Explain the formation of chloride ion from chlorine atom.

Formation of chloride ion from chlorine atom:

1. Chlorine atom $\left(\mathrm{Cl}\right)$ has atomic number $17$. Thus, the electronic configuration of the chlorine atom is $(2,8,7)$.
2. Chlorine atom gains one electron in the outermost shell to complete the octet.
3. On gaining one electron, a negatively charged chloride ion $\left({\mathrm{Cl}}^{-}\right)$ is formed.

Thus, a chlorine atom $\left(\mathrm{Cl}\right)$ gains one electron and forms chlorine ion $\left({\mathrm{Cl}}^{-}\right)$.