Groups 5A – 8A

Group 5A Nitrogen and phosphorus are non-metals, arsenic is a semimetal, and antimony and bismuth tend to be metallic, typically forming ions with a +3 charge. The oxides of nitrogen and phosphorus are acidic when dissolved in water. Arsenic and antimony oxides are amphoteric, and bismuth oxide is basic.

Group 5A The elements of group 5A may form three, five or six covalent bonds, except for nitrogen which cannot expand its “octet.” Due to its small size, nitrogen readily forms π bonds. Thus elemental nitrogen, N2, has a triple bond. The other elements exist as larger molecules containing single bonds.

Nitrogen Elemental nitrogen is an extremely stable molecule due to the triple bond. As a result, many nitrogen containing compounds decompose exothermically (and sometimes explosively) to form nitrogen gas.

Explosives Nitrogen based explosives such as nitroglycerin, will rapidly decompose when ignited or exposed to a sudden impact.

Explosives C3H5(NO3)3(l)  6 N2(g) + 12CO2(g) + 12H2O(g) + O2(g) + energy Note the large number of moles of gaseous products. Explosives typically involve a very large volume change, producing many moles of small gaseous molecules.

Explosives Trinitrotoluene, TNT, is another nitrogen based explosive. 2C7H5(NO3)3(l)  12 CO2(g) + 5 H2(g) + 3N2(g) + 2C(s) + energy

Sodium Azide Sodium azide, NaN3(s), is used in air bags in automobiles. A small amount of sodium azide (100g) yields 56L of nitrogen gas at 25oC and 1 atm.

Sodium Azide NaN3(s)  2Na(l) + 3 H2(g) This reaction takes place in about 40ms. Other components are put in the air bag so that the molten sodium metal is deactivated into glassy silicates. 10 Na(l) +2KNO3(s) K2O(s) +5Na2O(s)+ N2(g) 2 K2O(s) + SiO2(s)  K4SiO4(s) 2 Na2O(s) + SiO2(s)  Na4SiO4(s)

Nitrogen Fixation Bacteria (in the nodules on the roots of pea plants) can readily convert nitrogen to ammonia at 1 atm and soil temperature. The Haber process requires a temperature of 400oC and a pressure of 250 atm.

Uses of Nitrogen Compounds Ammonia, synthesized from nitrogen and hydrogen using the Haber process, is used to make fertilizers, explosives and nitric acid. Dinitrogen oxide, N2O, is used as “laughing gas”, a dental anesthetic. The gas is highly soluble in fats, and its major use is as a propellant in cans of whipped cream.

Uses of Nitrogen Compounds Nitrogen monoxide, NO, is a biologically active molecule, and is crucial in controlling blood pressure and the dilation of blood vessels.

Environmental Issues Nitrogen dioxide (NO2) and dinitrogen tetra-oxide (N2O4) are in equilibrium with each other: N2O4(g) ↔ 2 NO2(g) colorless red-brown The oxides of nitrogen are the result of high temperature combustion in jet engines and automobiles.

Environmental Issues The compounds cause the reddish brown smog seen over Los Angeles and other cities on a sunny day. They also react with moisture in the air to produce nitric acid and nitrous acid. This “acid rain” is a respiratory irritant, and destroys facades of buildings and statuary.

Isolation of Phosphorus Phosphorus was initially isolated in an attempt to extract gold from urine. The element emits light and glows when exposed to oxygen.

Phosphorus Elemental phosphorus exists as several allotropes. All differ greatly in structure from nitrogen due to a lack of multiple bonding between the larger phosphorus atoms. Phosphorous can also use d orbitals to expand its bonding.

Elemental Phosphorus white phosphorus black phosphorus red phosphorus

Elemental Phosphorus

White Phosphorus White phosphorus exists as discrete P4 molecules. It is a waxy white solid that is very poisonous and reactive. It burns vigorously in air, and is stored under water.

White Phosphorus The element gets its name from the phosphorescent glow emitted by white phosphorus when it is exposed to air in the dark. White phosphorus has been used in weaponry. The pieces of phosphorus in bombs and grenades get embedded in the skin, where they burn.

Red Phosphorus Red Phosphorus is a polymeric chain of P4 units. It is stable in air to a temperature of 400oC. Red phosphorus is used in “safety” matches on the striking surface.

Red Phosphorus Red phosphorus is used in “safety” matches on the striking surface.

Black Phosphorus Black phosphorus is the most stable of the allotropes. It is formed from white phosphorus that is heated under very high pressures.

Source of Phosphorus Phosphorus must be extracted from its compounds, as it is not found in elemental form. The main source is phosphate rock, or calcium phosphate. Large deposits are found in central Florida, Morocco and the Pacific island of Nauru. The extraction process consumes large amounts of electricity and coke, and produces large amounts of dusts, flue gases and sludge.

Uses of Phosphorus Compounds Phosphates are used in fertilizers, and the salt of tripolyphosphoric acid is used in detergents that won’t form insoluble “scum” in hard water. Phosphoric acid is added to soft drinks where it prevents bacterial growth and also forms inert compounds with any metal ions that may be leached from the container walls.

Biological Aspects - Nitrogen All plant life requires nitrogen for growth and survival. Bacteria found in nodules on the roots of pea, bean, alder and clover plants convert nitrogen in the air to nitrogen compounds.

Biological Aspects - Phosphorus Phosphorus is essential for life. The hydrogen phosphate ion and dihydrogen phosphate ions are involved in buffering blood. Phosphate units link the sugar esters of DNA and RNA, and also make up part of ATP, the energy storage unit in living things.

Biological Aspects - Arsenic Arsenic, though generally considered toxic, is also essential to life. We only need trace amounts, and its role is still unknown. In the 19th century, before the discovery of antibiotics, arsenic was used as one of the first forms of chemotherapy to destroy the organism that causes syphilis.

The group 6A Elements Group 6A, oxygen, sulfur, selenium , tellurium and polonium, are sometimes called the chalcogens. O, S and Se are nonmetals, Te a semimetal, and polonium the only metal of the group. Except for oxygen, the other members of the group exhibit all even-numbered oxidation states from -2 through zero to +6.

Oxygen Oxygen contains a double bond that is much stronger than a single bond (494 kJ/mol vs. 142 kJ/mol). The lower elements in the group form much weaker π bonds due to their larger atomic size and greater bond length.

Oxygen Oxygen is a colorless, odorless gas that forms a pale blue liquid. The molecule is paramagnetic due to the presence of two unpaired electrons, and is attracted to a magnetic field.

Ozone Ozone, O3, is an allotrope of oxygen. It occurs naturally in the upper atmosphere of earth. The ozone layer absorbs ultraviolet light and serves to help screen out harmful, cancer causing, radiation.

Sulfur Sulfur is found in large deposits as the free element, or in a variety of ores. Elemental sulfur has a variety of forms and structures. At room temperature, the most stable form is rhombic sulfur, S8 rings.

Sulfur If molten sulfur is cooled slowly, the eight- membered rings stack into monoclinic sulfur, which has a needle-like appearance.

The Frasch Process The Frasch process is used to extract sulfur from underground deposits. Superheated water (under pressure) is pumped into the ground where the sulfur melts. The molten sulfur is then pumped up to the surface and into huge forms where it cools and solidifies. Explosives are used to break the huge chunks of sulfur into more useable sizes.

The Frasch Process

The Frasch Process

Sulfur Oxides Sulfur dioxide and sulfur trioxide are formed during the combustion of sulfur-containing fuels (coal). They are respiratory irritants, and, when combined with moisture in the air, produce acid rain.

Selenium Until the 1960s, the only major use of selenium was the addition of CdSe to glass to produce a ruby-red color. Cadmium selenide is also used as a semiconductor in photocells, because its conductivity varies with light intensity.

Selenium Selenium is now used in “xerox” machines and laser printers. They use a drum coated with selenium that is exposed to an electric field. The regions on the drum that are exposed to high light intensity lose their charge. Toner powder adheres only to the charged areas of the drum which correspond to the printed areas on the page.

Photocopiers

The Halogens – Group 7A

Group 7A Group 7A is the first group that consists of all non-metals. Fluorine, chlorine, bromine and iodine all exist as diatomic molecules at room temperature. Astatine, At, is unstable, and very rare. The elements illustrate a variety of group trends.

Group 7A

Group 7A The halogens are typically found in ionic form. The elements are prepared by electrolysis of salts (for F2 and Cl2) or using redox reactions.

Fluorine Fluorine is the most electronegative element, and often acts as an oxidizing agent. Fluorine exhibits oxidation numbers of zero and -1 only. The other halogens have positive oxidation numbers when bonded to fluorine or oxygen.

Fluorine Fluorine has only one isotope, 19F. F2(g) is used to separate isotopes of uranium. Uranium exists as two isotopes, 238U and 235U. Uranium-235 is used in bombs and nuclear reactors. Both isotopes of uranium are found in deposits as UO2(s).

UO2(s) + 4 HF(g)  UF4(s) + 2 H2O(g) Fluorine The uranium (IV) oxide is converted to the fluoride using HF(g). UO2(s) + 4 HF(g)  UF4(s) + 2 H2O(g)

Fluorine UF4(s) + 2 F2 (g) UF6(g) The ionic UF4 is reacted with fluorine gas to form covalent (and gaseous at 60oC) UF6. UF4(s) + 2 F2 (g) UF6(g) The gaseous mixture of 235UF6 and 238UF6 can be separated based on their different rates of effusion.

Hydrofluoric Acid HF(aq), is a weak acid, whereas the binary acids of the other halogens are all strong acids. HF has another unique property. It can be used to etch glass, and must be stored in plastic bottles.

SiO2(s) +6 HF(aq) SiF62-(aq) + 2 H+(aq) + Hydrofluoric Acid SiO2(s) +6 HF(aq) SiF62-(aq) + 2 H+(aq) + 2 H2O(l)

Fluoridation of Water The enamel of teeth contains hydroxyapatite, Ca5(PO4)3(OH). The compound is somewhat soluble in the acidic media of the mouth. If the hydoxide ion is replaced with fluoride, the less soluble and harder fluorapetite, Ca5(PO4)3(F) is formed. The harder enamel resists decay.

Chlorine Chlorine is a pale green poisonous gas, and was used as the first poison gas in warfare. In small doses, chlorine gas effectively kills harmful bacteria in drinking and swimming water.

Chlorine Chlorine is a pale green poisonous gas, and was used as the first poison gas in warfare. In small doses, chlorine gas effectively kills harmful bacteria in drinking and swimming water.

Bromine Bromine is a volatile, oily, orange-brown corrosive liquid at room temperature. Silver bromide, AgBr(s), is used in black and white photography. Upon exposure to light, the silver ion forms fine crystals of metallic silver.

Iodine Iodine is a dark purple metallic looking solid. In the human body, it is found primarily in the thyroid gland where if is used in the synthesis of the hormones thyroxine and triiodothyronine. These hormones are essential for growth, the regulation of neuromuscular functions, and the maintenance of reproductive functions.

Group 8A – The Noble Gases

Group 8A – The Noble Gases It was believed that the noble gases, He, Ne, Ar, Kr, Xe and Rn, were inert, and never formed compounds. During the past 50 years, several compounds of Kr, Ze and Rn have been synthesized.

The Discovery of the Noble Gases It had been known since at least 1785 that air contained something in addition to oxygen and nitrogen. The new elements was called argon, from the Greek word for “lazy.” Helium was identified in the emission spectrum of the sun (1868) before it was isolated on earth from uranium ores.

Group Trends All elements are odorless, colorless monatomic gases at room temperature. The boiling points are consistent with weak london dispersion forces and the increase in polarizability going down the group.

Group 8A – The Noble Gases

Uses of the Noble Gases Helium, due to its low density and chemical inertness, is used to provide lift for baloons and blimps. Helium is also used in breathing mixtures for deep sea diving. It is less soluble in the blood than nitrogen, and thus minimizes “the bends.”

Uses of Noble Gases Argon is chiefly used to provide an inert atmosphere for chemical reactions and in welding. Along with Ne, Kr and Xe, argon also produces characteristic colors in “neon” lights. Argon is also used between panes of glass in thermal insulating windows.

Compounds of Xe and Kr The first compound of a noble gas, XePtF6(s), was synthesized in the early 1960s using a very strong oxidizing agent, PtF6(g). Since then, several compounds containing Xe or Kr with fluorine and oxygen have been synthesized.

Radon Radon is radioactive, and is formed in rocks and soil by the radioactive decay of uranium and thorium. 222Rn can infiltrate the air in basements. Since it is gaseous, it is breathed into the lungs where it undergoes radioactive decay, bombarding lung tissue with alpha and beta particles. These particles disrupt living cells and can initiate lung cancer.