The doorbell is a funny thing. It's always there, but you never think about it until something calls your attention to it - usually, the bell. Nitrogen is much the same. After all, it's all around you. Roughly 78% of the air we breathe is nitrogen. However omnipresent it may be, it's colorless, odorless, tasteless, and easy to ignore. If you have found yourself in need of some reminding, do read on.
Nitrogen (chemical symbol N) is the seventh element on the periodic table, with (amazingly) the atomic number seven. It always has seven protons, and (in its neutral state) seven electrons as well. The typical nitrogen atom has 7 neutrons as well, making it the standard N-14 isotope with an atomic mass of 14.01, or 14.00674 if you want to get picky and look it up in the 82nd CRC handbook, like yours truly just did. (Most of the more detailed numerical facts to follow also come from the CRC - I don't know them all by heart you know.) The other naturally occurring isotope is N-15, with 8 neutrons. Only about 0.37% of nitrogen atoms have that extra neutron - lucky devils.
The nitrogen you breathe is nitrogen in its elemental form - a diatomic gas. Two nitrogen atoms are bound together by a triple bond - three pairs of electrons shared covalently between the atoms. If someone asks - the hybridization of each nitrogen atom in this situation is "sp". If they didn't ask, and you don't care about electrons, skip to the next paragraph. A nitrogen atom has the electron configuration 1s2, 2s2, 2p3, so it has 5 valence electrons in its outer "shell". When it hybridizes to the sp configuration, two electrons get stuck in one of the sp orbitals, and stay there as a lone pair (unbonded). The remaining sp orbital and the two p orbitals get one electron each, and these are the ones that pair up with the adjoining atom to form three bonds. The sp orbitals overlap end-on to make a sigma bond, and the two p-orbital pairs align in parallel, making two pi bonds.
With three bonds, nitrogen gas is a very stable molecule. Under most circumstances, it is inert, or unreactive. This makes it very useful in certain laboratory environments, as it can be used as a "blanket" to keep more reactive gasses away from unstable compounds. Other properties of nitrogen are fairly well known. It becomes a liquid once it reaches a temperature of -195.79 degrees Celsius. (At higher pressures, this can be achieved at warmer temperatures.) While not as common, solid nitrogen can also be made - at the chilly temperature of -210 degrees Celsius. Nitrogen is somewhat soluble in water, and therefore also in blood. This solubility increases as pressure increases, leading to the unfortunate circumstances divers know as "the bends". Deep underwater, pressure is increased, so a diver whose air supply contains nitrogen will dissolve more nitrogen into their blood than normal. If they rise quickly from the depths, the pressure decreases too rapidly for the gas to be released through the lungs, as is normal, and the extra nitrogen then forms bubbles in the bloodstream, disrupting blood flow quite painfully, and sometimes even fatally. (This is why divers often carry oxygen/helium blends rather than compressed air.)
Although we have mentioned that nitrogen gas is fairly unreactive, the nitrogen atom itself is quite reactive. After all, it has three unpaired electrons, and just like the historic father with three daughters, it wants to get them paired off quickly. It will take on oxidation states all the way from +5 down through -3 in order to achieve this goal. Because of this, nitrogen appears in many compounds. Several examples follow.
Ammonia is a common and simple nitrogen compound. Made of a central nitrogen atom with three hydrogen atoms attached, it is commonly found in household cleaners. Dissolved in water, it forms "ammonium hydroxide", and behaves as a basic solution. (It will turn litmus blue, and cheerfully eat away at your flesh - so be safe.)
Perhaps the most popular nitrogen compounds are the cyanides. Everyone knows that cyanide is a poison. The cyanide ion is simply a carbon and a nitrogen triply bound together, with an extra electron. It can pair up with a hydrogen ion (hydrogen cyanide, a toxic gas), a metal ion (sodium cyanide, for instance, which dissolves in water and is also toxic), or any positive ion (ammonium cyanide, anyone?).
The amines are a class of organic compounds that have nitrogen in them. The nitrogen is attached to carbon and hydrogen by single bonds, making it similar to ammonia in bonding structure. In smell though, amines tend to be more along the lines of fish oil. (Yech!)
As gross as amines may be, they are an integral part of life. The amino acid is a relatively small molecule with an amine on one end. By itself, it doesn't do much, but amino acids can make a chain, attaching the amine at the end of one to the carboxylic acid at the opposite end of the next. A chain of amino acids (also called peptides) is called a protein (or a polypeptide). Realizing that all life that we know of utilizes proteins, this in turn means that all life relies on nitrogen.
Our genetic codes also contain a good deal of nitrogen. DNA and RNA contain 5 "nitrogen bases", which pair up to encrypt all of our traits. Adenine, cytosine, guanine, thymine and uracil are all amines. You can probably even catch the hint in their names with the -ine endings. (All, that is, except for uracil. Somebody obviously didn't get the memo when it was time to name that one.)
That was nitrogen in a nutshell. With luck, you've learned what you came to learn, discovered that nitrogen is truly fascinating, and can go back to ignoring it with a good conscience - until someone brings it to your attention again.