Problem 22 - Introductory Chemistry Atoms First

Question

Without knowing electronegativity values, a student claims \(\mathrm{BaCl}_{2}\) is more ionic than \(\mathrm{BeCl}_{2}\). All she has access to is a periodic table. How does she know she is right?

Step-by-Step Solution

Answer

The student can determine that BaCl\(_2\) is more ionic than BeCl\(_2\) by understanding the periodic trends in electronegativity. Since electronegativity generally decreases down a group, Beryllium (Be) has a higher electronegativity than Barium (Ba). Both compounds contain Chlorine (Cl). The electronegativity difference between Be-Cl is smaller compared to Ba-Cl, indicating that BaCl\(_2\) has a greater ionic character. This conclusion can be made without knowing the exact electronegativity values and by just using the periodic table.

Step 1: Identify the location of Ba and Be in the periodic table

When looking at the periodic table, we can find the locations of the elements involved in the two compounds. Barium (Ba) is in Group 2 and Period 6, while Beryllium (Be) is also in Group 2 but in Period 2.

Step 2: Understand the electronegativity trend across the periodic table

Generally, electronegativity increases across a period (from left to right) and decreases down a group (from top to bottom) on the periodic table. Therefore, elements in the top-right corner have the highest electronegativity values, and elements in the bottom-left corner have the lowest electronegativity values.

Step 3: Compare the relative electronegativity values of Ba and Be

Considering the periodic trend in electronegativity, we can conclude that Beryllium (Be), which is higher up in Group 2, will have a higher electronegativity than Barium (Ba), which is further down in Group 2.

Step 4: Identify the location of Cl in the periodic table

Chlorine (Cl) can be found in Group 17 and Period 3 in the periodic table. Since both compounds contain Chlorine, the electronegativity of Chlorine will affect the ionic character of both compounds in the same way.

Step 5: Compare the electronegativity difference between Ba-Cl and Be-Cl

As previously determined, Be has a higher electronegativity than Ba. Therefore, the electronegativity difference between Be and Cl will be smaller compared to the difference between Ba and Cl. Smaller electronegativity differences generally correspond to greater covalent character, while larger differences imply greater ionic character.

Step 6: Conclude the relative ionic character of BaCl2 and BeCl2

The student can conclude that BaCl2 is more ionic than BeCl2 because the electronegativity difference between Ba and Cl is larger than the difference between Be and Cl. This conclusion can be reached without knowing the exact electronegativity values, but by understanding the periodic trends in electronegativity.

Key Concepts

Electronegativity

Electronegativity is a key concept in understanding the nature of chemical bonds between different atoms. It is a measure of an atom's ability to attract and hold electrons in a chemical bond. This property varies among elements and is crucial in determining whether a bond will be ionic or covalent.
On the periodic table, electronegativity tends to increase as you move from left to right across a period and decrease as you move down a group. This is due to the increasing effective nuclear charge, meaning more protons in the nucleus attracting electrons more strongly, and the increasing atomic radius, where more electron shells reduce the effective pull the nucleus has on the outermost electrons.
Understanding these trends can help predict the behavior of compounds like \( ext{BaCl}_2\) and \( ext{BeCl}_2\) without specific electronegativity values. Beryllium (Be), being higher in the periodic table than Barium (Ba), has a higher tendency to attract electrons than Ba, making it less ionic when paired with chlorine (Cl).

Ionic Character

The ionic character of a bond is influenced by the difference in electronegativity between the two bonding atoms. A larger electronegativity difference suggests a higher ionic character, implying that one atom donates electrons to the other, typical in ionic bonds. Conversely, a smaller difference is indicative of a more covalent character, where electrons are shared more equally between the atoms.
For example, in the compound \( ext{BaCl}_2\), the larger difference in electronegativity between Barium (Ba) and Chlorine (Cl) results in a more ionic compound compared to \( ext{BeCl}_2\). The student was able to interpret that despite not knowing specific electronegativity values by understanding periodic trends.
Thus, \( ext{BaCl}_2\) is considered more ionic due to Barium's lower electronegativity compared to Beryllium, making the electron transfer to Chlorine more complete.

Periodic Trends

Periodic trends are predictable patterns observed within the periodic table concerning various elemental properties such as atomic radius, ionization energy, and electronegativity. These trends provide essential insights into the chemical behavior of elements and are a valuable tool for predicting and explaining the nature of chemical compounds.
Some key periodic trends include:

• Electronegativity increases across a period and decreases down a group.
• Atomic radius decreases across a period and increases down a group.
• Ionization energy increases across a period and decreases down a group.

Recognizing these trends is crucial when predicting compound characteristics, such as the case where \( ext{BaCl}_2\) is deemed more ionic than \( ext{BeCl}_2\). Understanding that elements' properties change predictably across the periodic table can help one foresee bonding types without needing exact numerical values.

Group and Period

Groups and periods are the two main ways in which elements in the periodic table are organized. A group is a vertical column on the periodic table, where elements have similar electronic configurations, particularly in their outer shell, resulting in similar chemical properties. For instance, both Beryllium (Be) and Barium (Ba) are in Group 2, known as the alkaline earth metals, sharing properties due to having two electrons in their outer shell.
A period is a horizontal row, where elements show a progressive change in chemical and physical properties. These changes occur because, as you move from left to right across a period, the number of protons and electrons increases, changing the atomic properties.
Knowledge of an element's group and period is vital when determining its behavior in compounds. For instance, although Be and Ba belong to the same group, their differing periods explain their varied electronegativity, with Ba being lower and thus forming more ionic bonds with chlorine in \( ext{BaCl}_2\). Understanding the periodic table's structure helps clarify why certain chemical properties manifest, supporting predictions about compound behavior.