Question

Write the following ions in order of decreasing capacity to accept electrons. \(\mathrm{H}^{+}, \mathrm{Mg}^{2+}, \mathrm{K}^{+}, \mathrm{Ag}^{+}, \mathrm{Zn}^{2+}\) (a) \(\mathrm{Ag}^{+}>\mathrm{H}^{+}>\mathrm{Zn}^{2+}>\mathrm{Mg}^{2+}>\mathrm{K}^{+}\) (b) \(\mathrm{H}^{+}>\mathrm{Zn}^{2+}>\mathrm{Mg}^{2+}>\mathrm{K}^{+}>\mathrm{Ag}^{+}\) (c) \(\mathrm{K}^{+}>\mathrm{Mg}^{2+}>\mathrm{Zn}^{2+}>\mathrm{H}^{+}>\mathrm{Ag}^{+}\) (d) \(\mathrm{Mg}^{2+}>\mathrm{Zn}^{2+}>\mathrm{K}^{+}>\mathrm{Ag}^{+}>\mathrm{H}^{+}\)

Short answer

The correct order is (b) \(\mathrm{H}^{+}>\mathrm{Zn}^{2+}>\mathrm{Mg}^{2+}>\mathrm{K}^{+}>\mathrm{Ag}^{+}\)

Step-by-step solution

Understand the Concept of Electropositivity

Electropositivity refers to the tendency of an atom to donate electrons to become a cation. Metals are typically electropositive. The capacity of an ion to accept electrons is the opposite, often related to electronegativity, and hydrogen ions (protons) are great electron acceptors due to their small size and the fact they are just protons without electrons.

Compare Charges and Electronegativities

Generally, ions with higher positive charges have a greater capacity to accept electrons. However, the size of the ion and its electronegativity also play important roles. Higher electronegativity means a greater ability to attract electrons. The hydrogen ion, being essentially a bare proton, has a high capacity to accept electrons, while larger ions with the same charge are less electronegative and have a lower capacity to accept electrons.

Order the Ions

Considering their charges and typical electronegativities: Hydrogen ion (H+) can most readily accept an electron as it has no electrons to repel the incoming electron. Among the divalent ions, Zn2+ is smaller and has a higher charge density than Mg2+, which means it can attract electrons more effectively. K+ is larger with a lower charge density, thus has the least tendency to attract electrons. Silver ion (Ag+) has a filled d10 configuration making it fairly stable and hence less likely to accept electrons compared to a bare proton like H+.

Choose the Correct Order

Based on the previous steps, the correct order of these ions in decreasing ability to accept electrons is: H+ > Zn2+ > Mg2+ > Ag+ > K+. This corresponds to option (b).

Key concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that measures the ability of an atom to attract electrons in a chemical bond. It is a dimensionless quantity, often discussed in the context of the Pauling scale, where fluorine is assigned the highest value of 4.0, being the most electronegative element.

When comparing the electron accepting capacity of ions, electronegativity plays a vital role as highly electronegative ions have a stronger pull on electrons. This results in a greater ability to attract and accept electrons from other species. In ions, this can be influenced by the atomic number and the distance of the outermost electrons from the nucleus; the closer the electrons are to the nucleus, the greater the electronegative pull.

For example, in the context of the exercise, hydrogen ions (H⁺) have a significant capacity to accept electrons due to their high effective nuclear charge, which stems from their small size and lack of electron cloud. This concept is typically used to predict the charge distribution within molecules and the outcome of reactions, lending insight into molecular shape, polarity, and even the types of intermolecular forces that can occur between molecules.

Ion Charge Density

Ion charge density refers to the charge of an ion relative to its size, which means that ions with higher charges and smaller radii have higher charge densities. To put it simply, it's a measure of the charge concentration on an ion. A higher charge density indicates a stronger electrostatic attraction for electrons, which generally enhances the ion's capacity to accept electrons.

This concept is significant when distinguishing between ions with the same charge but of different sizes, such as divalent ions like Zn²⁺ and Mg²⁺ in the given exercise. Zinc ion (Zn²⁺), being smaller than magnesium ion (Mg²⁺), has a higher charge density and therefore a higher affinity for electrons. Charge density can also influence properties such as solubility and the lattice energy of ionic compounds. Understanding charge density provides insights into how strongly ions attract one another within a crystal lattice and how effectively they can attract additional electrons.

Electropositivity

Electropositivity is the propensity of an atom to donate electrons and form positive ions, or cations. It is commonly associated with metals, due to their tendency to lose electrons and participate in ionic bonding. Electropositivity is inversely related to electronegativity; elements that are highly electropositive have low electronegativity values and are usually found towards the left side and the bottom of the periodic table.

In our exercise, potassium ion (K⁺) displays electropositivity by readily losing its valence electron to achieve a stable noble gas electron configuration. However, despite being electropositive, this does not correlate with a high electron accepting capacity. This is because, once potassium has lost its electron and acquired a positive charge, there is less of a tendency to gain back that electron compared to ions like hydrogen or zinc, which have higher charge densities and electronegativities.

Electropositivity has practical implications in predicting the types of bonds that elements will form and their reactivity, particularly in redox reactions where electron transfer occurs. It provides a framework for understanding chemical behavior, like why certain elements prefer to give up electrons and why others are prone to accepting them to form stable compounds.