Question

The electron affinity for sulfur is more exothermic than that for oxygen. How do you account for this?

Short answer

The more exothermic electron affinity in sulfur compared to oxygen can be attributed to two factors: the shielding effect of inner electrons and the larger size of the 3p orbitals in sulfur. Due to these factors, sulfur experiences weaker electrostatic attractions, making it easier to add an electron to its outermost shell and releasing more energy than oxygen during the process.

Step-by-step solution

Define Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a neutral atom in the gaseous state, forming a negative ion. A more negative, or more exothermic, electron affinity indicates that the atom releases more energy when gaining an electron.

Compare Electron Configurations

Let's look at the electron configurations for oxygen and sulfur. Oxygen (atomic number 8): 1s² 2s² ₂p⁴ Sulfur (atomic number 16): 1s² 2s² ₂p⁶ 3s² ₃p⁴ Oxygen is in Period 2 and Group 16, whereas sulfur is in Period 3 and Group 16 in the periodic table. Both elements have the same outermost p orbitals, which are 2p orbitals for oxygen and 3p orbitals for sulfur.

Explain Shielding Effect

As we move down a group (from Period 2 to Period 3 in this case), the atomic number increases and additional electron orbitals are added. These inner shells are filled with electrons which "shield" or protect the outer electrons from the attraction of the nucleus. This shielding effect makes it easier for an electron to be added to the outermost shell as the repulsion from the inner electrons can counteract the electrostatic attraction from the nucleus.

Understand Orbital Size

In the case of oxygen and sulfur, the 3p orbitals in sulfur are larger than the 2p orbitals in oxygen. This means that electrons in the 3p orbitals are farther away from the nucleus, and the electrostatic attraction is weaker. As a result, it is easier to add an electron to the sulfur atom than the oxygen atom.

Combine the reasons

The shielding effect of the inner electrons and the larger size of the 3p orbitals in sulfur make it easier to add an electron to the atom. Consequently, sulfur has a more exothermic electron affinity than oxygen. This is because more energy is released when an electron is added to sulfur's 3p orbitals than when an electron is added to oxygen's 2p orbitals due to weaker electrostatic attractions in sulfur.