(VA)15 Group - The nitrogen family includes the following

compounds: Nitrogen (N), Phosphorus (P), Arsenic (Ar), Antimony (Sb), and Bismuth (Bi).

- All Group 15 elements have the electron configuration ns2np3 in their outer shell.

- There is a transition from non-metallic to metallic character as we go down the group. Nitrogen and phosphorus are non-metals; arsenic and antimony are semi-metals; bismuth is metallic. Besides nitrogen, the other important element of group 15 is phosphorus.

Occurrence - The elements of Group 15, except phosphorus do not occur very abundantly in nature.

- Nitrogen is the most abundant component of the Earth's atmosphere (78.1 %). It is not very abundant in earth’s crust.

- Nitrogen is an important constituent of proteins and amino acids.

- Phosphorus is the eleventh element in the order of abundance in crustal rocks of the earth.

- Phosphorous is not found pure in nature, but in the form of apatite ores. These include compounds such as fluorapatite (Ca5(PO4)3F), which in fluoridated water is used to strengthen teeth, and hydroxylapatite (Ca10(OH)2(PO4)6), a major component of tooth enamel and bone material.

- The elements As, Sb and Bi are not very abundant. Their important source is as sulphides occurring as traces in other ores.

Selected physical properties are given in the following table

Oxidation States

- The common oxidation states are +3, +5, –3.

- The tendency to show –3 oxidation state decreases down the group due to decrease in electronegativity which is due to increase in atomic size.

- Nitrogen is a colorless diatomic gas. - The dinitrogen molecule (N2) has a nitrogen-nitrogen triple bond of unusual stability which is responsible for the low reactivity of this molecule (not totally inert). - Nitrogen is therefore suitable as an inert atmosphere for many chemical reactions that are either oxygen or moisture sensitive. - Liquid nitrogen, is used for studying low temperature reactions, trapping of solvent vapors.

- The stability of +3 state increases down the group whereas that of +5 state decreases due to INERT-PAIR effect.

Nitrogen

- In 1999, species, N5+, was reported, not stable at room

temperature but can be preserved for weeks at -78 °C.

- The N5+ ion has a V-shaped structure, bent at the central

nitrogen and linear at the neighboring atoms.

Nitrogen Chemistry 1) Hydrides:

- Ammonia (NH3), the most important hydride of nitrogen, is made by the Haber process, which uses finely divided iron as catalyst (500 0C and pressures of 250 atm):

N2 + 3 H2 ↔ 2 NH3

- In addition to ammonia, nitrogen forms the hydrides :

- N2H4 (hydrazine);

- N2H2 (diazene or diimide); and

- HN3 (hydrazoic acid).

- Ammonia, is used : - In fertilizers; - Synthesis of explosives; - Synthetic fibers such as nylon; - Synthesis of many organic and inorganic compounds.

- Oxidation of hydrazine is highly exothermic, so hydrazine is used as a rocket fuel:

- Nitrous oxide, N2O:

Is commonly used as a mild dental anesthetic, environmentally acceptable .

- Nitric oxide, NO:

Is has many biological functions.

- In the atmosphere, NO is oxidized to NO2. These gases, make the problem of acid rain, because NO2 reacts with atmospheric water to form nitric acid:

3NO2 + H2O → 2HNO3 + NO

- The gases N2O4 and NO2:

At ordinary temperatures and pressures, both exist in equilibrium:

Oxyionsand Oxides ) 2

Preperation:

- First, ammonia is reacted with oxygen using a platinum-rhodium catalyst to form nitric oxide, NO:

Nitrogen Oxoacids ): 3acid (HNO) Nitric 1

- Industrially important compound. - It undergoes Autoionization:

- The nitric oxide is then oxidized by air and water:

2) Nitrous acid (HNO2):

- Can be obtained in solution by acidifying solutions of nitrites:

- HNO2 is unstable and has not been obtained pure:

Phosphorous

- Phosphorus exists as tetrahedral P4 molecules in the liquid and gas phases. At very high temperatures, P4 can dissociate into P2:

- Other forms:

1) white phosphorus: - Exists in two forms ɑ-P4 (cubic) and β-P4 (hexagonal). - Condensation of phosphorus from the gas or liquid phases (tetrahedral P4 molecules) gives primarily the ɑ-form, which slowly converts to the β-form at temperatures above -76.9C.

- There are three basic allotropes and many intermediate forms.

- Can be stored under water to slow its oxidation, because it inflames in air to give phosphorus pentoxide (P4O10):

+ 5O2

2/ Red phosphorus:

- Much less toxic.

- Obtained by heating of white P in the absence of air.

3/ Black phosphorus :

- The most stable form.

- Obtained from white phosphorus by heating at very high pressures.

- Converts to other forms at higher pressures.

Phosphine, PH3: - Is a highly poisonous gas. - Its melting and boiling points are much lower than

those of ammonia.

Phosphoric acid (H3PO4):

- Two methods are commonly used.

1/ The first is the combustion of molten phosphorus to give P4O10 which can be converted into H3PO4 by reaction with steam:

2/ The second; is the reaction of phosphate salts with sulfuric acid. For example:

- Arsenic have a variety of allotropes.

The most stable allotrope of arsenic is the:

- Gray alpha (α-form), which is similar to the rhombohedral form of phosphorous.

- In the vapor phase, arsenic, like phosphoms, a tetrahedral As4.

- Antimony and bismuth also have similar α-forms.

- These three elements have a somewhat metallic appearance and moderately good conductors:

Arsenic, for example, is the best conductor in this group.

Bismuth is the heaviest element to have a stable, nonradioactive nucleus .

Group 16 (VI) - The Oxygen Family, also called the Chalcogens, consists of the elements found in Group 16 of the periodic table and is considered part of the Main Group elements. It consists of the elements Oxygen (O), Sulfur (S), Selenium (Se), Tellurium (Te) and Polonium (Po).

- All group elements are solids except oxygen.

- Oxygen, Sulfur, and Selenium = nonmetals. - Tellurium = metalloid. - Polonium = metal.

- All elements of the Oxygen family have 6 electrons in their outermost shell, with a valence electron configuration of ns2np4.

- Oxygen and Sulfur are essential for life. - Selenium conducts electricity when exposed to light.

- Polonium is radioactive.

Occurrence - Oxygen, the colorless gas that comprises about 21% of the earth's atmosphere.

- Sulphur occurs in earth’s crust to the extent of 0.05 % mostly as metal sulphides and sulphates. Another major

source of sulphur is H2S present in natural gas and crude

oil. Sulphur exists in combined state in living matter

and is a constitutent of some amino acids (e.g cysteine),

proteins and enzymes .

- Selenium and Tellurium are less abundant than sulphur

and occurs as selenides and tellurides in sulphide ores.

- Polonium does not exist in nature in compounds.

:StatesOxidation - The elements of Group 16 exhibit a number of oxidation states (see Table below):

Po Te Se S O Property

+2; +4 -2; +2; +4; +6

-2; +2; +4; +6

-2; +2; +4; +6

-2; -1; +1; +2

Oxidation states

- The stability of -2 oxidation state decreases down the group

due to increase in atomic size and decrease in electronegativity.

- Since electronegativity of oxygen is very high, it shows only

negative oxidation state as –2 except in the case of OF2 and

O2F2 where its oxidation state is +2 and +1 respectively.

This is because fluorine is more electronegative than oxygen.

Physical properties are summarized in the following table:

- Po does not show +6 oxidation state due to inert pair effect.

- The stability of +6 oxidation state decreases down the group and stability of +4 oxidation state increases (inert pair effect).

Allotropy:

- Oxygen exists in the diatomic form (O2)g and triatomic form (O3)g(Ozone).

Oxygen

- (O2)g is stable, Paramagnetic, contains two unpaired electrons and the structure is linear .

- (O3)g is produced by passing an electric discharge through (O2)g:

- Both O2 and O3 are powerful oxidizing agents.

- Ozone also absorbs ultraviolet radiation and decomposes

back to oxygen: 2O3 3O2

- Pollutants in the upper atmosphere such as nitrogen oxides (occur naturally and from high-flying aircraft) and chlorine atoms (from decomposition of chlorofluorocarbons used as aerosols, refrigerants, and other sources) causes the decomposition of ozone:

Types of oxygen ions

Allotropy:

- Allotropes of known structure include cyclic S6, S7, S8, S9, S10, S11, S12, S18 and S20.

Sulfur

- The most stable form at room temperature is the: (orthorhombic, or α-S8).

- Above 159 oC, the S8 rings begin to open; the resulting S8 chains can react with other S8 rings to open them and form SI6 chains, S24 chains, and so on.

- Large rings can also form, by the linking of ends of chains.

- Chains exceeding 200,000 sulfur atoms can be formed at 180 0C.

- Selenium, a highly poisonous element, and Tellurium also exist in a variety of allotropic forms, whereas Polonium, a radioactive element, exists in two metallic allotropes.

& Po Teelements: Se, Other

Compounds 1) Hydrides: - Binary hydrides of general formula H2X are known for all elements (X = O, S, Se, Te, Po).

- Oxygen and Sulfur forms another hydride, H2O2 and H2S2.

- Group 16 elements form mono-halides of the type M2X2; di-halides of the type MX2; tetra-halides of the type MX4; and hexa-halides of the type MX6 (Where M = S, Se, Te; X = halogen).

2) Halides:

- Except oxygen all the other group 16 elements form hexa-fluorides. (see Table below)

3) Oxides:

- Group 16 elements form two types of oxides, dioxides of the type MO2 and trioxides of the type MO3 (see Table below).

Commercial Preperation:

1/ Synthesis of SO2 by combustion of sulfur:

S + O2 → SO2

2/ SO2 is then converted to SO3 by the exothermic reaction (using a ctalyst):

SO2 + O2 → SO3

3/ The SO3 then reacts with water to form sulfuric acid:

SO3 + H2O → H2SO4

It is very dangerous to pass SO3 directly into water (causes burns). To avoid this, the SO3 is absorbed into 98% H2SO4 solution to form disulfuric acid, H2S2O7 (oleum):

SO3 + H2SO4 → H2S2O7

4SO2acid: HSulfuric

- The oleum is then mixed with water to form sulfuric acid:

H2S2O7 + H2O → 2H2SO4

Sulfuric acid reacts very exothermically with water. When concentrated sulfuric acid is diluted with water, it is therefore essential to add the acid carefully to water; adding water to the acid the solution at the top may boil and can cause serious burns to human tissue.