Hydrogen Bonding

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In this lesson we will explore another type of chemical bonding, hydrogen bonding. Specifically, we will answer the following questions: What are hydrogen bonds, how do they form, and why are they important? In other lessons, we explored ionic and covalent bonds. Those bonds allow atoms to join together to form molecules and compounds, so they are bonds that occur within a molecule or compound. Intra means within, so bonds that occur within a molecule or compound are called intramolecular bonds. For example, polar covalent bonds between two hydrogen atoms and an oxygen atom form a molecule of water. The polar covalent bonds hold the molecule together. But hydrogen bonds provide another option. Hydrogen bonds not only can occur within a single molecule as intramolecular bonds, as seen here, but they can also occur between different molecules. Inter means between, so bonds between different molecules are called intermolecular bonds. A hydrogen bond is a weak bond that occurs only when there are polar covalent bonds in the area. Recall that in polar covalent bonds, electrons are shared unequally between the atoms. A classic example of this is in water. A water molecule has two hydrogen atoms and one oxygen atom. Hydrogen is tiny, usually just a single electron and a proton. Oxygen is much bigger, though, and has eight positive protons. Each hydrogen atom binds to oxygen by sharing its lone valence electron. But its electron is more strongly attracted to the oxygen and spends more time hanging out with the oxygen than it does with its own nucleus. This causes a slight imbalance in the electrical charge across the molecule. Oxygen takes on a slight negative charge because of its greater claim to the shared electrons, and the hydrogen atoms take on slight positive charges because of this imbalance. The molecule is said to be polar. These slight charges set the stage for hydrogen bonds. When multiple water molecules are in the same area, the slightly positive hydrogen atoms are attracted to nearby and slightly negative oxygen atoms from other water molecules, and a weak bond forms between them. This is the hydrogen bond. Hydrogen bonds don't just form in water, though, they can form any time hydrogen is present in polar molecules. In truth, a hydrogen bond is not really a true chemical bond. It's more of an electrical attraction. The strength of a hydrogen bond varies depending on to whom the hydrogen is bonding, which in our cells is typically oxygen or nitrogen. But in general, hydrogen bonds are weaker than either covalent or ionic bonds. They are also transient. That means that they form, release, and reform with that or another atom. This is somewhat like being at a crowded party and meeting many people. You shake a whole lot of hands, but rarely any one for a very long period. Although they are weak, hydrogen bonds are essential for our existence. For example, water is loaded with hydrogen bonds that give it many unique characteristics, and our body fluids are largely water. Physiology involves chemical reactions, and chemical reactions in our bodies occur in water. Water's hydrogen bonds, for example, help ionic compounds, such as this salt crystal, dissociate, or separate, in our body fluids, making ions, such as sodium and chloride, available for our cells to use. Hydrogen bonds also determine the shapes of many molecules in our bodies, and their shapes, in turn, control how they function. Proteins, for example, are shaped in part by their covalent bonds, shown here, but also by their hydrogen bonds, shown here. Most chemical reactions in our bodies are assisted by enzymes, which are proteins, that must have very precisely shaped binding sites in order to attach the chemicals they need to so that the reactions can occur. Antibodies also have specific shapes, to allow them to bind to and rid your body of disease-causing agents, such as bacteria. The hydrogen bonds in the proteins in you hair help determine if your hair is straight or if, instead, it's curly. As a final example, hydrogen bonds also hold together your DNA molecules. The DNA contains all of your genetic information and that makes you who you are.