Question

Consider the following energy changes: Magnesium oxide exists as \(\mathrm{Mg}^{2+} \mathrm{O}^{2-}\) and not as \(\mathrm{Mg}^{+} \mathrm{O}^{-}\). Explain.

Short answer

In conclusion, magnesium oxide exists as Mg²⁺O²⁻ because both Mg²⁺ and O²⁻ ions achieve a stable noble gas configuration, identical to neon (Ne). They form a stable ionic compound, magnesium oxide (MgO), whereas Mg⁺ and O⁻ ions are less stable and do not achieve a noble gas configuration, making it unfavorable for magnesium oxide to exist as Mg⁺O⁻.

Step-by-step solution

Recalling the electron configuration of magnesium and oxygen

Recall the electron configuration of magnesium and oxygen. Magnesium (Mg) has an atomic number of 12, and its electron configuration is: \[1s^2 \, 2s^2 \, 2p^6 \, 3s^2\] Oxygen (O) has an atomic number of 8, and its electron configuration is: \[1s^2 \, 2s^2 \, 2p^4\]

Understanding the tendency to achieve noble gas configuration

Atoms tend to achieve a noble gas configuration by gaining, losing, or sharing electrons. For magnesium, the nearest noble gas is neon (Ne), with an electron configuration of: \[1s^2 \, 2s^2 \, 2p^6\] For oxygen, the nearest noble gas configuration is also neon (Ne).

Explain the formation of Mg²⁺ and O²⁻ ions

Magnesium can achieve a noble gas configuration by losing its two valence electrons in the 3s shell. This will result in an Mg²⁺ ion with an electron configuration identical to neon (Ne): \[1s^2 \, 2s^2 \, 2p^6\] Oxygen can achieve a noble gas configuration by gaining two electrons to fill its 2p subshell, resulting in an O²⁻ ion: \[1s^2 \, 2s^2 \, 2p^6\] With both Mg²⁺ and O²⁻ ions achieving a noble gas configuration, they can now form a stable ionic compound, magnesium oxide (MgO).

Discuss the instability of Mg⁺ and O⁻ ions

If magnesium were to lose only one electron, it would form an Mg⁺ ion: \[1s^2 \, 2s^2 \, 2p^6 \, 3s^1\] Likewise, if oxygen were to gain only one electron, it would form an O⁻ ion: \[1s^2 \, 2s^2 \, 2p^5\] Both Mg⁺ and O⁻ ions would not achieve a stable noble gas configuration and would hence be less stable than Mg²⁺ and O²⁻ ions. Consequently, magnesium oxide would not form as Mg⁺O⁻ due to the unstable nature of the ions.

Conclusion

In conclusion, magnesium oxide exists as Mg²⁺O²⁻ because both Mg²⁺ and O²⁻ ions achieve a stable noble gas configuration. The Mg⁺ and O⁻ ions are less stable, making it unfavorable for magnesium oxide to exist as Mg⁺O⁻.

Key concepts

Electron Configuration

Understanding electron configuration is crucial when it comes down to chemical bonding. In essence, electron configuration refers to the distribution of electrons among the different shells and subshells surrounding the nucleus of an atom. For instance, magnesium (Mg), with an atomic number of 12, has its electrons arranged as follows:

\[1s^2 2s^2 2p^6 3s^2\]
This means magnesium has two electrons in its innermost shell (the 1s orbital), two electrons in the 2s orbital, six in the 2p orbital, and its last two electrons are in the 3s orbital, which are its valence electrons. On the other hand, oxygen (O), which has an atomic number of 8, follows the configuration
\[1s^2 2s^2 2p^4\]
signifying two electrons in the 1s orbital, two in the 2s, and four in the 2p orbital. These arrangements dictate how atoms will interact with each other to form compounds through ionic or covalent bonds.

Noble Gas Configuration

Atoms generally long for stability, and a full valence shell – known as noble gas configuration – is the most stable state they can achieve. The noble gases (group 18 elements) like helium, neon, and argon, naturally have complete outer shells, which makes them inert or non-reactive. Other elements achieve this blissful stable state by gaining, losing, or sharing electrons to complete their outer shells.

For example, magnesium, after losing two electrons, reaches the noble gas configuration of neon (Ne), becoming a positively charged ion (\(Mg^{2+}\)). Oxygen, by gaining two electrons, also attains the electron configuration of neon and becomes negatively charged (\(O^{2-}\)). This transfer of electrons towards achieving a noble gas configuration is the driving force behind the formation of ionic compounds, including magnesium oxide (MgO).

Ionic Bonding

Ionic bonding is the electrostatic force of attraction between positively and negatively charged ions. It is characterized by the complete transfer of electrons from one atom to another, leading to the formation of oppositely charged ions.

In the case of magnesium oxide (MgO), the ionic bonding is formed between magnesium ions (\(Mg^{2+}\)) and oxide ions (\(O^{2-}\)). Although one might think that magnesium could simply lose one electron and oxygen could gain one, resulting in \(Mg^+\) and \(O^-\), this scenario does not lead to the stability that nature favors. The reason behind this is the quest for noble gas configuration—magnesium needs to lose two electrons, not just one, to achieve the stability of neon, and oxygen needs to gain two to complete its valence shell. So while \(Mg^+\) and \(O^-\) could theoretically exist, they do not offer the lowest energy and most stable configuration for these atoms. That's why you find \(Mg^{2+}\) and \(O^{2-}\) coming together to form the strong ionic bond seen in magnesium oxide.