Comparision of Nitrogen and Phosphorus 

Three major features:
 
- the radius of phosphorus is 50% greater than that of nitrogen
 
- the ionization energies decrease from N to P
 
- the bond energy trends are different
 
 
The oxidation states for both elements vary from +5 to -3,
but the stability trends are different.
 
 
 Valence shell expansion: the coordination number of nitorgen
is <= 4, but phosphorus is larger and the C.N. can be up to 6.
 PF6- and PCl6- but NH4+ and NF4+
 
 
Bond enthalpies:
 
N-H vs P-H chemistry
 
 The N-H bond is stronger than the P-H bond (386 vs 321 kJmol-l), so despite a lower DHo(atm) for P,
PH3 is thermodynamically unstable. PH3 is pyrophoric (kinetics). NH3 also a stronger base.
 
Lone-pair repulsion
 
 Nitrogen:
 
 Weakens N-X bonds in many nitrogen compounds resulting in thermodynamic instability,
e.g., the oxides and halides.
 
Phosphorus:
 
 Much reduced in P-X bonds (also dp-pp bonding, see below)
 hence stronger bonds and thermodynamically stable halides and oxides.
 
Compare:
 
 Hydrolysis of NCl3 gives NH3 and Cl2O
whereas PCl3 gives H3PO3 and HCl.
 
pp-pp bonding
 
 Nitrogen:
 
 Very important feature of nitrogen chemistry. Coupled with weak N-X single bonds,
nitrogen compounds tend to be monomeric or dimeric,
e.g., elemental nitrogen (N2), oxides, nitrates and nitrites.
 
Phosphorus:
 
 Quite strong multiple bonds but single bonds even more important.
Hence many phosphorus compounds "polymerize", e.g., the oxides (N2O3 and N2O5 vs P4O6 and P4O10),
oxy-anions, suphides [note: NO is monomeric but N4S4 and P4S3, P4S4 etc],
elemental phosphorus (P4), and nitrogen compounds [e.g., (NPCl2)4].
 
dp-pp bonding
 
 Open only to phosphorus, and particularly in compounds with the more electronegative elements N, O, F.
Strengthens both P-X and P=X bonds and hence chain or cyclic structures for the oxides and oxy-anion salts.
 
Kinetic effects too:
 
X3N-O     X3P=O
 
 Normal N-O bond length; Short P=O bond;
 high polarity; low energy; low polarity; high energy;
 high reactivity inert
 
Oxidation states
 
 - Negative oxidation state stability N > P
 
 - Positive oxidation states P > N
 
 - Oxidation states: NCl3 very unstable but PCl3 stable;
 N2O5, NO3 , NF4 but P4O10, PF5, PCl5, PBr5; nitric acid
 is very oxidizing but phosphoric acid is not.
 
 - In aqueous solution: refer to oxidation state diagram.
 Note: H3PO2 is in fact H2P(=O)OH, and phosphorous acid is
 not P(OH)3 but HP(=O)(OH)2 - an indication of the P=O
 bond strength
 
 - Donor properties of NR3 and PR3 in complex formation.