Group VA (15) Elements: N, P, As, Sb, Bi Nitrogen family
Group 15 of the periodic table includes the elements nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). Like other groups, these elements also show gradation in properties, but the properties of nitrogen differ markedly from other members. It is mainly due to the small size and absence of d-orbitals in nitrogen. (Nitrogen family)
1- Electronic configuration of nitrogen family :
The outer electronic configuration of VA group elements is ns2np3 in which all the p-orbitals are half-filled. The electronic configurations of VA groups are given in the following table (4.9).
Table 4.9: Electronic configuration of VA elements
|Element||Symbol||Atomic No.||Electronic configuration with an inert gas core|
|Arsenic||As||33||[Ar]3 d10,4 s2,4p3|
|Antimony||Sb||51||[Kr]4 d10,5 s2,5p3|
|Bismuth||Bi||83||[Xe]4ff14,5 d10,6 s2,6p3|
2- Metallic property of nitrogen family:
3- Atomic and Ionic radii of the nitrogen family:
4- Ionization potential and electronegativity of nitrogen family :
7- Oxidation states and valency:
8- Catenation property:
Although this is the characteristic property of carbon, the elements of this group which lie close to carbon in the periodic table also show this property to some extent. Nitrogen which has the minimum for its N−N bond energy (163.1KJmol−1 ) shows this property to the maximum extent. Thus, N atoms can form many compounds which contain straight chains of two or more N-atoms, for example.
9- Allotropy :
Red phosphorus is an amorphous solid, It is less reactive than white phosphorus. It is non toxic. Thermodynamically, the most stable form of phosphorus is black phosphorus. It is obtained by heating white phosphorus at 200∘C to a pressure of 1200MPa. If extreme pressure (10,000MPa) is used, a short impact is enough to convert white phosphorus quantitatively to black. It is denser (density =2.69gcm−3) than white phosphorus (density = 1.82gcm−3) ar violet phosphorus (density =2.36gcm−3 ). Black phosphorus (orthorhombic) has a parallel, puckered double layers structure as shown in figure (4.16).
10- Hydrides :
Fig. 4.16: Structure of double-layer orthorhombic black phosphorus. The boldface letters indicate atoms above the plane of the paper while the regular type indicates atoms below the plane of the paper.
NH3 is a strong Lewis base due to the presence of a lone pair of electrons. It has also a great tendency to form H-bond. PH3 is a weaker base than ammonia whereas AsH3, SbH3, and BiH 3 do not show any basic properties. It is because in these hydrides M has an empty nd-orbital. In these hydrides, M undergoes sp3 hybridization. They have pyramidal shapes but their HMH bond angles are different. trihalides in the MX3 series (M=N, P, As, Sb or Bi and X=F, Cl, B or I) are known.
Like the hydrides, the trihalides of group VA have a pyramidal structure in the gaseous state NBr3 and Nl3 are unstable. The instability of halides can be explained on the basis of the small size of the nitrogen atom. Trihalides of N and P are covalent whereas trihalides of Sb and Bi have more ionic, less covalent character. Thus, the covalent character of MX3 decreases on moving from N to Bi.
Nitrogen can not form pentahalides of the type NX5 because nitrogen does not have d-orbitals in its valency shell and can not expand its octet. Phosphorus forms the pentahalides with all the four halogen atoms (F, Cl, Br, and I ), arsenic forms only AsF F5 while antinomy forms SbF5 and SbCl5. On the other hand, bismuth does not give any Penta halide because due to the inert pair effect +3 oxidation state of Bi is more stable than its +5 oxidation state. P, As and Sb form their Penta halides because of the presence of vacant d− orbitals in their valence shell. These pentahalides are trigonal bipyranidal in shape.
(iii) Di element tetra halides :
P, As and Sb also form element tetrahalides of the type M2X4. The halides P2Cl4 and P2Br4 are least well characterized, while P2 F4, P2I4, As2l4, and Sb2l4 are well defined. They all have an X2M−MXX2 type of structure and all are reactive. Diarsenic tetraiodide As2I4 and Sb2l4 decompose on standing to MX3+M.
No Bi2X4 compound is known, but it has long been known that when metallic bismuth is dissolved in molten BiCl3, a black solid of approximate composition BiCl can be obtained.