Problem 42 - Chemistry The Central Science In Si Units

Question

(a) What is the trend in first ionization energies as one proceeds down the group 17 elements? Explain how this trend relates to the variation in atomic radii. (b) What is the trend in first ionization energies as one moves across the fourth period from \(\mathrm{K}\) to \(\mathrm{Kr}\) ? How does this trend compare with the trend in atomic radii?

Step-by-Step Solution

Answer

(a) Ionization energy decreases down Group 17 due to increasing atomic radii. (b) Ionization energy increases across Period 4 as atomic radii decrease.

Step 1: Understanding the First Ionization Energy

The first ionization energy is the energy required to remove the outermost electron from an atom in its gaseous state. This energy helps understand how strongly an atom holds onto its electrons.

Step 2: Trend of Ionization Energy Down Group 17

As we go down group 17 (the halogens), the first ionization energy decreases. This is because atomic radii increase, resulting in the outermost electron being farther from the nucleus, thus less tightly bound.

Step 3: Relating Trend to Atomic Radii in Group 17

Atomic radii increase as you move down a group because additional electron shells are added. As a result, the outermost electrons are further from the nucleus and experience less electrostatic force, leading to lower ionization energies.

Step 4: Trend of Ionization Energy Across Fourth Period

As you move across the fourth period from potassium (K) to krypton (Kr), the first ionization energy generally increases. This is due to increasing nuclear charge with a relatively constant shielding effect, which pulls electrons closer together.

Step 5: Comparing with Atomic Radii Across the Fourth Period

Across the fourth period, atomic radii decrease as the electrons are added to the same energy level while the nuclear charge increases. This decrease in atomic radii results in an increase in ionization energy, contrasting the trend seen as you move down a group.

Key Concepts

Atomic Radii

The atomic radius is a measure that defines the size of an atom. It commonly refers to the distance from the nucleus to the outermost electron cloud. Understanding atomic radii is essential because it relates to how electrons are arranged around the nucleus and influences several atomic properties.

Atomic radii tend to increase as we move down a group in the periodic table. This happens because each row down adds a new electron shell, making the atom's overall size larger. As a result, outer electrons are further away from the nucleus, experiencing less attraction.

Conversely, atomic radii tend to decrease as you move across a period from left to right. Despite the addition of more protons and electrons, these electrons enter the same shell without increasing size. Therefore, the added protons increase the nuclear charge, pulling the electron cloud closer to the nucleus and reducing the atomic radius.

Periodic Trends

Periodicity in the periodic table refers to the repeating patterns of chemical and physical properties of elements—essentially how certain properties change in cycles.

Ionization Energy Trends

• Ionization energy is the required energy to remove the outermost electron from an atom.
• As you move across a period from left to right, ionization energy tends to increase. This is due to a higher nuclear charge, which more strongly attracts electrons.
• Down a group, ionization energy decreases as the atomic radii increase, reducing the nuclear force on the outer electrons.

Relating to Atomic Radii

• There is an inverse relationship between atomic radii and ionization energy, especially evident as you move down a group.
• Larger atomic radii mean electrons are further from the nucleus and thus easier to remove, leading to lower ionization energy.
• Smaller atomic radii, with electrons closer to the nucleus, require more energy for removal.

Group 17 Elements

Group 17 of the periodic table is known as the halogens and includes elements like fluorine, chlorine, bromine, iodine, and astatine. These are highly reactive nonmetals with distinct properties.

Properties and Trends

• Halogens have high electronegativities and affinities for electrons, indicating strong tendencies to gain electrons and complete their outer shells.
• As you move down Group 17, atomic radii increase with added electron shells, and ionization energy decreases due to decreased nuclear attraction.

Significance of Trends

• The increased atomic size influences the physical state of halogens; for instance, fluorine and chlorine are gases, while iodine is solid at room temperature.
• Despite decreasing ionization energy, halogens maintain strong reactivity because they are close to achieving a stable electron arrangement.