Hydrogen Bonds — Overview & Examples

What Are Hydrogen Bonds?

They are a form of dipole-dipole interaction between polar molecules that contain hydrogen atoms. The hydrogen atoms must be bonded to either nitrogen, oxygen, or flourine. The bond itself happens when these molecules are attracted to these same atoms in other molecules.

Polar molecules get their partial charges, or dipoles, from the uneven sharing of the electrons in the bonds between atoms. These electrons stick closer to atoms that have higher electronegativity, or need for electrons. Therefore, how polar or ionic a bond is depends on the difference between the electronegativities of the two elements involved.

The highest electronegativities on the periodic table are found in the top right (red), with an electronegativity of 3.98 for fluorine ($F$), 3.44 for Oxygen ($O$), and 3.04 for nitrogen ($N$). These atoms are more electronegative than hydrogen ($H$). This means the electrons will move to the stronger elements like fluorine, oxygen, and nitrogen, creating a partial negative charge. The hydrogen side will have a partial positive charge.

When multiple molecules containing hydrogen bound to $N$, $O$, or $F$ are near each other, the more positive $H$ will stick to the more negative atom of another molecule. This interaction is called a hydrogen bond or H-bonding. It is important to note that the bond directly between $N$, $O$, or $F$ and hydrogen ($H$) in the same molecule is not a hydrogen bond. H-bonding is not a true bond between atoms but a strong interaction between atoms on other molecules.

The most classic example of hydrogen bonds is in water, $H_{2}O$. H-bonding occurs in three places on one molecule, which connects it to three other water molecules. This leads to a lot of interactions between water molecules and is referred to as cohesion.

In this picture of the crystal structure of ice, H-bonding is shown with gray dotted lines. The $O$ of each molecule forms the negative dipole of the molecule, while the $H$s are the positive dipoles.

Hydrogen bonds are what holds the separate molecules in ice packed in a solid structure. They also keep the molecules in liquid water close together. In steam, the water molecules are too far apart to do H-bonding.

They are much stronger than the other types of intermolecular bonds. This results in unique properties of water necessary for biology.

What Is a Hydrogen Bond?

It is an interaction between the positive pole of one polar molecule and the negative pole of another.

These molecules must contain nitrogen ($N$), oxygen ($O$), or fluorine ($F$) atoms, and of course hydrogen ($H$). Recall that polar molecules have two ends (poles): one that has a slight positive charge ($δ+$) (hydrogen side) and one that has a slight negative charge ($δ-$) ($N$, $O$, or $F$). Water is a good example of a polar molecule. Let's take a look at its poles:

The electron pairs or lone pairs (electrons not bonding) are attracted to the oxygen atom of the water molecule due to oxygen's electronegativity. This word just means it has an affinity (liking) for negative charges.

We mentioned that hydrogen bonds form between the positive pole of one polar molecule and the negative pole of another. The positive pole involved will always be a hydrogen atom.

The positive pole (hydrogen) of one molecule and the negative pole ($N$, $O$, or $F$) of another molecule are attracted to each other through the simple "opposites attract" idea.

Even though we've used two water molecules to illustrate hydrogen bonding above, remember that the negative pole involved in the hydrogen bond can come from any polar molecule. What we mean is that it doesn't have to be water. This type of bonding also plays an important role in the structure of DNA. Hydrogen bonds form between the nucleic acids of DNA, holding together both sides of the double helix.

Key Vocabulary:

1. Polar
2. Cohesion
3. Adhesion
4. Surface tension
5. Capillary action
6. Specific heat
7. Density

(Video) Hydrogen Bonds

by Whats Up Dude

Hydrogen bonds or H-bonds are relatively weak compared to ionic and covalent bonds, but they can make a significant impact if there are many of them. When comparing H-bonds to other interactions between molecules, they are actually pretty strong.

Water is a polar molecule and can make H-bonds with different molecules or other water molecules. Non-polar molecules, which do not have charge differences in the molecule, will not mix with polar molecules. This is why water (polar) and oil (non-polar) will not mix, no matter how much you try.

The video above by Whats Up Dude describes how hydrogen bonds form.

How Do Hydrogen Bonds Form?

Hydrogen bonds are an attraction between an electronegative atom and a proton (hydrogen) atom. Hydrogen bonds are weaker than both ionic and covalent bonds, but they are stronger than other weak forces between molecules such as Van der Waals.

Hydrogen bonds occur because certain elements such as oxygen, nitrogen, and fluorine are more electronegative than other elements. These atoms are greedy and hog electron density, making them more negative. Hydrogen atoms, which have a positive charge, are then attracted to these electronegative atoms and form weak bonds.

Hydrogen bonds can occur between two different molecules (intermolecular) or between two atoms in the same molecule (intramolecular).

Hydrogen bonds are responsible for some very important concepts in biology.

• High specific heat of water: Water's ability to hold heat is due to hydrogen bonds. This is good because it allows for a stable world temperature. The Earth can't heat up or cool down entirely overnight, and we get a more stable temperature range.

• Protein folding: Hydrogen bonds are important in the formation of proteins. As the proteins are made, they have to twist and fold into specific shapes to be able to perform their functions. Hydrogen bonds are largely responsible for the way that the proteins fold. Without them, proteins would not be able to work correctly.