Metals, Metalloids, and Nonmetals

Nitrogen and Phosphorus

Nitrogen and phosphorus are elements from group 15 of the periodic table. Nitrogen is an essential element for living things on Earth because it is crucial for both plant and animal biological function, and phosphorus is very important in industry.

In nature, pure nitrogen is in the form of a diatomic molecule (N2). Nitrogen is abundant in the atmosphere, forming about 78% of it. The nitrogen-nitrogen bond in an N2 molecule is an extremely strong triple bond. This bond cannot be easily broken. This causes nitrogen to be an inert molecule in most situations. Nitrogen has five valence electrons. Nitrogen shows a variety of oxidation states in compounds, ranging from +5 to –3. The most common oxidation states are +5, 0, and –3. Nitrogen is highly electronegative but is less electronegative than oxygen and fluorine. Nitrogen commonly takes on positive oxidation states when in a compound with oxygen or fluorine and negative oxidation states when in a compound with other elements.

The low reactivity of N2 gas makes it important in many industries. N2 is obtained from the atmosphere by separating N2 from air.

Living organisms need a variety of nitrogen compounds. Plants are an important part of the nitrogen cycle, as animals can obtain nitrogen by eating plants. Plants cannot obtain nitrogen from air, due to the high strength of the nitrogen-nitrogen bond. In nature, plants rely on bacteria to break nitrogen down. The availability of non-N2 nitrogen in nature is a major factor limiting plant growth. The process of breaking down N2 into more usable nitrogen compounds is called nitrogen fixation. Nitrogen fixation is used to convert nitrogen in the air to molecules such as ammonia that can be used by plants.

Nitrogen-based artificial fertilizers can be used to overcome this limitation. Nitrogen-based artificial fertilizers use the Haber-Bosch process which breaks down the nitrogen-nitrogen bonds in N2 and converts nitrogen into ammonia (NH3). This process relies on the following reaction:
N2(g)+3H2(g)2NH3(g){\rm N}_2(g)+3{\rm H}_2(g)\rightleftharpoons2{\rm{NH}}_3(g)
This is an equilibrium reaction that heavily favors N2 in normal conditions. Under high pressure and specific catalysts, the equilibrium still favors N2 but becomes more balanced, and some NH3 can be recovered. The ammonia can then be used for various nitrogen compounds, including artificial fertilizers. The Haber-Bosch process also is the biggest consumer of commercial hydrogen gas.
The Haber-Bosch process produces ammonia (NH3) form nitrogen (N2), which is typically nonreactive due to its strong triple bonds, and hydrogen (H2) gases. This process uses a catalyst, high temperature, and high pressure to make ammonia, which was very difficult to make on an industrial scale before the process was developed.
Like nitrogen, phosphorus has five valence electrons and can take oxidation states between +5 and –3. Phosphorus is less electronegative than nitrogen. Phosphorus often takes positive oxidation states in compounds.

Pure phosphorus has multiple allotropes. White phosphorus is molecular, with molecules consisting of four phosphorus atoms tetrahedral in shape. White phosphorus is unstable and very reactive; it spontaneously combusts when it comes in contact with air. Red phosphorus is an amorphous solid and is much less reactive. Black phosphorus is a crystal and is more stable than either white or red phosphorus.

Phosphorus is extracted from minerals that have phosphates. The most common phosphate found in nature is calcium phosphate (Ca3(PO4)2). Phosphorus halides such as PCl3 and PCl5 are the most important commercial phosphorus compounds, used in lubrication oils, paints, pesticides, and flame retardants. These halides are obtained by reacting phosphorus with diatomic molecules of elements of group 17.
White and black phosphorus are allotropes of phosphorus. Phosphate ion is a commercially important form of phosphate.