Question

For an oxygen atom, which requires more energy, the addition of two electrons or the removal of one electron?

Short answer

The removal of one electron from an oxygen atom requires less energy than the addition of two electrons.

Step-by-step solution

Understand the nature of an oxygen atom

An oxygen atom (O) has 6 electrons in its outermost shell. Achieving a full outer shell typically requires the addition of 2 electrons, which would give oxygen a stable 'noble gas' electronic configuration similar to neon (Ne).

Understand electron affinity and ionization energy

Electron affinity refers to the energy change when an electron is added to a neutral atom, which is usually exothermic (releases energy) for nonmetals like oxygen. Ionization energy is the energy required to remove an electron from a neutral atom. The first ionization energy is always less than the second due to increased effective nuclear charge on the remaining electrons.

Consider the energy change for adding electrons to an oxygen atom

Adding the first electron to oxygen is exothermic and releases energy. Adding the second electron to the negatively charged oxide ion (O-) is endothermic and requires more energy than adding to the neutral atom due to electrostatic repulsion.

Consider the energy change for removing electrons from an oxygen atom

Removing the first electron from an oxygen atom requires a considerable amount of energy because electrons are being removed from a stable configuration.

Compare the energy changes

Considering both processes, the removal of an electron (which requires overcoming the first ionization energy) generally requires more energy than the addition of the first electron (which may release energy). However, if we are to compare adding two electrons (which involves overcoming electron-electron repulsion in the second addition) to removing one, removing one electron is typically less energetically costly than adding two to a nonmetal such as oxygen.

Key concepts

Oxygen Atom Electron Configuration

Understanding the electron configuration of an oxygen atom is crucial to conceptualizing its chemical behavior. Oxygen, with the atomic number 8, has 6 electrons in its outer shell and follows the electron configuration of 1s² 2s² 2p⁴. This arrangement means that oxygen is two electrons short of having a filled valence shell, which is the most stable state according to the octet rule.

For oxygen to reach the stable configuration similar to the noble gas neon, it would desire to gain two electrons yielding an oxide ion with the configuration of 1s² 2s² 2p⁶. This electron gain process is driven by the atom's electronegativity and its tendency to form stable molecules or ions.

First Ionization Energy

The first ionization energy is a pivotal concept to comprehend. It represents the amount of energy needed to remove the most loosely bound electron from a neutral atom in its gaseous state. For an oxygen atom, this entails ejecting an electron from its p orbital, which requires considerable energy due to the strong attraction between the nucleus and the valence electrons.

In a stepwise process, each subsequent ionization energy is higher than the previous one, because as you remove each electron, the electron-to-proton ratio decreases, causing the electrons to be held tighter by the nucleus. This increased effective nuclear charge makes it harder to strip away electrons, which in turn raises the ionization energy.

Electron-Electron Repulsion

The concept of electron-electron repulsion is rooted in the principle that like charges repel. In the context of atomic structure, this implies that electrons in the same orbital, or in close proximity within the atom, will experience a force that pushes them apart. This repulsion is significant when considering the energy required to add an electron to an already negatively charged ion.

When adding the first electron to a neutral oxygen atom, this process is often exothermic because the electron is attracted to the positively charged nucleus. However, adding a second electron to the negatively charged oxide ion (O^-) requires more energy. That's because this second electron will feel a repulsion from the negatively charged ion, which is the phenomenon of electron-electron repulsion making the process less favorable energetically.