4.9.1. Cause of Formation of Hydrogen Bond

When hydrogen is bonded to a strongly electronegative element ‘X’, the electron pair shared between the two atoms moves far away from hydrogen atom. As a result, the hydrogen atom becomes highly electropositive with respect to the other atom ‘X’. Since there is displacement of electrons towards X, the hydrogen acquires fractional positive charge (δ +) while ‘X’ attains fractional negative charge (δ–).

In this part, we explain how the formation of a hydrogen bond begins with the bonding of hydrogen to a highly electronegative atom, such as fluorine (F), oxygen (O), or nitrogen (N). When hydrogen bonds with one of these electronegative elements, it shares its electron with that element. However, because the electronegative element attracts the shared electrons much more strongly, the electrons spend more time near 'X' than near the hydrogen atom. This creates a situation where the hydrogen atom has a partial positive charge (represented as δ +), while the electronegative atom becomes partially negatively charged (δ–). This unequal sharing of electrons is what characterizes a polar bond.

Think of the relationship between a dog and its owner. Imagine the dog (hydrogen) trying to pull a toy away from its owner (the electronegative atom). The owner, being stronger, manages to keep the toy closer to themselves, but the dog still holds onto it. As a result, the dog feels a bit 'left out' (similar to how hydrogen feels left out when the shared electrons are closer to the electronegative atom). In a similar way, hydrogen ends up with a positive charge because the shared electrons are closer to the electronegative atom.

This results in the formation of a polar molecule having electrostatic force of attraction which can be represented as: Hδ+ – Xδ– –– Hδ+ – Xδ– –– Hδ+ – Xδ–.

Here, we discuss the outcome of the displacement of electrons and the creation of partial charges. The polar nature of the bond between hydrogen and the electronegative atom leads to the formation of what is known as a hydrogen bond. In this context, the hydrogen bond is the attractive force that arises due to the positive charge of hydrogen (δ +) being attracted to the negative charge of electronegative atoms (δ–) in adjacent molecules. The hydrogen bond can thus be depicted with the illustrated notation showing the positive and negative charges, indicating that there is attraction between the molecules.

Imagine two friends standing close to each other, where one friend has a shiny, attractive object (the electronegative element) that draws the attention of the other friend (the hydrogen). Just like how the friend with the attractive object pulls the other friend closer due to their interest, the electrostatic force due to the positive and negative charges pulls the hydrogen and electronegative atoms together, hence forming a bond that is not as strong as a covalent bond but still significant in strength.

The magnitude of H-bonding depends on the physical state of the compound. It is maximum in the solid state and minimum in the gaseous state. Thus, the hydrogen bonds have a strong influence on the structure and properties of the compounds.

This final segment details how hydrogen bonds behave differently based on the state of matter. In solids, hydrogen bonds are strong and contribute significantly to the rigidity and structure of the substance, while in gases, these interactions become weaker due to increased molecular spacing and movement, resulting in fewer or weaker hydrogen bonds. Consequently, the strength and number of hydrogen bonds play a critical role in determining properties such as melting and boiling points, solubility, and molecular stability.

Consider ice versus steam. In ice (solid state), water molecules are held together tightly by hydrogen bonds, making it a solid crystal structure. Conversely, in steam (gaseous state), the water molecules are far apart and can move freely, resulting in negligible hydrogen bonding. Just as ice feels solid and water vapor feels light and dispersed in the air, the differences in hydrogen bonding affect the state and properties of water significantly.

--