Question

We have seen that the reactivity of an element is determined by its electron configuration. What is the electron configuration of the ion \(\mathrm{Cl}^{-}\)? (Hint: You must add one additional electron beyond the number of electrons that chlorine would normally have.) How does its reactivity compare with neutral Cl? How would the reactivities of \(\mathrm{Na}\) and \(\mathrm{Na}^{+}\)compare?

Short answer

The electron configuration for Cl^- is 1s^22s^22p^63s^23p^6, which is less reactive than neutral Cl due to its full valence shell. Na is more reactive than Na+ since Na tends to lose an electron to achieve a stable noble gas configuration, while Na+ already has it.

Step-by-step solution

Understanding Chlorine's Electron Configuration

First, determine the electron configuration for a neutral chlorine atom. Chlorine has an atomic number of 17, which means it has 17 protons and, when neutral, 17 electrons. Its electron configuration can be expressed as 1s^22s^22p^63s^23p^5.

Adding an Electron to Chlorine

To find the electron configuration of Cl^-, add one electron to the neutral chlorine configuration. The additional electron will occupy the next available orbital, which is the 3p orbital, resulting in the configuration 1s^22s^22p^63s^23p^6.

Writing the Electron Configuration for Cl-

With the additional electron, the electron configuration of the resulting Cl- ion is 1s^22s^22p^63s^23p^6. This is now a filled shell configuration, similar to that of an argon atom.

Comparing Reactivity of Cl and Cl-

Chlorine (Cl) is more reactive than Cl- because Cl has an unfilled shell and energetically favours gaining one electron to achieve a stable configuration, whereas Cl- already has a stable, full outer shell and is less inclined to react.

Understanding Sodium's Electron Configuration

Sodium (Na) has an atomic number of 11, which means it has 11 electrons. The electron configuration for a neutral sodium atom is 1s^22s^22p^63s^1.

Removing an Electron from Sodium

To find the electron configuration of Na+, one electron is removed from the neutral sodium configuration. The electron from the outermost shell (3s orbital) will be removed, resulting in the configuration 1s^22s^22p^6, which is similar to neon's configuration and stable.

Comparing Reactivity of Na and Na+

Neutral sodium (Na) is highly reactive because it can easily lose one electron to achieve the stable, noble gas configuration of neon. In contrast, Na+ has already achieved a stable electron configuration and will be less reactive.

Key concepts

Reactivity of Elements

The reactivity of an element largely depends on its ability to achieve a stable electron configuration. Elements tend to react in ways that will fill or empty their outermost electron orbitals to reach a noble gas configuration, which is the most chemically stable arrangement. For instance, chlorine (Cl) needs to gain a single electron to complete its outer shell and reach the stability of an argon configuration, hence it is quite reactive.

When chlorine gains an electron to become Cl^-, it achieves this desired stable configuration, significantly reducing its reactivity. The Cl^- ion does not readily participate in further chemical reactions, because its outer electron shell is full, making it energetically unfavorable to either gain or lose electrons.

In contrast, elements with only one or two electrons in their outer shell, such as sodium (Na), are also highly reactive. Sodium, with its single outer electron, will easily lose that electron to attain the electron configuration of neon, a noble gas. Once it loses an electron to become Na⁺, it becomes far less reactive, as its outer shell is now full and the ion is more stable.

Ionic Electron Configuration

Ionic electron configuration refers to the arrangement of electrons in an ion, which is an atom or molecule with a net electrical charge due to the loss or gain of one or more electrons.

For the Cl^- ion, the electron configuration is 1s²2s²2p⁶3s²3p⁶, mirroring the stable configuration of argon. During ion formation, chlorine gains one electron and enters a state of lower energy, which is more chemically stable and favored by nature.

Electron Configuration of Sodium Ions

On the other hand, a Na⁺ ion loses an electron, resulting in an electron configuration of 1s²2s²2p⁶. This new configuration resembles the electron structure of neon, another noble gas. Sodium achieves this through the process of ionization where it loses its single 3s electron. The ionic electron configuration can greatly influence the properties and reactivity of the ion.

Chemical Stability

Chemical stability is a term that describes how resistant a substance is to changing its chemical structure. Stable configurations are typically those that have full valence electron shells, as seen in noble gases such as helium (He), neon (Ne), and argon (Ar).

Atoms seek to reach a state of maximum stability through reactions that result in full outer electron shells. For example, in ionic bonds, atoms achieve chemical stability by transferring electrons to one another, resulting in ions like Cl^- and Na⁺ which possess electron configurations analogous to noble gases.

Stable Ions Versus Reactive Atoms

Comparatively, reactive atoms or ions, such as neutral chlorine or sodium, have electron configurations that are energetically poised to lose or gain electrons to reach stability. This quest for electron configuration stability underpins many chemical reactions. Ionic electron configurations are a measure of chemical stability for ions, and understanding this can provide insights into the potential reactivity or resistance to reaction of a substance.