Question

The electron affinity of each group 5 A element is more positive than that of each corresponding group 4 A element. Use the outer electron configurations for these columns to suggest a reason for this observation.

Short answer

Group 4A elements have half-filled p orbitals and can more easily gain an additional electron, making the electron affinity more negative. In contrast, Group 5A elements have one more electron, leading to electron-electron repulsion when gaining another electron, thus a more positive electron affinity.

Step-by-step solution

Understanding Electron Affinity

Electron Affinity (EA) is the energy change that occurs when an electron is added to a gaseous atom. A more positive EA indicates that less energy is released when the atom gains an electron, suggesting it is less favorable for the atom to gain an electron.

Identifying Electron Configurations

Identify the outer electron configurations of group 4A and 5A elements. Group 4A elements have an outer electron configuration ending in p2 (e.g., C: 2s2 2p2). Group 5A elements have an outer electron configuration ending in p3 (e.g., N: 2s2 2p3).

Comparing Electron Configurations and Stability

Elements in Group 4A have one less electron in their p orbital than elements in Group 5A. This means that Group 4A elements can more easily accommodate an additional electron in the half-filled p orbital, making the process more exothermic (more negative EA) than for Group 5A elements, where adding an electron results in a paired p orbital situation, which is relatively less stable due to electron-electron repulsion.

Key concepts

Electron Affinity

Electron affinity (EA) is a measure of the energy released when an additional electron is added to an isolated atom in the gas phase. Essentially, it's a way to gauge how much an atom 'wants' an extra electron.

If an atom releases a large amount of energy when it gains an electron, it has a high (or less positive, more negative) electron affinity, indicating that it is energetically favorable for the atom to acquire that additional electron. Conversely, if an atom releases less energy, it has a lower (or more positive) electron affinity and is less inclined to accept an extra electron.

This property is crucial in predicting how atoms will interact, especially when forming chemical bonds, as atoms with higher electron affinities are more likely to attract electrons and form negative ions.

Outer Electron Configurations

Outer electron configurations refer to the distribution of electrons in the outermost shell of an atom, which can significantly influence chemical reactivity and properties. Group 4A elements, such as carbon, have the outer electron configuration ending in p2, while Group 5A elements, like nitrogen, have it ending in p3.

The outer configuration is particularly important because electrons in the outer shell are the ones involved in bonding and chemical reactions. The difference in the number and arrangement of these outer electrons can change an element's behavior when interacting with other substances, as they determine the element's ability to gain or lose electrons during chemical processes.

Group 4A Elements

Group 4A elements, also known as the carbon group, have four electrons in their outermost shell. The typical configuration for these elements is ns2 np2. This configuration is effectively one electron short of a half-filled p orbital, which is a relatively stable state.

As a result, these elements can easily accept another electron to reach a half-filled, and thereby more stable, configuration. This increases their electron affinity as the process is more energetically favorable, leading often to the release of more energy compared to atoms that already have a half-filled p orbital.

Group 5A Elements

The elements in Group 5A, like nitrogen and phosphorus, have outer electron configurations that end in p3. This configuration means that their p orbitals are exactly half-filled, providing a greater degree of stability due to the symmetrical distribution and minimized electron-electron repulsions.

When these elements gain an additional electron, it forces the pairing of electrons in one of the p orbitals, which is less favorable due to increased repulsion. Consequently, the electron affinity of Group 5A elements is less negative (more positive) than that of Group 4A elements because adding an electron disrupts the stable half-filled configuration.

Orbital Stability

Orbital stability plays a pivotal role in the behavior of atoms during chemical reactions. In the context of electron affinity, atoms gain stability when their outer orbitals achieve a half-filled or full-filled configuration. These are energetically favorable states due to the distribution of electrons minimizing repulsion.

Group 4A elements are one electron away from a half-filled p orbital, while Group 5A elements have a half-filled p orbital. Adding an electron to a half-filled p orbital in a Group 5A element results in less stability, which is why these elements have a more positive electron affinity compared to Group 4A elements. Understanding these subtle differences in orbital stability can help explain the trends in electron affinity and predict the relative reactivity of different elements.