Problem 34 - Chemistry and Chemical Reactivity

Question

Compare the elements \(\mathrm{B}, \mathrm{Al}, \mathrm{C},\) and \(\mathrm{Si}\). (a) Which has the most metallic character? (b) Which has the largest atomic radius? (c) Which has the most negative electron attachment enthalpy? (d) Place the three elements \(\mathrm{B}, \mathrm{Al}\), and \(\mathrm{C}\) in order of increasing first ionization energy.

Step-by-Step Solution

Answer

(a) Al, (b) Al, (c) C, (d) Al < B < C.

Step 1: Analyzing Metallic Character

The metallic character of an element typically increases as you move down a group and decreases as you move across a period from left to right in the periodic table. Thus, among the elements given: Boron (B), Aluminum (Al), Carbon (C), and Silicon (Si), Aluminum ( Al ) has the most metallic character since it is located in Group 13 and is below Boron.

Step 2: Comparing Atomic Radius

Atomic radius tends to increase down a group and decrease across a period. For the elements B , Al , C , and Si , Aluminum ( Al ) is the farthest down Group 13, indicating it has the largest atomic radius.

Step 3: Evaluating Electron Attachment Enthalpy

Electron attachment enthalpy becomes more negative as you move from left to right across a period. Among these elements, C and Si are in Group 14, and Carbon ( C ), being higher up the periodic table, has a more negative electron attachment enthalpy than Silicon. Boron and Aluminum have less negative values.

Step 4: Ordering by First Ionization Energy

First ionization energy increases across a period and decreases down a group. Order of increasing first ionization energy is Al < B < C , since Al is further down a group, and C is furthest to the right on the periodic table among the given elements.

Key Concepts

Metallic Character

The metallic character of an element is a measure of its tendency to lose electrons and form positive ions. This characteristic is indicative of how "metal-like" an element behaves. As you move down a group in the periodic table, the metallic character typically increases because the additional shells of electrons reduce the effective nuclear charge on the outer electrons, making them easier to lose. Conversely, moving across a period from left to right, the metallic character decreases as elements become less inclined to lose electrons in favor of sharing or gaining them to achieve full outer shells.
Among the elements in question—Boron (B), Aluminum (Al), Carbon (C), and Silicon (Si)—Aluminum (Al) demonstrates the greatest metallic character. This is because it is further down in Group 13 than Boron, thus having more electron shells which allows it to more easily shed its outer electrons compared to its periodic companions. Aluminum's metallic nature outshines the others, making it distinctly metal-like under these conditions.

Atomic Radius

The atomic radius of an element refers to the size of its atoms, measured from the nucleus to the boundary of the surrounding cloud of electrons. As we move down a group in the periodic table, the atomic radius increases because there are more electron shells around the nucleus.
Conversely, as we move across a period from left to right, the atomic radius decreases. This occurs because more protons are added to the nuclei without a corresponding increase in shielding, pulling the electron cloud closer to the nucleus.
In the context of Boron (B), Aluminum (Al), Carbon (C), and Silicon (Si), Aluminum (Al) has the largest atomic radius. Located further down Group 13 than Boron and Silicon, Aluminum benefits from additional electron shells that result in a larger radius. Hence, its atomic size surpasses those of the other mentioned elements.

Electron Attachment Enthalpy

Electron attachment enthalpy, also known as electron affinity, refers to the energy change that occurs when an electron is added to a neutral atom in the gas phase, often expressed in terms of becoming more negative. This property becomes more favorable (i.e., more negative) as you move from left to right across a period because atoms are approaching a filled valence shell, where the added electron can be tightly held.
In the elements Boron (B), Aluminum (Al), Carbon (C), and Silicon (Si), Carbon (C), being early in Group 14 and located further left compared to Silicon (Si), shows the most negative electron attachment enthalpy. This reflects its stronger tendency to gain an extra electron compared to Silicon, Boron, and Aluminum, making it the most likely among these elements to achieve a stable electron configuration by adding an electron.

Ionization Energy

Ionization energy is the amount of energy required to remove the most loosely held electron from a neutral atom in the gas phase. This energy tends to increase across a period due to a growing effective nuclear charge, attracting electrons more strongly. It decreases down a group as atomic size increases, since outer electrons are further away from the nucleus and less tightly bound.
When arranging Boron (B), Aluminum (Al), and Carbon (C) in order of increasing ionization energy, the sequence is Aluminum < Boron < Carbon.

• Aluminum, being further down the group, has the lowest first ionization energy.
• Boron follows since it's higher up yet not as far across the period as carbon.
• Carbon, furthest right, outshines in holding its electrons.

Thus, Carbon displays the strongest retention for its electrons, demanding more energy to eject them compared to the others.