Problem 46 - Chemistry and Chemical Reactivity

Question

Rank the following in order of increasing ionization energy: Cl, \(\mathrm{Ca}^{2+}\), and \(\mathrm{Cl}^{-}\). Briefly explain your answer.

Step-by-Step Solution

Answer

Increasing ionization energy order: \(\mathrm{Cl}^{-} < \mathrm{Cl} < \mathrm{Ca}^{2+}\).

Step 1: Understand Ionization Energy

First, let's understand that ionization energy is the energy required to remove an electron from an atom or ion in its gaseous state. Generally, ionization energy increases across a period (from left to right on the periodic table) and decreases down a group.

Step 2: Analyze Ionization Energies of Cl, Ca²⁺, and Cl⁻

Consider their general positions and charges. Chlorine (Cl) is a neutral atom, calcium ion (\(\mathrm{Ca}^{2+}\)) is a cation missing two electrons, and chloride ion (\(\mathrm{Cl}^{-}\)) is an anion with an extra electron.

Step 3: Compare Ionization Energies

\(\mathrm{Ca}^{2+}\) has a high ionization energy because it's already lost electrons and holds remaining electrons tightly. \(\mathrm{Cl}^{-}\) is more stable with extra electrons, so it has lower energy requirements for electron removal. \(\mathrm{Cl}\), being neutral, has moderate ionization energy.

Step 4: Rank the Species

Therefore, the order of increasing ionization energy is: \(\mathrm{Cl}^{-} < \mathrm{Cl} < \mathrm{Ca}^{2+}\). This order reflects how tightly electrons are held by each species.

Key Concepts

Periodic Trends

Periodic trends refer to patterns or regularities within elements on the periodic table. One key property that shows clear trends is ionization energy.
Ionization energy, the energy required to remove an electron, typically increases as you move from left to right across a period. This occurs because the number of protons in the nucleus increases, pulling electrons closer and making them harder to remove.

• Across a period, ionization energy increases.
• Down a group, ionization energy decreases.

Conversely, ionization energy usually decreases as you move down a group. This is due to the addition of electron shells, which increase the distance between the nucleus and the outermost electrons, making them easier to remove. Understanding these trends is crucial for predicting element behavior and comparing different atoms and ions' ionization energies.

Chlorine

Chlorine, a member of the halogen family, is located in group 17 of the periodic table. As a non-metal, chlorine has high electronegativity and high ionization energy compared to many other elements.
In its neutral state, chlorine has seven valence electrons, making it eager to gain one additional electron to achieve a stable octet configuration.
This tendency to gain electrons is reflected in its ionization energy.

• Neutral chlorine atoms have moderate ionization energy.
• They have a strong pull from the nucleus on the outermost electrons.

When comparing to other species, chlorine's ionization energy is typically higher than anions like chloride but lower than cations like calcium ions.

Calcium Ion

The calcium ion, (\( \mathrm{Ca}^{2+} \)), is a cation formed when calcium, a group 2 alkaline earth metal, loses two electrons. This removal occurs because its outer shell only has two electrons that it can easily lose to achieve a stable noble gas configuration.

• The ionization of calcium to form (\( \mathrm{Ca}^{2+} \)) significantly increases the ion's overall ionization energy.
• Once electrons are removed, the remaining electrons experience a stronger nuclear pull.

Due to the absence of these electrons, the ion has a notably higher ionization energy. It's important to understand that compared to neutral atoms or anions, cations like the (\( \mathrm{Ca}^{2+} \)) hold their electrons much more tightly.

Anion and Cation

Anions and cations are terms used to describe ions with positive and negative charges, respectively. Anions like (\( \mathrm{Cl}^{-} \)) have gained additional electrons, while cations like (\( \mathrm{Ca}^{2+} \)) have lost electrons.

• Anions result from a gain of electrons and generally have lower ionization energy, as extra electrons are added, making it easier to remove one.
• Cations form through electron loss and have higher ionization energy because they hold remaining electrons more tightly.

Understanding the nature of these ions is essential because it influences their chemical reactivity and stability. Anions tend to gain additional electrons, while cations release them, impacting their respective ionization energy values and making them distinct in behavior.