Question

All the isoelectronic species illustrated in the text had the electron configurations of noble gases. Can two ions be isoelectronic without having noble-gas electron configurations? Explain.

Short answer

Yes, two ions can be isoelectronic without having noble-gas electron configurations. This is possible when two ions have the same total number of electrons but these electrons do not occupy a completely filled electron shell configuration, as is the case with noble gasses.

Step-by-step solution

Understanding Isoelectronic Terms

Isoelectronic refers to different atoms, ions, or molecules that have the same total number of electrons. The challenge here is to think about whether these atoms or ions must necessarily have a noble-gas electron configuration.

Analyzing the Electron Configurations

The electron configurations of noble gases are characterized by completely filled electron shells. An atom becomes an ion by losing or gaining electrons to achieve a 'stable' configuration, usually similar to the nearest noble gas. However, this process is not limited to imitating noble gas configurations.

Conclusion

So, two ions can indeed be isoelectronic without having noble gas electron configurations. This becomes possible when two ions have the same total number of electrons, but those electrons do not fully fill their respective shells (which would be the case for a noble gas configuration). In other words, they may both lack electrons to become truly stable (resembling a noble-gas configuration), but they could still have the same total number of electrons.

Key concepts

Electron Configuration

Electron configuration is the distribution of electrons in an atom or ion. It tells us how electrons are arranged around the nucleus in different shells and subshells. The order in which electrons fill various orbitals follows the rules based on increasing energy levels and principles like the Aufbau principle, Hund's rule, and the Pauli exclusion principle.
For example, the electron configuration for hydrogen is written as \(1s^1\), indicating one electron in the first shell's s subshell. Understanding this concept is crucial as it helps us predict chemical properties and reactivity of elements.
For ions, electron configurations change as they gain or lose electrons. An ion's electron configuration can mirror that of a noble gas when it reaches a more stable form, reflecting the completion of an energy shell.

Noble Gases

Noble gases belong to Group 18 of the periodic table and are known for their characteristic full electron shells, making them chemically inert under most conditions. Their general electron configuration is \(ns^2np^6\), which indicates completely filled valence orbitals, contributing to their lack of reactivity.
Noble gases are the standard for electronic stability, which many other elements strive to achieve by forming ions. For example, sodium (Na) loses one electron to resemble neon (Ne), thus, achieving a noble gas electron configuration. This desired stable state makes noble gases essential in understanding the concepts of ion formation and chemical bonding.

• Helium (He): \(1s^2\)
• Neon (Ne): \(1s^2 2s^2 2p^6\)
• Argon (Ar): \(1s^2 2s^2 2p^6 3s^2 3p^6\)

These configurations display full shell configurations, which are energetically favorable.

Ion Stability

Ion stability relates to how energetically favorable an ion's electron configuration is, often aiming for a state similar to that of a noble gas. Stability in ions generally happens when they have a complete set of electrons filling their outer shell, reducing the energy of the system.
However, ions can achieve stability even when their configurations do not match those of noble gases. For example, the sulfide ion (S²⁻) and chloride ion (Cl⁻) are isoelectronic, each having 18 electrons, but only chloride matches a noble gas configuration (argon).
Ionic stability is also influenced by factors such as:

• Charge Balance: The overall energy reduction in a system when oppositely charged ions balance each other.
• Coordination Number: The number of ions bonded to a central ion.
• Crystal Lattice: Arrangement in solid structures that provide stability.

Understanding ion stability helps us comprehend how atoms interact, form compounds, and stabilize in different environments.