Physical Properties Chemistry Notes
Physical Properties :
Main physical properties of group -15 are listed in taMe 7.1
Table 7.1 : Some atomic and physical properties of group-15 elements
→ aEIII single bond (E = element); bb E3+; cE3+, dWhite phosphorus; eGrey a-form at 38.6 atom; Fsublimation temperature: gAt 63K; hGrey α-form; Molecular N2
→ Electronic configuration The general valence shell electronic configuration of the elements of group 15 is ns2np3. The valence p-sub shell has three half filled p-orbitais in accordance with the Hund’s rule i.e..
→ According to Hund’s rule, the electronic configurations involving fully filled or exactly half filled orbitais are the most stable. So the elements of group 15, having half filled orbitais are fairly stable and not so reactive.
The electronic confturation of the elements of this family is given in table 7.2
Table 7.2 : Electronic configuration of elements in group -15 (Nitrogen family)
→ Physical State : In this family, nitrogen, the first member is a gas at room temperature. Phosphorus, the nextelement is a waxy solid whih the rest elements are solid and have metallic lustre.
→ Atomiclty : Nitrogen is a diatomic gaseous molecule (N2) at ordinary temperature, while phesporus, arsenic, antimony and bismuth exista as discrete tetraatomic tetrahedral molecules. viz. P4, As4, Sb4 etc.
→ Nitrrigr.n exist as düitomic molecule duc to its unique ability Lo form pπ – pπ multiple bonding, In nitrogen molecule nitrogen atoms are linked by triple bond. The tñple bond in nitrogen molecule ta very atable as the dissociation energy is very high i.e.945 kJ mol-1 Therefore, nitrogen molecule is inert at room temperature.
→ AtomIc and Ionic redii: On moving down the group, the atomic radii increases with increase in atomic number, It is because of addition of a new shell in each succeeding element.
N < P < As < Sb < Bi (Atomic radii)
→ Along the period, the atomic and ionic radii of the elements of group I S are smaller than the atomic radii of the corresponding group 14 elements, It is because of larger effective nuclear charge in case of the elements of group 15 than the elements of corresponding group 14.
→ As we know that as effective nuclear charge increase, the electrons are more strongly attracted towards the nucleus. This resulta is decrease in covalent radii. Atomic radii of the elements of group- 14> Atomic radii of the elements of group. 15. The ionic radii in particular oxidation state of the elements of gmup -15 increases on moving down the group (ie., Either all the elements are cationic or anionic in nature)
N3- < P3- < As3- < Sb3- < Bi3-
→ Ionisation Energy : On moving down the group, the first ionisation enthsilpy decnases regularly. It is because of increase in the sire of atoms and screening effect of the intervening electrons.
N > P > As > Sb > Bi (Ionisation Enthiilpy)
→ The ionisation energy or ionisation enthalpy ot’ the elements of group 15 is much higher than those of correaponding elements of carbon family.
→ Reason : In general, the ionisation enthalpy increases along a period because of decrease in atomic size and increase in effective nuclear charge but thu ionisation enthalpy of the elements of group 15 is larger than the elements of group 14.
→ It is because of highly stable half filled electronic configuration (np3) of p-orbitais of the elements of group-15. Due to stable half filled configuration, the value electrons of groups elements are strongly attracted by the nucleus and hence they have very valence electrons of group 15 elements are strongly attracted by the pucleus and hence they have very less tendency to lose electrons. As a result, the ionisation energy of the elements of nitrogen family increases.
Group 14 < Group -15 > Group – 16 (First ionisation enthalpy)
∆iH1, < ∆iH2 < ∆iH3
First Ionisation Enthalpy
→ Electronegativity Electronegativity decreases gradually on moving down the group from N to Bi. This is due to increase in atomic size from N to Bi.
N< P< AsSb 21 > 20 > 1.9 ≈ 1.9
→ Metallic Character: Metallic nature increases down the group. In group N and P are purely non-metal, As and Sb are metalloids or semimetals and Bi is a metal.
→ On moving down the group metallic character increases because atomic size increases and ionisation energy decrease due to which the electrons can be removed easily. Hence, electropositive character also increases.
→ Melting and Boiling Points : The melting points of the elements of group -15 first increases from Nitrogen (N) to Arsenic (As) and then decreases from Antimony (Sb) to Bismuth (Bi).
However the boiling points increase regularly from N to Bi.
→ Reason: The increase in melting points of the elements from N to As is due to increase in their atomic size. However, after As the atomic size even though increases but the melting point decreases from Sb to Bi.
→ It is because of their tendency to form three covalent bonds instead of five covalent bonds, due to inert pair effect. As a result, the forces of attraction among their atoms decrease, hence melting point decreases. The melting point of Bi still lower than Sb, it is due to large size of atoms of Bi which decrease the inter-atomic force of attraction.
N < P < Bi < Sb < As (melting point)
→ However, the regular increase in boiling points of elements of group-15 from N to Bi is because of their atomic size. N < P < As < Sb < Bi (Boiling points)
Density : It increases gradually on going down the group
Allotropy All the elements except bismuth show allotropy. For example:
→ Catenation The tendency to form bonds with itself (self linking of atoma) is known as catenation. The elements of group 15 also shows catenation property but to a much smaller extent than carbon. Only three elements i.e., N.P and As of this family show this catenation property. Some examples of nitrogen are as follows:
→ Phosphorus has the maximum tendency for catenation among the elements of group -15. Phosphorus can form cyclic as well as open chain compounds having many phosphorus atoms. Actually catenation property depends upon the bond dissociation energy. Larger the bond dissociation energy greater will be tendency to show catenation. Catenation property bond dissociation enthalpy
Oxidation states There are five electrons in the valence shell, so it show oxidation states from – 3to + 5. It can be discussed as follow.
→ Negative Oxidation States All the elements of group – 15 have 5 electrons in its valence shell with configuration of ns2np3. Therefore, these elements require three more electrons to acquire the nearest noble gas configuration. Although gain of three electrons form M3- ions require large amount of energy yet it takes place only with Nitrogen (N). It is because of its smallest size and highest electronegativity among the group. Thus, nitrogen form N3-ion (nitride ion) and show-3 oxidation state.
→ Example Magnesium nitride (Mg3 N2) and calcium nitride (Ca3 N2). Other elements of this group form covalent compounds with metals and show – 3 oxidation state,
- For example Calcium phosphide (Ca3P2)
- Magnesium bismuthide (Mg3 Bi2)
- Sodium arsenide (Na3 As)
- Zinc antimonide (Zn3 Sb2)
- Calcium arsenide (Ca3 As2)
→ But the tendency of these elements to show – 3oxidation state decreases as we move down the group from N to Bi. It is due to decrease in their electronegativity and ionisation energy
→ Positive Oxidation States In addition to – 3 oxidation state, the elements of this group also exhibit + 3 and + 5 oxidation states. However, they cannot form Motions due to the requirement of tremendous amount of energy On moving down the group, the stability of +5 oxidation state decreases while that of +3 oxidation state increases due to inert pair effect.
Due to inert pair effect, + 5 oxidation state of Bismuth (Bi) is less stable than + 3 state. Nitrogen cannot form compounds having + 5 oxidation state such as NF5 NCl5 etc. Because it does not have vacant d-orbitals in its valence shell so it can not extend its octet.
However, Nitrogen can exist in various oxidation states from-3 to +5 in its hydrides, oxides etc. It has been shown below
Table 7.3: Compounds of nitrogen and their oxidation states
|Compounds||Oxidation states of nitrogen|
|Ammonia (NH3)||– 3|
|Hydrazine (N2H4)||– 2|
|Hydroxylamine (NH2OH)||– 1|
|Nitrous oxide (N2O)||+ 1|
|Nitric oxide (NO)||+ 2|
|Nitrogen trioxide (N2O3)||+ 3|
|Nitrogen dioxide (NO2)||+ 4|
|Nitrogen pentaoxide (N2O5)||+ 5|
→ Disproportionation : It is a type of redox reaction in which the same compound is simultaneously reduced and oxidised. It is observed that the compounds of nitrogen and phosphorus in + 1 to + 4 oxidation states tends to disproportionate in acid solution.
For example :
But + 3 oxidation state in compounds of As, Sb and Bi is quite stable. They do not disproportionate.:
→ Maximum Covalency The maximum covalency of nitrogen is four because it does not have vacant d-orbitals in its outermost shell. Due to the absence of empty d-orbitals nitrogen cannot form penta halides like NF5, NCl5 etc.
→ While other elements of this group can extend their covalency beyond 4 as they have empty d-orbitals. Except nitrogen all other elements of group 15 can use all their valence orbital to exhibit five or six covalency.
For example PF5, [PF6]–, AsF5, PCl5, [SbF6]–, [BiF6]–