When do hydrogen bonds form between molecules

The donor in a hydrogen bond is usually a strongly electronegative atom such as N, O, or F that is covalently bonded to a hydrogen bond. The hydrogen acceptor is an electronegative atom of a neighboring molecule or ion that contains a lone pair that participates in the hydrogen bond. Since the hydrogen donor N, O, or F is strongly electronegative, it pulls the covalently bonded electron pair closer to its nucleus, and away from the hydrogen atom.

The hydrogen atom is then left with a partial positive charge, creating a dipole-dipole attraction between the hydrogen atom bonded to the donor and the lone electron pair of the acceptor. This results in a hydrogen bond.

Although hydrogen bonds are well-known as a type of IMF, these bonds can also occur within a single molecule, between two identical molecules, or between two dissimilar molecules. Intramolecular hydrogen bonds are those which occur within one single molecule. This occurs when two functional groups of a molecule can form hydrogen bonds with each other.

In order for this to happen, both a hydrogen donor a hydrogen acceptor must be present within one molecule, and they must be within close proximity of each other in the molecule. For example, intramolecular hydrogen bonding occurs in ethylene glycol C 2 H 4 OH 2 between its two hydroxyl groups due to the molecular geometry.

Intermolecular hydrogen bonds occur between separate molecules in a substance. They can occur between any number of like or unlike molecules as long as hydrogen donors and acceptors are present in positions where they can interact with one another.

When we consider the boiling points of molecules, we usually expect molecules with larger molar masses to have higher normal boiling points than molecules with smaller molar masses. This, without taking hydrogen bonds into account, is due to greater dispersion forces see Interactions Between Nonpolar Molecules. Larger molecules have more space for electron distribution and thus more possibilities for an instantaneous dipole moment. However, when we consider the table below, we see that this is not always the case.

We see that H 2 O, HF, and NH 3 each have higher boiling points than the same compound formed between hydrogen and the next element moving down its respective group, indicating that the former have greater intermolecular forces. The same effect that is seen on boiling point as a result of hydrogen bonding can also be observed in the viscosity of certain substances.

Substances capable of forming hydrogen bonds tend to have a higher viscosity than those that do not for hydrogen bonds.

Generally, substances that have the possibility for multiple hydrogen bonds exhibit even higher viscosities. Hydrogen bonding cannot occur without significant electronegativity differences between hydrogen and the atom it is bonded to. Thus, we see molecules such as PH 3 , which no not partake in hydrogen bonding. PH 3 exhibits a trigonal pyramidal molecular geometry like that of ammonia, but unlike NH 3 it cannot hydrogen bond. This is due to the similarity in the electronegativities of phosphorous and hydrogen.

Both atoms have an electronegativity of 2. This prevents the hydrogen bonding from acquiring the partial positive charge needed to hydrogen bond with the lone electron pair in another molecule. The size of donors and acceptors can also effect the ability to hydrogen bond. This can account for the relatively low ability of Cl to form hydrogen bonds. When the radii of two atoms differ greatly or are large, their nuclei cannot achieve close proximity when they interact, resulting in a weak interaction.

Hydrogen bonding plays a crucial role in many biological processes and can account for many natural phenomena such as the Unusual properties of Water. In addition to being present in water, hydrogen bonding is also important in the water transport system of plants, secondary and tertiary protein structure, and DNA base pairing.

The cohesion-adhesion theory of transport in vascular plants uses hydrogen bonding to explain many key components of water movement through the plant's xylem and other vessels. Within a vessel, water molecules hydrogen bond not only to each other, but also to the cellulose chain which comprises the wall of plant cells. Since the vessel is relatively small, the attraction of the water to the cellulose wall creates a sort of capillary tube that allows for capillary action.

This mechanism allows plants to pull water up into their roots. Furthermore, hydrogen bonding can create a long chain of water molecules which can overcome the force of gravity and travel up to the high altitudes of leaves. Hydrogen bonding is present abundantly in the secondary structure of proteins , and also sparingly in tertiary conformation. The secondary structure of a protein involves interactions mainly hydrogen bonds between neighboring polypeptide backbones which contain Nitrogen-Hydrogen bonded pairs and oxygen atoms.

Since both N and O are strongly electronegative, the hydrogen atoms bonded to nitrogen in one polypeptide backbone can hydrogen bond to the oxygen atoms in another chain and visa-versa. Though they are relatively weak, these bonds offer substantial stability to secondary protein structure because they repeat many times and work collectively.

In tertiary protein structure, interactions are primarily between functional R groups of a polypeptide chain; one such interaction is called a hydrophobic interaction.

A European Space Agency astronaut Pedro Duque of Spain watches a water bubble float between him and the camera, showing his image refracted, on the International Space Station. B A large water sphere made on a 5 cm diameter wire loop by U. Weird Science. Adhesion is similar to cohesion, but it involves unlike i. Water is very adhesive ; it sticks well to a variety of different substances.

Water sticks to other things for the same reason it sticks to itself — because it is polar so it is attracted to substances that have charges. Water adheres to many things— it sticks to plants, it sticks to dishes, and it sticks to your eyebrows when you sweat. In each of these cases water adheres to or wets something because of adhesion.

This is why your hair stays wet after you shower. Molecules of water are actually sticking to your hair Fig. Adhesion also explains why soil is able to hold water and form mud. Investigate the cohesive and adhesive properties of water.

The cohesion of water creates surface tension where air and water meet. You observed this in Activity 2 when you looked at the ability of water to pile on top of a penny without spilling over see Fig.

The hydrogen bonds between water molecules at the surface are analogous to the to members of a red rover team holding hands. When playing red rover, team members line up to form a chain to try and prevent someone from running through their joined hands Fig.

The linked hands represent the hydrogen bonds between water molecules that can prevent an object from breaking through. Of course, a faster or heavier person can more easily break through the hand bonds during a game of red rover. Where air and liquids meet there are unbalanced forces. Water molecules very near the surface are being pulled down and to the side by the strong cohesion of water to itself and the strong adhesion of water to the surface it is touching.

The result is a net force of attraction between water molecules a very flat, thin sheet of molecules at the surface see Fig. Because of hydrogen bonding, water can actually support objects that are more dense than it is.

Water molecules stick to one another on the surface, which prevents the objects resting on the surface from sinking. It is also what allowed you to float a paper clip on water and the reason why a belly flop off the high dive into a pool of water is painful. In Activity 2, you tried to stick two rulers together using a thin film of water between the rulers. Water acted like glue, and you were able to use one ruler to lift the other ruler using the adhesiveness of water see Fig.

This particular resource used the following sources:. Skip to main content. Liquids and Solids. Search for:. Hydrogen Bonding. Learning Objective Describe the properties of hydrogen bonding. Key Points Hydrogen bonds are strong intermolecular forces created when a hydrogen atom bonded to an electronegative atom approaches a nearby electronegative atom.

Greater electronegativity of the hydrogen bond acceptor will lead to an increase in hydrogen-bond strength. The hydrogen bond is one of the strongest intermolecular attractions, but weaker than a covalent or an ionic bond. Hydrogen bonds are responsible for holding together DNA, proteins, and other macromolecules.