Question

Which of the following ions are likely to be found in chemical compounds: \(\mathrm{Na}^{2+}, \mathrm{Li}^{+}, \mathrm{Al}^{4+}, \mathrm{F}^{2-},\) or \(\mathrm{Te}^{2-} ?\) Explain briefly.

Short answer

\(\mathrm{Li}^{+}\) and \(\mathrm{Te}^{2-}\) are the ions likely to be found in chemical compounds.

Step-by-step solution

Analyze the Sodium Ion

Sodium (\(\mathrm{Na}\)) normally has an oxidation number of +1 since it has one electron in its outer shell that it easily gives away to reach a stable, full shell. Therefore, \(\mathrm{Na}^{2+}\) is not a common ion.

Analyze the Lithium Ion

Similar to sodium, Lithium (\(\mathrm{Li}\)) has one electron in its outer shell. Therefore, \(\mathrm{Li}^{+}\) is a common ion.

Analyze the Aluminium Ion

Aluminium (\(\mathrm{Al}\)) has three electrons in its outer shell. Therefore, \(\mathrm{Al}^{3+}\) is a common ion, not \(\mathrm{Al}^{4+}\).

Analyze the Fluorine Ion

Fluorine (\(\mathrm{F}\)) has seven electrons in its outer shell and typically gains one electron to reach a stable, full shell. Therefore, \(\mathrm{F}^{1-}\) is a common ion, not \(\mathrm{F}^{2-}\).

Analyze the Tellurium Ion

Tellurium (\(\mathrm{Te}\)) is in the same group as oxygen and sulfur and typically forms a 2- charge by gaining two electrons. Therefore, \(\mathrm{Te}^{2-}\) is a common ion.

Key concepts

Oxidation Number

The oxidation number of an atom refers to the charge an atom would have if the compound it is in were composed of ions. It helps us in understanding how electrons are distributed among atoms within a molecule, focusing primarily on the transfer or sharing of electrons in chemical bonds. In chemical reactions, atoms try to achieve more stable configurations by either losing electrons and becoming positively charged ions or gaining electrons to become negatively charged.
These changes in electron number are directly related to oxidation numbers. Some important points to remember about oxidation numbers are:

• Metals typically have positive oxidation numbers as they tend to lose electrons.
• Non-metals tend to have negative oxidation numbers, as they usually gain electrons.
• In compounds or ions, the sum of oxidation numbers of all atoms is equal to the total charge of the molecule or ion.

For example, sodium (\(\mathrm{Na}\)) usually has an oxidation number of +1, as it loses one electron to gain a full outer shell, making \(\mathrm{Na^+}\) a common ion. Understanding these numbers is key to predicting which ions are likely to form in a reaction.

Stable Electron Configuration

Atoms tend to seek a stable electron configuration, often resembling the nearest noble gas, because this state is associated with the lowest energy and maximal stability. The concept of stable electron configurations is essential in predicting the formation of ions and explaining why certain charges are more common than others in ions.
An atom's stability is largely determined by the following factors:

• The Octet Rule: Most elements attempt to acquire the electron configuration of the nearest noble gas, which involves having a completely filled outer shell.
• Electrons in outermost shell: For example, sodium has one electron in its outer shell and achieves a stable configuration by losing it to become \(\mathrm{Na^+}\).
• Halogens, like fluorine, have seven electrons in their outer shells and typically gain one more electron to reach the stable octet configuration, forming \(\mathrm{F^-}\) in the process.

Understanding these principles helps you rationalize why some ions like \(\mathrm{Li^+}\) and \(\mathrm{Te^{2-}}\) are common, whereas others like \(\mathrm{Na^{2+}}\) or \(\mathrm{F^{2-}}\) are not.

Common Ions

Common ions are ions that frequently appear in chemical compounds due to their favorable energy states and stable configurations. The predictability of which ions will form primarily depends on the atoms' desire to achieve a low-energy, stable electronic configuration through the exchange of electrons.
Some typical features of common ions include:

• Elements on the left side of the periodic table (metals) often form positive ions, such as \(\mathrm{Li^+}\) and \(\mathrm{Na^+}\), by losing their outer electrons.
• Elements on the right side (non-metals) frequently form negative ions by gaining electrons, as seen with \(\mathrm{F^-}\) and \(\mathrm{Te^{2-}}\).
• Transition metals can have multiple common oxidation states, leading to various common ions.

Aluminium typically forms \(\mathrm{Al^{3+}}\) because it loses three electrons to achieve a stable configuration. While tellurium, part of the oxygen group, usually forms \(\mathrm{Te^{2-}}\) by gaining two electrons. Recognizing these patterns among common ions aids in predicting the types and charges of ions you might encounter in different chemical settings.