Problem 92 - Chemistry

Question

(a) Which of the following compounds has the greatest ionic character: \(\mathrm{PCl}_{5}, \mathrm{SiCl}_{4}, \mathrm{CCl}_{4}, \mathrm{BCl}_{3} ?\) (b) Which of the following ions has the smallest ionic radius: \(\mathrm{F}^{-}, \mathrm{C}^{4-}, \mathrm{N}^{3-}, \mathrm{O}^{2-} ?(\mathrm{c})\) Which of the following atoms has the highest ionization energy: \(\mathrm{F}, \mathrm{Cl}, \mathrm{Br}, \mathrm{I} ?(\mathrm{~d})\) Which of the following oxides is most acidic: \(\mathrm{H}_{2} \mathrm{O}, \mathrm{SiO}_{2}, \mathrm{CO}_{2} ?\)

Step-by-Step Solution

Answer

(a) \(\mathrm{BCl}_{3}\) (b) \(\mathrm{F}^{-}\) (c) \(\mathrm{F}\) (d) \(\mathrm{SiO}_{2}\)

Step 1: Determine the compound with the greatest ionic character

Ionic character is influenced by the difference in electronegativity between ions in a molecule. In this case, \(\mathrm{BCl}_{3}\) has the highest ionic character, since Boron has the lowest electronegativity and therefore the biggest difference compared to Chlorine.

Step 2: Identify the ion with the smallest ionic radius

Ionic radius tends to decrease across a period as effective nuclear charge increases. Also, it increases down a group due to the addition of energy levels. Among the given, \(\mathrm{F}^{-}\) has the smallest ionic radius.

Step 3: Find the atom with the highest ionization energy

Ionization energy is the energy required to remove an electron from an atom. It tends to increase across a period and decrease down a group. Here, Fluorine (\(\mathrm{F}\)) has the highest ionization energy.

Step 4: Determine the most acidic oxide

The acidity of an oxide depends on the electronegativity and oxidation states. In general, non-metal oxides are acidic, and among them, \(\mathrm{SiO}_{2}\) is more acidic compared to \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{CO}_{2} \)

Key Concepts

Understanding Ionic Character

Ionic character refers to how "ionic" a bond is between two atoms in a molecule. This is largely influenced by the difference in electronegativity between the atoms involved. Electronegativity is a measure of how strongly an atom attracts electrons in a bond.
The larger the electronegativity difference between two bonded atoms, the greater the ionic character. Ionic bonds are characterized by one atom completely transferring an electron to another, leading to positive and negative ions. However, most real-world "ionic" bonds have some degree of covalent character due to the sharing of electrons.
When comparing compounds for ionic character, look for:

• Large differences in electronegativity.
• Presence of metal and non-metal atoms as they often form ionic bonds due to significant electronegativity differences.

For example, in the compounds listed ( 𝑃𝐶𝑙_{5}, 𝑆𝑖𝐶𝑙_{4}, 𝐶𝐶𝑙_{4}, 𝐵𝐶𝑙_{3}), the electronegativity difference between Boron and Chlorine is the greatest, giving 𝐵𝐶𝑙_{3} the highest ionic character.

Exploring Ionic Radius

The ionic radius is a measurement of the size of an ion's electron cloud. It can vary depending on the ion's charge and position in the periodic table. An ion's size:

• Decreases across a period due to increased effective nuclear charge attracting the electrons more strongly and pulling them closer to the nucleus.
• Increases down a group as additional electron shells are added.

Anions (negatively charged ions) are generally larger than neutral atoms, and this size increases further with more negative charge because of increased electron-electron repulsion.
In our exercise, the ion 𝐹^{-} has the smallest ionic radius among 𝐹^{-}, 𝐶^{4-}, 𝑁^{3-}, 𝑂^{2-}. This is because it's higher up in the periodic table and has the least electron shielding effect, holding its electron more tightly due to a strong nuclear charge.

Decoding Ionization Energy

Ionization energy is the amount of energy needed to remove an electron from an isolated atom in its gaseous state. It follows trends similar to atomic size:

• Increases across a period as electrons are more tightly bound to the atom due to increasing nuclear charge.
• Decreases down a group as electrons are further away from the nucleus and experience greater shielding from inner electron shells.

The atom with the highest ionization energy in our list (𝑭, 𝑪𝒍, 𝑩𝒓, 𝑰) is 𝑭 (Fluorine). As you move from left to right across the periodic table, ionization energy increases. Fluorine is located in the second period, hence it has a notably higher ionization energy than its counterparts because the electrons are pulled closer to the nucleus, making them harder to remove.

The Nature of Acidic Oxides

Acidic oxides react with water to form acids or with bases to form salts. These are typically oxides of nonmetals. Such oxides have high electronegativity and strongly polar bonds, which help in reacting with water to give acidic solutions.
There are several tiers of acidity, generally relying on electronegativity:

• Non-metal oxides are usually more acidic than metal oxides.
• Acidity increases with higher oxidation states, meaning molecules like 𝐶𝑂_{2} (carbon dioxide) can form acids like carbonic acid when dissolved in water.

In our exercise, 𝑆𝑖𝑂_2 is more acidic than 𝐻_2𝑂 and 𝐶𝑂_2. While 𝑆𝑖𝑂_2 might not dissolve in water as readily as 𝐶𝑂_2 does to form an acid, its oxide structure supports the formation of strong acidic solutions when interacting with bases, compared to the other options.