Problem 7 - Chemistry The Central Science

Question

The atomic and ionic radii of the first three group \(6 \mathrm{~A}\) elements are (a) Explain why the atomic radius increases in moving downward in the group. (b) Explain why the ionic radii are larger than the atomic radii. (c) Which of the three anions would you expect to be the strongest base in water? Explain. [Sections 22.5 and 22.6\(]\)

Step-by-Step Solution

Answer

(a) The atomic radius increases in moving downward in the group because there is an increase in the number of electron shells (principal quantum number, n), which leads to an increased distance between the nucleus and the valence electrons. (b) The ionic radii are larger than the atomic radii because when an element gains an electron to form an anion, there is an increase in electron-electron repulsion, causing the electron cloud to expand and resulting in a larger ionic radius. (c) The strongest base in water would be the anion of the third element in Group 6A (bottommost element) because it has the largest atomic and ionic radii, making it more polarizable and better able to donate an electron pair to a proton.

Step 1: (a) Explanation for Atomic Radius Increase

As we move down a group in the periodic table, the atomic radius generally increases. This is because there is an increase in the number of electron shells (principal quantum number, n) in each subsequent element. As n increases, the distance between the nucleus and the valence electrons also increases, leading to an increase in the atomic radius.

Step 2: (b) Explanation for Larger Ionic Radii

Ionic radii are larger than atomic radii for the anions of the elements in question. This is because when an element gains an electron to form an anion, there is an increase in electron-electron repulsion within the ion. This increased repulsion causes the electron cloud to expand and, as a result, the ionic radius to be larger than the atomic radius.

Step 3: (c) Strongest Base in Water

To determine which of the three anions will be the strongest base in water, we need to consider their ability to donate an electron pair to a proton (H+ ion). Anions that have larger radii are generally more polarizable and less tightly held to the nucleus. This means they have a more considerable ability to donate an electron pair to a proton, making them stronger bases. Considering that the atomic radius increases when moving down the group, we can expect the bottommost element to have the largest atomic and ionic radii, thus being the strongest base in water. In this case, considering elements in Group 6A, that would be the anion of the third element.

Key Concepts

Understanding Periodic Table Trends

Grasping the trends of the periodic table is fundamental in predicting the behaviors and properties of elements. As we move from left to right across a period, the atomic number increases, and electrons are added to the same energy level while the nucleus gains more protons, resulting in a greater nuclear charge. Consequently, electrons are pulled closer to the nucleus, generally decreasing the atomic radius.

In contrast, when we move down a group in the periodic table, additional electron shells are added which increases the distance between the nucleus and the outermost electrons. This increase in the number of energy levels, or shells, outweighs the increasing nuclear charge, which leads to an increase in atomic size. Therefore, the atomic radius increases down a group, leading to different reactivity and bonding characteristics among the elements.

Electron-Electron Repulsion and Its Effects

Electron-electron repulsion is a key concept that impacts the size of atoms and ions. Within an atom or ion, negatively charged electrons repel one another due to their like charges. This repulsive force becomes particularly noticeable when an atom gains additional electrons, transforming into an anion.

When an element becomes an anion, the number of electrons increases while the number of protons in the nucleus remains constant. The increased number of electrons leads to more significant repulsion among them, causing the electron cloud to expand. This expansion results in the ionic radius being larger than the atomic radius of the neutral atom. Understanding this concept is crucial for explaining variations in ionic sizes and overall chemistry among different elements and ions.

Base Strength in Water and Ionic Radii

The strength of a base in water is determined by its tendency to accept protons. In the case of anions, larger ions with greater ionic radii tend to be stronger bases. This is because the larger anions are more polarizable, meaning their electron cloud can be more easily distorted.

A large, polarizable anion has loosely held outer electrons that are more available to bond with protons, making them more reactive as a base. Therefore, in a group of anions, we anticipate that the anion with the largest ionic radius (typically the one lowest on the periodic table) will exhibit the strongest base characteristics. This is not only due to the size but also how loosely electrons are held, which increases the anion's ability to attract and react with hydrogen ions in water.