoms.

Explanation:
The process of anion formation involves the gaining of electrons. Let's understand this in more detail:

1. Electron Gain:
- An atom or a group of atoms requires additional electrons to achieve a stable electron configuration.
- By gaining electrons, the atom or group of atoms can fill their outermost energy level and attain a stable configuration similar to that of a noble gas.
- This stable configuration is energetically favorable and leads to the formation of an anion.

2. Formation of Negative Charge:
- When an atom or a group of atoms gains one or more electrons, the number of negatively charged electrons exceeds the number of positively charged protons.
- This imbalance of charges results in an overall negative charge for the ion.
- The negative charge indicates that there are more electrons than protons in the ion.

3. Examples:
- For example, consider the chlorine atom (Cl). It has 17 protons and 17 electrons, resulting in a neutral charge.
- When chlorine gains one electron, it forms a chloride ion (Cl-) with a total of 18 electrons and 17 protons.
- The extra electron causes the chloride ion to have a negative charge, making it an anion.

Conclusion:
Anions are formed by the gaining of electrons by an atom or a group of atoms. This process allows the atom or group of atoms to achieve a stable electron configuration and results in the formation of a negatively charged ion.