The p- Block Elements
The p- block elements are placed in groups 13 to 18 of the periodic elements. Their general configuration or valence shell electron configuration is ns2np6 (except helium which has 1s2 configuration) Since they belong to the groups 13, 14, 15, 16, 17 and 18, they have 3, 4, 5, 6, 7 and 8 electrons in the valence shell. P ? block elements contain all 3 types of elements: 1. Metals
2. Non Metals
3. Metalloids
The Group 15 Elements The group 15 elements include: 1. Nitrogen Non metals
2. Phosphorus
3. Arsenic Metalloids
4. Antimony
5. Bismuth ------> Typical Metal
This can be remembered by:
Nit PAAB (read as night pub)
NITROGEN: 1. Occurrence:
Molecular nitrogen comprises 78% by volume of the atmosphere.
In the earth?s crust: It occurs as:
1. Sodium Nitrate (NaNO3) or Chile Saltpetre
2. Potassium Nitrate (KNO3) or Indian Saltpetre
It occurs in the form of Proteins in:
1. Animals
2. Plants
2. Atomic and Physical Properties:
| PROPERTY | NITROGEN |
| Atomic No. | 7 |
| Atomic Mass No. | 14.01 |
| Electronic Configuration (E. C.) | 1s2, 2s2, 2p3 |
| Ionisation Enthalpy 1H/ kJ mol-1 I | 1402 |
| II | 2856 |
| III | 4577 |
| Electronegativity | 3.0 |
| Covalent Radius/ pm2 | 70 |
| Ionic radius/ pm | 171 e3- |
| Melting Point / K | 63 |
| Boiling Point / K | 77.2 (molecular nitrogen) |
| Density/ [g cm-3 (298 K)] | 0.879 (at 63 K) |
3. Chemical Properties:
Nitrogen exhibits +1, +2, +4 apart from -3, +3 and +5 when it reacts with oxygen. In the case of nitrogen, all oxidation states from +1 to +4 tend to disproportionate in acid solution.
Eg. 3HNO2+ H2O+ 2NO
Nitrogen is restricted to a maximum covalency of 4 since only 4 (1 s and 3 p) orbitals are available for bonding. This is also the reason why it cannot form a pentahalide inspite of having +5 oxidation state.
4. Anomalous Properties Of Nitrogen:
Nitrogen differs from the rest of the members of this group because of the following reasons :
1. Smaller Size
2. High electronegativity
3. ?? ?? ionization enthalpy
4. ?? ?? non availability of d- orbitals
5. Unique ability to form p?- p? multiple bonds with itself and with other elements having small size and high electronegativity (Eg. C, O).
6. Heavier elements of this group do not form p?- p? bonds as their atomic orbitals are so large and diffuse that they cannot have effective overlapping
7. Thus, Nitrogen exists as a diatomic molecule with a triple bond (1s and 2p) between the two atoms. Thus, its bond enthalpy is very high.
8. However, the single N ? N bond formed is weaker as compared to other elements of Group 15 because of high interelectronic repulsion of the non bonding electrons which occurs due to the small bond length. Thus, the catenation tendency is weaker in Nitrogen.
9. Also, Nitrogen cannot form d?- p?. And the reason for this is quite obvious.
10. Reactivity Towards Hydrogen: Nitrogen forms hydrides with hydrogen of the type EH3 (where E = Group 15 elements) i.e it forms NH3. NH3 is the most stable hydride due to its bond dissociation enthalpy while it is the mildest reducing agent and strongest basic hydride in Group 15.
11. Reactivity Towards Oxygen: Nitrogen forms oxides of two types with oxygen: E2O3 and E2O5. N2O3 is purely acidic.
12. Reactivity Towards Halogens: Nitrogen reacts to form Halides of the type EX3. Although elements of Group 15 form a series of two halides, EX3 and EX5, Nitrogen does not form EX5 because its covalency is restricted to 4 due to the non availability of d- orbitals. Trihalides of Nitrogen are unstable except for NF3.
13. Reactivity Towards Metals: nitrogen reacts with metals to form their binary compounds exhibiting -3 oxidation state such as Calcium Nitride (Ca3N2).
1. Preparation of Dinitrogen:
COMMERCIALLY:
DINITROGEN
LIQUEFACTION or FRACTIONAL DISTILLATION OF AIR
HERE, LIQUID DINITROGEN (BP.77.2 K) DISTILLS OUT FIRST, LEAVING BEHIND LIQUID NITROGEN (BP. 90 K)
LABORATORY PREPARATION:
DINITROGEN
TREATING AQ. SOLN. OF NH4Cl WITH NaNO2
N2 (g) + 2H2O(l) + NaCl (aq) ALONG WITH SMALL AMOUNTS OF NO AND HNO3
IMPURITIES CAN BE REMOVED BY PASSING THE GAS THROUGH H2SO4 (aq) CONTAINING K2Cr2O7
OR
THERMAL DECOMPOSITION OF (NH4)2Cr2O7
OR
FOR VERY PURE NITROGEN, THERMAL DECOMPOSITION OF Ba(N3)2 OR NaN3.
1. Properties of dinitrogen:
1. Colourless
2. Odourless
3. Tasteless
4. Non toxic
5. 2 Stable Isotopes: 14 N and 15 N
6. Very low solubility in water
7. Low freezing and low boiling points
8. Inert at room temperature because of the high bond enthalpy of N? N.
9. Reactivity increases rapidly with rise in temperature.
At higher temperatures, it directly combines with samoe metals to form predominantly ionic nitrides and with non metals covalent nitrides. Some reactions are:
6Li + N2 2 Li3N
3Mg + N2 Mg3N2
Haber?s Process for formation of AMMONIA:
N2 + 3H2 773 K 2NH3
Nitric Oxide:
N2 + O2 2NO
2. Uses:
1. Manufacture of NH3 and other industrial chemicals containing nitrogen (eg. Calcium cyanamide)
2. Where inert atmosphere is required (eg. Iron and steel industry, inert diluent for reactive chemicals).
3. Liquid Dinitrogen is used as a refrigerant to preserve biological materials, food items and in cryosurgery.
AMMONIA:
1. Preparation:
(a) Small Quantities:
DECAY OF NITROGENOUS ORGANIC MATTER EG. UREA |
(a) Small Scale Manufacture:
WHEN TREATED WITH CAUSTIC SODA OR LIME |
DECOMPOSE
(b) Large Scale Manufacture:
H2
N2 + H2
CATALYST: IRON OXIDE AL2O3 + K2O |
At 700 K
N2 + 3H2 + 2NH3 LIQUID NH3
2. Properties:
1. Colourless Gas
2. Pungent Odour
3. Freezing and Boiling Pts are 198.4 and 239.7 respectively
4. Molecule is trigonal pyramidal with nitrogen atom at the apex. It has three bond pairs and one lone pair.
5. Highly soluble in water. Aqueous solution is weakly basic due to formation of OH- ions
6. It forms ammonium salts with acids
7. Since it is a weak base, it precipitates the hydroxides of many metals from their salt solutions
8. The presence of a lone pair of electrons on the nitrogen atom of ammonia molecule makes it a Lewis base. It donates the electron pair and forms linkage with metal ions and the formation of such complex compounds finds application in detection of metal ions such as Cu2+ , Ag+
3.Uses:
1. Various nitrogenous fertilizers (urea, ammonium nitrate, ammonium phosphate, ammonium sulphate, etc. )
2. In the manufacture of some inorganic nitrogen compounds eg. Nitric acid
3. Liquid ammonia is used as a refrigerant.
| Name | Formula | Oxidation state of N2 | Common Methods of Preparation | Appearance and Nature |
| Dinitrogen oxide [Nitrogen (I)Oxide] | N2O N- N- O 113pm-119pm Linear | +1 | NH4NO3 ----> N2O + 2H2O | Colourless gas, neutral |
| Nitrogen Monoxide [Nitrogen (II)Oxide] | NO N ? O 115 pm | +2 | 2NaNO2 + 2FeSO4 + 3H2SO4 ------> Fe2(SO4)3 + 2NaHSO4 + 2H2O + 2NO | Colourless gas, neutral |
| Dinitrogen trioxide [Nitrogen (III)Oxide] | N2O3 | +3 | 2NO + N2O4
|
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