Inorganic Chemistry - Model Chapter 6 continued

Model Chapter 6 continued

D. Energy is released during the formation of anions. For fluorine, the addition of up to 0.95 charge of an electron is exothermic. Further addition of electronic charge on fluorine is endothermic. For other halogens the addition of up to one electron is exothermic. These ions remain stable in solution. But the effective charge on the halogen atoms in crystalline halides is always less than unity. Even in crystalline fluoride of alkali metals, the effective charge of fluorine is less than 1^.. Thus, the ionicity for NaF is less than 75%. In this connection, it is extremely interesting to note that addition of more than one electron to any atom is always endothermic due to the repulsion between the negative charges. In fact, there can be no atom with more than one negative charges in it. Even the environmental energy will fail to stabilize these multi-charged anions. Thus, we cannot expect the oxide ion to exist in a solution or in a lattice. With alkali and alkaline earth metals, oxygen gives its best ionic compounds. In these crystalline oxides, the effective charge of oxygen is less than 1^-. Even Cs₂0, calculations show, is less than 50 percent ionic and the partial charge on the oxygen is only - 0.94. But, a group or cluster of atoms, for example C0₃^2-, PO₄^3-, S0₄^2- etc, can carry more than one negative charge. As the total negative charges on the cluster of atoms remain distributed over two or more atoms, the repulsion between the like charges are reduced and hence these ions remain stable in solutions. Thus, multi-charged negative ions with a cluster of atoms do exist in solutions and in

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lattices when there is possibility for the distribution of negative charges over two or more atoms. Here also, the effective negative charge on a constituent atom of the complex ion never exceeds 1^-.In sharp contrast; atoms with more than two positive charges are possible and are indeed very common since it is a question of the removal of the electron. In fact, ionization potential depends mainly not on the number of charges an ion carries but on from where the electrons are removed. For instance, removal of an electron from Mg⁺ is far easier than the removal of an electron from Na⁺ ion.

Further, when an atom acquires an electron there is no corresponding increase in its nuclear charge. Hence, the electrons of the anions must be less tightly bound and hence more diffused than the electrons of the original atoms. Therefore, electron pairs on the anions must be more available for donor activity than that on the respective neutral atoms. It is evident that their polarizability will be greater than the respective neutral atoms.

Will the anions remain as solvated ions in solution? The answer is 'yes'. As the anions, in general, are bigger than cations, the solvation sheaths will not be as thick as that existing around cations although F^- ion has exceptionally high hydration energy. The very great oxidizing power of fluorine in solution is, in fact, related to the extraordinarily small heat of dissociation of the molecule and the very high hydration energy of F^- ion rather than with the electron affinity of fluorine atom.

Table 6.

	F	CI	Br	I
I.P. (e V)	17.4	13	11.8	10.4
Electron affinity (e V) •	3.74	4.02	3.78	3.44
Hydration energy of X	122	89	81	72

As an illustration, it may be noted that among halogens, the nucleophilicity increases in the order Cl < Br < I even though basisity decreases (the strength of the hydrohalic acids increases in the order HCl < HBr < HI). This is partly due to size. As size is increased, polarizability is also increased and hence the ability to attack the centre increases. But, the order may be reversed with the change of the solvent. As the size of the negative ions decreases, the solvation increases and therefore the energy required to break the solvation sheath increases ie the energy required for activation increases and so the rate decreases. Thus, in aprotic polar solvents such as dimethyl formamide the

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Table 7		ELECTRON AFFINITIES (kJmol-1)
Element		Value	Element		Value
1	H•	72.76	36	Kr	0
2	He.	0	37	Rb	46.89
3	Li	59.8	38	Sr	0
4	Be	0	39	Y	0.0
5	B	27	40	Zr	50
6	C	122.3	41	Nb	100
7	N           N-1	-7	42	Mo	100
	N-1            N-2 N	_-800b -1290b	43 44	Tc Ru	70 110
8	O              O.1	141.0	45	Rh	120
	O.1            O-2	-780'	46	Pd	60
9	F	327.9	47	Ag	125.7
10	Ne		48	Cd	0?
11	Na	52.7	49	In	29
12	Mg		50	Sn	121
13	Ad	44	51	Sb	101
14	Si	133.6	52	Te	190.14
15	P	71.7	53	1	295.3
16	S             S-1	200.40	54	Xe	0
	S-1             S-2	-590	55	Cs	45.49
17	CI	348.8	56	Ba	0
18	Ar	0	57	La	50
19	K	48.36	58-71	Ln	50
20	Ca	0	72	Hf	0
21	Sc	0	73	Ta	60
22	Ti	20	74	W	60
23	V	50	75	Re	15
24	Cr	64	76	Os	110
25	Mn	0	77	Ir	160
26	Fe	24	78	Pt	205.3
27	Co	70	79	Au	222.73
28	Ni	111	80	Hg	0
29	Cu	118.3	81	T1	30
30	Zn	0	82	Pb	110
31	Ga	29	83	Bi	110
32	Ge	120	84	Po	180
33	AS	77 •	85	At	270
34	Se              se-1     194.9 Se-1                     Se-2               -420		86	Rn	0
35	Br	324.6

Source : Inorganic Chemistry by Huheey

/U

order is Cl^- > Br^- > I^-. But, in water it is in the order                Cl^- < Br^- < I^-. When the

solvent molecules are more tightly bound with the nucleophile, it is clear that the solvation sheath remains as a barrier between the nucleophile and the substrate.

Though mono negative ions such as F^-, Cl^-, Br^- are formed by the liberation of energy, these ions are several times less stable in vacuum than their respective neutral atoms. These ions occur in solutions or in the fused state (except in the case of gases). Though positive ions are formed, (existence very brief), when strongly heated or when an electric current is passed through the gas, negative ions are never really formed. (This amounts to questioning the concept of electron affinity). In fact, unlike ionization energy, it is very difficult to obtain the affinity energy experimentally. Carefully taken values are available only for halogens and most of these values are obtained by indirect methods, the oldest being the use of Born - Haber cycle.