Question

What justification is there for regarding the ammonium ion as an analog of a Group 1 metal cation? Consider properties such as solubility, charge, and radius. The radius of \(\mathrm{NH}_{4}^{+}\)is \(151 \mathrm{pm}\).

Short answer

The ammonium ion (H_4^+) is considered analogous to Group 1 metal cations due to its +1 charge, high solubility in water, and an ionic radius that falls within the range of Group 1 cations' radii.

Step-by-step solution

Understanding Analogous Traits

To justify the ammonium ion (H_4^+) as an analog of a Group 1 metal cation, we must compare its properties like charge, solubility, and ionic radius with those of Group 1 cations.

Comparing Charge

The ammonium ion has a charge of +1, which is the same as the charge found on Group 1 metal cations like Na+, K+, and so on. This makes it comparable in terms of electrical charge.

Comparing Solubility in Water

Ammonium ion and Group 1 cations typically form salts that are highly soluble in water. This similarity in solubility is another factor that supports the analogy.

Comparing Ionic Radius

The ionic radius of the ammonium ion is 151 pm, which is within the range of ionic radii for Group 1 cations (e.g., Li+ is around 76 pm, Na+ is around 102 pm, and K+ is around 138 pm). Even though H_4^+ has a larger radius compared to some Group 1 cations, its size is still comparable.

Consolidating the Comparison

From the comparison of the above properties, we can conclude that the ammonium ion shares significant similarities in charge, solubility, and ionic radius with Group 1 metal cations, justifying its consideration as an analog.

Key concepts

Ionic Charge

In the world of chemistry, the term 'ionic charge' refers to the electrical charge that an ion carries. Ions are atoms or molecules that have lost or gained electrons, which results in a net positive or negative charge. For example, when an atom loses an electron, it becomes positively charged and is known as a cation. The ammonium ion (\(\text{NH}_{4}^{+}\)), which carries a positive charge of +1, is categorized as a cation much like those found in the Group 1 metals of the periodic table - including lithium (\(\text{Li}^{+}\)), sodium (\(\text{Na}^{+}\)), and potassium (\(\text{K}^{+}\)).

The similarity in the charge is critical because it directly impacts the ion's chemical reactivity and ability to form compounds. For instance, the +1 charge allows these cations to readily pair with negatively charged ions (anions) to form stable ionic compounds. It's the same charge that enables the ammonium ion to participate in chemical reactions in a manner akin to Group 1 cations. Due to this shared characteristic, the ammonium ion's behavior in forming ionic bonds is often comparable to that of its Group 1 counterparts.

Solubility in Water

When scientists discuss 'solubility in water,' they're referring to the ability of a substance to dissolve in water and form a homogeneous solution. This property is crucial for many biological and environmental processes. For ions, solubility varies depending on multiple factors such as temperature, pressure, and the presence of other ions.

Group 1 metal cations are known for their high solubility in water, a trait they share with the ammonium ion. For instance, common table salt (\(\text{NaCl}\)), which contains the sodium ion (\(\text{Na}^{+}\)), dissolves readily in water. Similarly, the ammonium ion forms salts like ammonium chloride (\(\text{NH}_{4}\text{Cl}\)) that also exhibit a high degree of solubility. This parallel in water solubility is significant because it means that the ammonium ion can play similar roles in aquatic systems and biological organisms, where the solubility of ions is key for processes like nutrient transport and cellular function.

Ionic Radius

The term 'ionic radius' is used to describe the size of an ion. Specifically, it is the measure from the center of the nucleus to the outer boundary of the surrounding cloud of electrons. The size of an ion affects its physical and chemical properties, including its density, boiling and melting points, and the distances between ions in a crystal lattice.

With an ionic radius of 151 pm, the ammonium ion is comparable in size to Group 1 cations. The ionic radii of Group 1 cations increase down the group; for example, lithium (\(\text{Li}^{+}\)) at around 76 pm, sodium (\(\text{Na}^{+}\)) at around 102 pm, and potassium (\(\text{K}^{+}\)) at around 138 pm. Although there is a range of sizes, the ammonium ion falls within the spectrum. This similarity in ionic radius is another reason that the ammonium ion can be seen as analogous to Group 1 cations. Their comparable sizes mean that they may fit similarly into crystal lattices and impact reactivity and solubility in comparable ways.