Question

Oxygen and selenium are members of Group \(6 \mathrm{~A}(16) .\) Water forms \(\mathrm{H}\) bonds, but \(\mathrm{H}_{2}\) Se does not. Explain.

Short answer

Water forms hydrogen bonds due to oxygen's high electronegativity; \(\text{H}_{2} \text{Se}\) does not because selenium's electronegativity is lower.

Step-by-step solution

Identify the elements and their groups

Oxygen and selenium are both members of Group 16 in the periodic table. This group also includes sulfur, tellurium, and polonium.

Understand hydrogen bonding

Hydrogen bonding occurs under two main conditions: (1) a hydrogen atom is covalently bonded to a highly electronegative atom, typically nitrogen, oxygen, or fluorine; (2) there is a lone pair of electrons on the electronegative atom.

Compare electronegativities

The electronegativity of oxygen is 3.44, which is significantly higher than that of selenium, which is 2.55. This high electronegativity in oxygen creates a stronger partial negative charge when it is bonded to hydrogen.

Explain the presence of hydrogen bonding in water

Because oxygen is highly electronegative, when it forms a bond with hydrogen, the hydrogen atom becomes partial positively charged while the oxygen becomes partial negatively charged. This allows for the formation of hydrogen bonds between water molecules.

Explain the absence of hydrogen bonding in \(\text{H}_{2} \text{Se}\)

Selenium is less electronegative than oxygen, which means the bond between hydrogen and selenium will be less polar. Therefore, hydrogen bonds do not form in \(\text{H}_{2} \text{Se}\) because the intermolecular attraction is not strong enough.

Key concepts

Electronegativity

Electronegativity is a measure of how strongly an atom can attract electrons in a chemical bond. Elements with high electronegativity values tend to pull shared electrons closer to themselves, creating partial negative charges. Oxygen, for example, has an electronegativity of 3.44, which is quite high. On the other hand, selenium has an electronegativity of 2.55, making it less effective at attracting electrons in a bond.

This difference in electronegativity plays a critical role in chemical properties and interactions. When you compare oxygen and selenium, the higher electronegativity of oxygen means it can create stronger partial charges and, subsequently, stronger intermolecular forces like hydrogen bonds.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole attraction between molecules. For hydrogen bonding to occur, two main conditions must be met:

• A hydrogen atom must be covalently bonded to a highly electronegative atom, such as nitrogen (N), oxygen (O), or fluorine (F).
• There must be a lone pair of electrons on the electronegative atom.

In the case of water (H₂O), oxygen is highly electronegative. When it bonds with hydrogen, it creates a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atom. This allows water molecules to attract each other through hydrogen bonds.

However, in H₂Se (hydrogen selenide), selenium's lower electronegativity means the partial charges are weaker. Thus, the intermolecular attractions are not strong enough to form hydrogen bonds.

Intermolecular Forces

Intermolecular forces are the attractions between molecules that govern physical properties like boiling points and solubility. These forces are generally weaker than the bonds within a molecule but are crucial for understanding how substances behave. Some of the key types of intermolecular forces include:

• Van der Waals (dispersion) forces: Weakest and present in all molecules.
• Dipole-Dipole interactions: Occur between molecules that have permanent dipoles.
• Hydrogen bonds: Strongest type of dipole-dipole interaction, occurring when hydrogen is bonded to N, O, or F.

Because water can form hydrogen bonds due to its highly polar nature, it has higher boiling and melting points compared to H₂Se. In contrast, H₂Se lacks significant hydrogen bonding, leading to weaker intermolecular forces and lower boiling and melting points.

Group 16 Elements

Group 16 elements in the periodic table include oxygen, sulfur, selenium, tellurium, and polonium. These elements are often referred to as the chalcogens. They share some common properties but also have distinct differences. Here are a few points about Group 16 elements:

• They have six valence electrons, which affects their chemical reactivity and bonding behavior.
• Oxygen is the most electronegative and is essential for life, playing a crucial role in respiration and water formation.
• Sulfur is less electronegative than oxygen but can still form strong bonds, especially in compounds like H₂SO₄.
• Selenium, while chemically similar to sulfur and oxygen, has lower electronegativity and does not form hydrogen bonds as effectively.

Understanding these elements' properties and behaviors helps explain why water is unique in its ability to hydrogen bond, whereas compounds like H₂Se do not exhibit this property.