Group 5A: Pnicogens
The word pnicogen (also said pnictogen) comes from Greek words pnigien and genes, translating to “choking producer.” The pnicogens consist of nitrogen, phosphorus, arsenic, antimony, and bismuth. These elements have some uniform properties within the group, but there is still great diversity in their properties such as their ability to have various oxidation states varying from -3 to +5.
History and Discovery of the elements
Antimony (Sb) was the first of the pnicogens to be isolated. Antimony (III) sulfide was used as a cosmetic by ancients to darken and beautify eyebrows. Antimony was also used in early times as a vomit inducer, a bell metal additive to make a silver sound, and a type metal alloy.
The word Arsenic (As) comes from the Greek word Arsenikon which is in some way an adaptation of a Persian word for “yellow orpiment” (a common sulfide ore of arsenic). Although the first isolation of arsenic is not known, Albert Magnus is credited for its discovery. Magnus was a German scholar who wrote clear descriptions of this element around 1250. Early reduction of arsenic (III) sulfide to free arsenic involved heating it with eggshells followed by charcoal. Arsenic has a poisonous nature that has been know for centuries. Some Chinese has been poisoned by drinking beverages from tin containers. Arsenic occurs often with tin and measures can be taken to separate the two. Intentional poisonings have also occurred.
Hennig Brandt, a German alchemist and physician isolated Phosphorus (P) in 1669. Brandt was investigating a way to turn silver into gold and chose to study human urine. He boiled down 60 pails of urine and put the produced paste in water. If the paste was removed, it burst into flames. Phosphorus was named after a German word meaning “light-bearer.” Later Phosphorus was found to be in human bones. Now, Phosphorus is produced by heating phosphate rock with sand and coke.
2Ca3(PO4 + 10C + 6SiO2 -> 6CaSiO3 + 10CO(g) P4(s)
The word bismuth (Bi) may have been derived from a German word for white metal. Bismuth was used as a secret alloy in the Gutenberg printing press by 1450. Alchemists believed bismuth was an early stage of silver and that it would transform over time. Claude-Francois Geoffrey is often credited for the discovery of bismuth because of his thorough investigation of the element. Bismuth was used to produce a deeper sound in bells and to increase the hardness and brilliancy of tin. This element is used today in safety devices for fire detection and sprinkler systems.
Nitrogen (N) compounds were known long before the element was first isolated. Ammonium chloride was characterized in the fifth century. Nitric acid was used to separate gold and silver in the sixteenth century. Potassium nitrate and sodium nitrate have been used as both fertilizer and in gunpowder. Daniel Rutherford discovered air left over after the removal of carbon dioxide from a space. Rutherford called this “phlogisticated air.” Nitrogen, meaning “nitron producer” was named after it was found to be a main component of nitric acid. Nitrogen is produced by liquefying common air and fractionally distilling it.
Nitrogen is much like the other elements in its group, but it has many properties that make it unique. Nitrogen is much more like its congeners than carbon is like the other group 4A elements. Nitrogen is about the same strength catenator as phosphorus is, much like carbon is about the same as silicon. Nitrogen-nitrogen bonds are much weaker than carbon-carbon bonds due to lone pair repulsion. Nitrogen differs from phosphorus with oxidation state. Nitrogen has a high ability to form pi bonds a lack of availability of a 3d orbital. Nitrogen’s ability for form pi bonds is shown in the stability of the diatomic N2. The triple bond is responsible for N2 making up 80 percent of the atmosphere, the difficult process of fixing nitrogen in other compounds, and the very exothermic reactions that result when N2 is produced. The ability of Nitrogen to form triple bonds results in a variety of nitrogen compounds that don’t correspond to any compounds with heavier pnicogens. These compounds include catenated chains involving alternating single and double N-N bonds, oxygen compounds involving N=O bonds, cyanides, thiocyanates, and sulfer and phosphorus nitrides involving nitrogen double bonds. The chemistry of the heavier pnicogens is based on single bonds and the availability of d orbitals. Phosphorus particularly forms pi double bonds with oxygen, nitrogen, and sulfur. Availability of d orbitals also helps to make expanded octets. Phosphorus, arsenic, and antimony display +3 and +5 oxidation states. Due to the inert pair effect, bismuth mainly displays the +3 oxidation state. The acidic properties of the pnicogens are that nitrogen and phosphorus are acidic, arsenic and antimony are amphoteric, and bismuth is basic.
Hydrides, Oxides, and Halides
NH3 is the most stable of the hydrides. Hydrogen bonding can occur between nitrogen and hydrogen. A fourth hydrogen can be added to the compound with more stability than analogs with other congeners. PH3 is somewhat stable and like NH3 is pyramidal. But P-H bonds are nonpolar and no hydrogen bonding can occur. PH3 reacts with oxygen in the air immediately and ignites. AsH3 is much less stable and when heated decomposes to metallic arsenic. SbH3 and BiH3 are even less stable that lighter hydrides. Nitrogen forms various oxides with oxidation states varying from +1 to +5. Among these are Nitric acid and other compounds involving both single and double N-O bonds. Phosphorous oxides are P2O3 and P2O5. The oxidation states of phosphorus are +3 and +5. The naturally occurring forms of the oxides are P4O6 and P4O10. When water is added to the oxides phosphorous and phosphoric acid are produced respectively.
P4O6(g) + 6H2O(l) -> 4H3PO3
P4O10(g) + 6H2O(l) -> 4H3PO4
Arsenic and Antimony are similar to phosphorous and have +5 oxidation state oxides. Although it doesn’t occur naturally when burned in air, antimony can be oxidized to the +5 oxide. Bismuth on the other hand will not for the +5 oxide. The acidity of pnictogens decreases down the column. This is shown by the basic nature of bismuth in its +3 oxide. Nitrogen, as expected, forms only pyramidal trihalides due to the lack of available d orbital to expand its octet. All pnicogens form the possible trihalides, but only phosphorus can form pentahalides with all the halogens. This shows the decreasing stability of the +5 oxidation state.
Oxidation States of Nitrogen
Nitrogen displays all nine oxidation states ranging from -3 to +5. Phosphorus displays -3, 0, +3, +4, and +5. Arsenic and antimony display -3, 0, +3, and +5. Bismuth is the most limited with only 0, +3, and +5 oxidation states. N3- occurs majorly in two compounds, nitrides and ammonia. Hydrazine is an N2- compound. This compound is used as a reducing agent and is also used as rocket fuel. The N1- oxidation state is the least stable of all nine possible oxidation states. Hydroxylamine is a colorless, thermally unstable, hygroscopic white solid. N1+ exists in dinitrogen oxide, also known as nitrous oxide. This gas is known more famously as laughing gas and was used as the first modern anesthetic. N2+ is found in nitric oxide. Nitric oxide has an odd number of electrons. It readily loses an electron and forms a triple bond much like CO. Dinitrogen trioxide and nitrous acid contain N3+. Nitrites with the 3+ oxidation state are used as meat preservatives. Nitrogen dioxide has N4+ and like nitric oxide has one electron in a lone pair position. This causes the bond angle of the O-N-O to increase from 120O to 134.1O. Since NO2 has one unpaired electron, it readily dimerizes with itself and forms N2O4. The highest oxidation state of nitrogen is N5+. Dinitrogen pentoxide and nitric acid contain this oxidation state of nitrogen.
Reactions and Uses
Three ways to fix nitrogen in compounds are by lightning, bacteria, and humans. Lightning provides enough energy to fix nitrogen to the oxygen in the air. Some bacteria in plants are able to fix nitrogen and deposit the compounds in the soil. These plant-bacteria combinations are used to fertilize the soil in rotation with other crops. Humans have found many ways to fix nitrogen in compounds. One way is executed by blowing air through an electric arc duplicating the effect of lightning. The main way of nitrogen fixation is the Haber process. Nitrogen and hydrogen are combined to directly produce ammonia gas. Once the ammonia is produced it can be converted into many other liquid of solid compounds.
Nitrates, Nitrites, and Phosphates
Nitrates are strong oxidizing agents and are used as explosives. Gunpowder was the first use of nitrates as explosives in potassium nitrate. Nitric acid is used to form trinitrotoluene and nitroglycerine. Along with the destructive uses of nitrates and nitrites, they can also be used as food preservatives. Phosphates have been used as fertilizers for 150 years. Simple phosphates have been outdated by the use of ammonium phosphates which provide both nitrogen and phosphorus to the soil. Phosphates can also be used in processing food such as in the leavening of bread. Phosphates are also used in detergents and as surface cleaners to prevent metal corrosion.
Rodgers, Glen E. Descriptive Inorganic, Coordination, and Solid-State Chemistry. Canada: Thomas Learning, 2002.