Download Group 15: The Nitrogen Family

1 Group 15: The Nitrogen Family
Group 15: The Nitrogen Family
Contents
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The Nitrogen Family
Nitrogen
Phosphorus
Others
The Nitrogen Family
Group 15 (VA) contains nitrogen, phosphorous, arsenic, antimony, and bismuth.
Elements in Group 15 have five valence electrons. Because the elements can
either gain three electrons or lose five to gain a stable configuration, they
more often form covalent compounds unless bonded to an active metal. Their
electron affinities are not very large. Metallic properties increase markedly
from gaseous nitrogen to barely-metallic bismuth with increasing size and
mass. Nitrogen and phosphorus are non-metallic, and arsenic and antimony are
metalloids.
Elements of this group are known as pnicogens and their compounds as pniconides. The name is derived
from the Greek word pnicomigs meaning suffocation.
The Group 15 elements are unique enough to be studied individually, rather than as a group.
These elements are much less reactive than the elements of Group 16, and their chemistries are more
complicated. Most of the chemistry of these elements is in the +3 or +5 oxidation states, although they form
gaseous compounds with hydrogen in the -3 oxidation state: ammonia NH3, phosphine PH3, arsine AsH3,
stibine AsH3, and bismuthine BiH3; these all burn in oxygen to give oxides or the free element (in the case of
nitrogen).
All form oxides—nitrogen with difficulty, the others with ease. Most of the oxides are acidic, exceptions being
nitrous oxide N2O, nitric oxide NO, and bismuth oxide Bi2O3. Except for nitrogen, typical oxides are in the +3 or
+5 (bismuth excluded) oxidation states. All form halides—nitrogen with difficulty, but those of phosphorus,
arsenic, and antimony fully hydrolyze in water.
Nitrogen and phosphorus form important acids in the +5 oxidation state. Nitrogen forms nitric acid HNO3, a
substance used to create medicines and explosives (but this acid is corrosive and dangerous, so don't touch it
or even spill it on anything), and salts known as nitrates such as potassium nitrate KNO3, an important
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fertilizer. Phosphorus forms phosphoric acid, H3PO4; phosphates are salts of phosphoric acid. Some
phosphates are essential to respiration and thus life itself.
Nitrogen
The nitrogen cycle shows how nitrogen is passed along organisms and the atmosphere.
Nitrogen occurs naturally as the diatomic gas N2. It composes about 70% of the air we breathe. The bond
holding the two nitrogen atoms together is triple covalent, so it is very strong. Because of that, nitrogen is very
unreactive. It is used in many places when an inert gas is needed. However, nitrogen will react with some
substances:
Although nitrogen gas is usually considered
inert, it does react with some elements by
burning.
The nitrides react violently with water to
form ammonia gas and a basic solution.
In its pure form, nitrogen is not very useful. It is much more important when it is a component of ammonia,
nitrate, oxides, or biomolecules like protein. Due it is very unreactive nature, it is difficult to get nitrogen to react
and form these useful substances. Any process that can convert elemental nitrogen into a nitrogen compound
is called nitrogen fixation. Nitrogen fixation is biologically important because amino acids, proteins, and
enzymes contain nitrogen. It is commercially important because it is used in explosives, rocket fuels, and
fertilizers.
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There are many nitrogen fixation reactions:
This occurs in bacterial enzymes. The ammonia
(NH3) quickly becomes ammonium (NH4+). The
nitrogen in the bacteria enters the soil where plants
can absorb it. Humans and animals that eat those
plants can get the nitrogen.
The Haber process is used for commercially
producing ammonia. This reaction only occurs at
very high pressures and temperatures (around 20
MPa and 500 °C) and in the presence of an iron
catalyst. Also, the reaction occurs in somewhat
complex equipment that must input pure reactants
and extract the ammonia.
Keep in mind that ammonia is a gas at STP. The household product called "ammonia" is actually an aqueous
solution of ammonium hydroxide (NH4OH) that forms when ammonia gas is dissolved in water. Ammonia, quite
unlike hydrogen compounds of Groups 16 and 17, is a base in its reactions, forming salts with weak and strong
acids alike. Such a substance as ammonium chloride (NH4Cl) is a soluble, strongly ionic salt.
Nitrogen compounds are often extremely unstable because nitrogen atoms in nitrogen compounds tend to seek
each other to recombine as nitrogen gas. Many nitrogen compounds are literal explosives, including TNT and
nitroglycerin. These explosives are in common use in construction projects for the demolition of buildings and
other obstacles to new construction, or to get access to minerals in mining operations.
Handling of any explosive or making them is appropriately left to experts.
Phosphorus
White phosphorus atomic structure
Phosphorus has two common allotropes: red phosphorus and white phosphorus. White phosphorus (P4) has a
waxy appearance and turns yellow when exposed to light. When exposed to oxygen in the dark, it glows pale
green.
White phosphorus is extremely dangerous. It causes severe burns and is very toxic. It ignites with a very
hot flame and can be explosive. Leave all experiments with white phosphorus to experienced chemists
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White phosphorus ignites under all but the most delicate conditions. The combustion of white phosphorus
produces phosphorus(V) oxide:
One of its most common uses is in military weapons that cause severe burning of the object hit by the weapon.
Red phosphorus is an amorphous solid. It is more stable and explodes at temperatures higher than those of
white phosphorus. It is still, however, dangerously reactive. Both forms of phosphorus are insoluble in water
and can be interconverted with various applications of heat, pressure, and light.
There also exist black phosphorus and violet phosphorus. Unlike nitrogen, phosphorus will not readily form a
diatomic molecule with a triple bond. Diphosphorus does exist, but only between a temperature range of 1200
°C and 2000 °C.
Phosphorus is essential to life in the form of phosphates in bones and in substances known as ADP and ATP
that transform food into useful energy in cells.
Others
A crystal of bismuth, showing its colorfuliridescent tarnish.
Arsenic is similar to phosphorus. It has three allotropes: grey arsenic, yellow arsenic, and black arsenic. Grey
arsenic is the most common form. Its structure is similar to graphite.
Antimony has the physical properties of a metal, but behaves chemically as a non-metal.
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Arsenic and antimony, as well as practically all of their compounds, are dangerous poisons.
Bismuth is a brittle, silvery metal. Bismuth is actually radioactive, decaying into thallium-205. Because its halflife is 19 x 1018 years, about a million times the age of the universe, bismuth is usually considered stable.
Bismuth is much less radioactive than the nearly-harmless and unavoidable radioactive isotopes of carbon and
potassium in living things. Unlike arsenic and antimony, its compounds aren't toxic unless something else in the
compound is itself toxic or the substance is very acidic or alkaline. In fact, a bismuth compound is very common
in a heavily-used stomach medication that requires no prescription.