Question

Identify the true statements concerning the atoms and ions of the group 6 A elements. [Sections 22.5 and 22.6] (a) The ionic radii are larger than the atomic radii because the ions have more electrons than their corresponding atoms. (b) Atomic radii increase going down the group because of increasing nuclear charge.(c) The ionic radii increase going down the group because of the increase in the principal quantum number of outermost electrons. (d) Of these ions, Se \(^{2-}\) is the strongest base in water be- cause it is largest.

Short answer

In summary, the true statement concerning the atoms and ions of the Group 6A elements is: - Statement (c): The ionic radii increase going down the group because of the increase in the principal quantum number of outermost electrons.

Step-by-step solution

Statement (a) Analysis

The statement says that the ionic radii are larger than the atomic radii because the ions have more electrons than their corresponding atoms. This is partially true. In general, when a neutral atom gains an electron to become a negative ion, its radius increases because the additional electron increases the electron-electron repulsion, which causes the electrons to spread out further from the nucleus. However, the statement doesn't cover the whole truth, because it doesn't consider the role of effective nuclear charge in determining the atomic radius.

Statement (b) Analysis

The statement says that atomic radii increase going down the group because of increasing nuclear charge. This statement is false. Atomic radii actually increase going down a group because of the increasing number of electron shells, which outweighs the increase in nuclear charge. The extra electron shells cause the electrons to be further away from the nucleus, so the effective nuclear charge decreases, making the atomic radius larger as we go down the group.

Statement (c) Analysis

The statement says that the ionic radii increase going down the group because of the increase in the principal quantum number of outermost electrons. This statement is true. As we go down a group in the periodic table, the ionic radii increase due to the increasing principal quantum number of the outermost electrons. This is because more electron shells are being added as we go down the group, resulting in a larger distance between the nucleus and the outermost electrons.

Statement (d) Analysis

The statement says that, of these ions, Se \(^{2-}\) is the strongest base in water because it is the largest. This statement is false. While it is true that larger ions generally have a greater ability to accept a proton (i.e., act as a base), there is no specific mention of other ions of Group 6A elements in this statement. We cannot conclude whether Se \(^{2-}\) is the strongest base compared to other group 6 A ions based on size alone without knowing the specific ions in question. So, after analysis, the correct statements are: 1. Statement (c): The ionic radii increase going down the group because of the increase in the principal quantum number of outermost electrons.

Key concepts

Atomic Radii

When discussing the atomic radii of elements, we're referring to the distances from the center of an atom to the outer boundary of its electrons. Understanding how atomic radii change across the periodic table is crucial for predicting properties and behavior of elements.

In Group 6A elements, atomic radii increase as you move down the group. This can seem counterintuitive, as one might assume that more protons would pull electrons closer, reducing the size. However, with each step down the group, an additional electron shell is added, which increases the distance of the outermost electrons from the nucleus. Despite the increase of positive charge in the nucleus with every added element, the effect of additional shells is more significant than the increase in nuclear charge, resulting in larger atomic radii for elements down the group. This reaches beyond just Group 6A and is a trend observed across all groups in the periodic table.

Ionic Radii

Ionic radii measurements are similar to atomic radii but are concerned with ions rather than neutral atoms. An ion is an atom that has gained or lost one or more electrons, giving it a net charge. In the context of Group 6A elements, these typically become anions—they gain electrons.

The ionic radii of these anions are larger than their atomic counterparts because the additional electrons increase the electron-electron repulsion within the ion. This repulsion pushes electrons further apart, expanding the radius of the ion. However, it must be remembered that this isn't the only factor at play. The overall size of an ion also depends on the balance between the electron-electron repulsion and the attraction of electrons to the nucleus—known as the effective nuclear charge. An increased nuclear charge can draw electrons closer, while a decreased charge, due to additional electron shells, results in larger radii.

Electron-Electron Repulsion

Electron-electron repulsion is a significant factor in determining the size of both atoms and ions. This phenomenon is a consequence of the like-charge repulsion described by Coulomb's law, which states that like charges repel each other with a force that decreases with the increasing distance between them.

In Group 6A anions, electron-electron repulsion increases with additional electrons, which forces the electron cloud to expand and increase the radius of the ion. This concept helps to explain why, when an atom gains electrons to become a negatively charged ion, its size increases despite the nucleus' unchanged charge. It is the balance between the repulsion among the electrons and the attraction between electrons and the nucleus that ultimately governs the size of the ion.

Principal Quantum Number

The principal quantum number, denoted as 'n', is a fundamental concept in quantum mechanics and chemistry. It essentially indicates the relative size and energy of atomic and ionic orbitals. The number corresponds to the electron shells or energy levels of an atom. For instance, the principal quantum number of the first electron shell is 1, the second is 2, and so on.

In Group 6A elements, as mentioned earlier, moving down the group increases the principal quantum number because additional electron shells are being added. Each new shell falls into a higher principal quantum number. This accounts for the larger ionic and atomic radii observed down the group, because an increase in 'n' means electrons are located in orbitals farther from the nucleus. This is in harmony with the idea that the extension of these outer electrons due to higher energy levels and the greater distanced orbitals imparts a larger overall size to the atom or ion.