Question

Explain why \(\mathrm{Cl}^{-}\)is known in nature, but \(\mathrm{Cl}^{2-}\) is not.

Short answer

Cl- ion is known in nature because chlorine atom can easily gain one electron to its 'p' orbital in the 3rd energy shell to achieve maximum stability. However, chlorine atom won't further accept another electron to form Cl2- ion as it would enter an energetically unfavorable higher energy shell (4th shell), rendering Cl2- ion unstable.

Step-by-step solution

Structure of a Chlorine Atom

A chlorine atom has 17 electrons. The electron configuration of a chlorine atom is \(1s^2 2s^2 2p^6 3s^2 3p^5\). This indicates that in its outermost shell (3rd energy shell), there are 5 electrons in the 'p' orbital. An atom is most stable when its outermost energy shell is filled to its maximum capacity.

Formation of Cl- ion

For the 3rd shell of a chlorine atom, the 'p' orbital can accommodate a maximum of 6 electrons. Since it currently has 5, it readily gains one electron to attain a full outer energy level and subsequently forms a Cl- ion. This ion is stable since the outermost shell is completely filled with its maximum of 8 electrons.

Possibility of Cl2- ion

On further addition of another electron to form Cl2-, the electron will have to go to the next energy level (4th shell), which is significantly higher in energy. Such an arrangement is not energetically favorable, making Cl2- ion unstable in nature.

Key concepts

Electron Configuration

The 'electron configuration' of an atom or ion describes how its electrons are distributed among different orbital shells and subshells. Electrons fill these orbitals in a manner that minimizes the energy of the atom, according to a set of rules known as the Aufbau principle, Hund's rule, and the Pauli Exclusion Principle.

In a chlorine atom, the electron configuration unfolds as follows: the first energy shell (1s) is filled with 2 electrons, the second shell (2s) also contains 2 electrons while the remaining 18 electrons are spread across the 2p (6 electrons), 3s (2 electrons), and 3p (5 electrons) subshells. This distribution leaves the chlorine atom with a partially filled outermost 'p' orbital, signaling a preference to gain one more electron to achieve stability.

Chloride Ion Cl-

A 'chloride ion Cl-' is a chlorine atom that has gained an extra electron, thus owning a net charge of -1. When chlorine accepts an electron, it's to complete its outermost 'p' orbital, transitioning from an electron configuration of ewline ewline ewline ewline$$[Ne]3s^23p^5$$ ewline ewline to $$[Ne]3s^23p^6$$. This configuration mimics that of a noble gas, giving the chloride ion a full octet and exceptional stability. The readiness of chlorine to accept an electron makes Cl- a common and essential ion in nature, playing vital roles in areas like the human body's cellular functioning and the earth's oceans.

Ionic Stability

The 'ionic stability' of an ion is influenced by its ability to achieve a noble gas electron configuration, which usually means having a complete set of electrons in its outermost shell. The stability is also affected by the energy of adding or removing electrons.

A Cl- ion, for instance, is stable because it achieves a full valence shell, resembling the nearest noble gas, argon. This is an energetically favorable condition because it involves minimal energy to gain a single electron compared to removing one or adding further beyond the complete octet. In contrast, Cl\(^{2-}\) is not stable as it would require placing an additional electron into a higher energy shell, which demands too much energy and is not sustained in nature.

Energy Shells

The concept of 'energy shells' comes from the quantum mechanical model of the atom where electrons occupy distinct energy levels, or 'shells'. As the energy level increases, the energy shells are located progressively further from the nucleus and have greater potential energy.

Each shell comprises one or more subshells, which further split into orbitals, each of which can hold a maximum of two electrons. Chlorine's electron configuration reflects electrons filling these shells and subshells in order of increasing energy level. When chlorine forms a Cl- ion, it adds one electron to the third shell, utilizing available space within that energy level. Attempting to form a Cl\(^{2-}\) ion would require an electron to jump to the fourth shell, which is undesirably high in energy and thus unstable.