By Ron B. Davis Jr., Georgetown University
Oxygen and nitrogen have many remarkable properties but their location on the periodic table also hints at yet another property of these two elements that makes them positively unique. Their powerful hunger for electrons leads to the ability of these elements to do something no other element can—to form what is known as a hydrogen bond.
What Is a Hydrogen Bond?
When fluorine, oxygen or nitrogen bonds to hydrogen, it forms a very polar covalent bond, drawing electrons toward itself, substantially creating what we call a polar covalent bond. This leaves the attached hydrogen with a partial positive charge that can interact favorably with partially negative fluorine, oxygen and nitrogen within another molecule.
Hydrogen bonds are the attraction between molecules that result from polar charges left over by covalent bonding—and these special bonds are crucial to the chemistry of our environment and even ourselves.
Hydrogen Bonds Hold Oceans Together
While molecular fluorine, oxygen and nitrogen have remarkably low boiling points, some very simple compounds of them, like hydrofluoric acid, water or ammonia, have much higher boiling points because of the hydrogen bonding that can occur between two or more molecules of the same kind.
This interaction makes water molecules attract one another much more strongly than one would otherwise predict. This is the reason it is possible for liquid water to exist here on Earth, opening the door for life to take hold at all. If you ever want to remember why hydrogen bonds matter, just remember that the entirety of the world’s oceans are held together by these simple but ubiquitous hydrogen bonds.
Hydrogen bonding also helps to explain why water expands when frozen, causing ice to float atop liquid water. As water freezes, its molecules arrange themselves in such a way that hydrogen bonding takes place, leaving some empty space within, making it less dense than its liquid.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Hydrogen Bonding in the Structure of DNA
The influence of hydrogen bonding on the chemistry around us isn’t limited to these two simple molecules: ammonia and water. The hydrogen bonds of oxygen and nitrogen also play indispensable roles in biochemistry, including the most famous example, the structure of DNA.
The famous double-helix structure that James Watson and Francis Crick proposed was two strands of DNA held together by networks of hydrogen bonds between base pairs. It is these hydrogen bonds that only allow particular bases to take up residence at particular locations within the double helix in so-called base pairs.
These Watson-Crick pairs, adenine with thymine and cytosine with guanine, create a strong interaction at the center of the double helix, holding it together. But try to create a different pair, and the hydrogen bonding partners do not align.
So, whether it is creating an environment that can support life, producing fertilizers to sustain that life, or even keeping the genetic code of life itself, oxygen and nitrogen are positively irreplaceable because of their hydrogen bonding abilities.
The Ability of Making Allotropes
Unlike the element from group 17, each atom of oxygen and nitrogen can form more than one chemical bond. In their most common diatomic form, all hydrogen bonds from one atom connect it to just one other discrete atom, making DI-atomic molecules with physical properties quite similar to fluorine.
But this is where the nonmetals get more interesting. Is it really necessary that all of the bonds to one oxygen or nitrogen atom must connect it to the same bonding partner? The answer to this question is ‘no’, and the resulting collection of bonding possibilities lead to a set of molecules called allotropes. Allotropes can be most simply explained as molecules of a given element containing different numbers or arrangements of atoms.
Oxygen’s most common allotrope in our world is by far diatomic oxygen. But oxygen has another naturally occurring allotrope in the form of ozone, a triatomic molecule of oxygen. This form of molecular oxygen is far less stable and only forms with an input of large amounts of energy.
Allotropes of Oxygen and Nitrogen
Ozone is best known for its tendency to form in our upper atmosphere, where it absorbs high-energy UV rays that might be harmful to organisms below. But, ozone is also an even more powerful oxidizing agent than diatomic oxygen, which makes ozone useful in disinfecting technologies like portable ozone generators, which turn diatomic oxygen from the air into ozone, which in turn damages or kills viruses and bacteria in the air and on surfaces.
Even more exotic allotropes of oxygen are possible, courtesy of its ability to make more than one bond. TETRA-oxygen, with four oxygen atoms, is one transient allotrope. Under immense pressures and high temperatures, oxygen can be forced to adopt an octa -oxygen allotrope, sometimes called ‘red-oxygen’.
In principle, nitrogen’s ability to make three hydrogen bonds might be exploited to produce a vast array of nitrogen allotropes. In practice, however, the remarkable stability of nitrogen’s diatomic form makes these allotropes difficult or even impossible to produce. But, that hasn’t stopped many chemical researchers from designing interesting and complex molecules using computer modeling techniques for study.
Common Questions about the Ability of Nitrogen and Oxygen to Make Hydrogen Bonds
The attraction between molecules that result from polar charges left over by covalent bonding are called hydrogen bonds. These bonds play an important role in chemistry in and around us. To understand the strength of hydrogen bonds, one should imagine how water molecules are attracted to one another making up all the seas and oceans of our planet.
The double-helix structure was proposed by Francis Crick and James Watson. This structure consists of two strands of DNA attached to one another by hydrogen bonds network between base pairs. These networks of hydrogen bonds keep the pair bases in their location within the double-helix, not allowing them to create different pairs.
Among the properties of oxygen and nitrogen, a unique one is their ability to create hydrogen bonds. This characteristic alone can positively make these two elements irreplaceable, allowing them create a life-friendly environment, keep the genetic code of life, and produce fertilizers.