Modern inorganic chemistry

34 STRUCTURE AND BONDING

Table 2.5

Electron affinity (kJ mol ')

Atomic number Element — Total

1st 2nd

8

16

0

s

• 142


• 200

+ 844

4- 532

+ 702

+ 332

the formation of the divalent ion is an endothermic process in spite

of the fact that a noble gas configuration is achieved.

Periodic trends

Table 2.6 shows the electron affinities, for the addition of one

electron to elements in Periods 2 and 3. Energy is evolved by many

atoms when they accept electrons. In the cases in which energy is

absorbed it will be noted that the new electron enters either a

previously unoccupied orbital or a half-filled orbital; thus in

beryllium or magnesium the new electron enters the p orbital, and

in nitrogen electron-pairing in the p orbitals is necessary.

Table 2.6

Period 2

Atomic number 3 4 5 6 7 8 910

Element Li Be B C N O F Ne

Electron affinity (kJ moP!) -57 +66 -15 -121 +31 -142 -333 +99

Period 3

Atomic number 11 12 13 14 15 16 17 18

Element Na Mg Al Si P S Cl Ar

Electron affinity (kJmor'I -21 +67 -26 -135 -60 -200 -364 —

The above discussion indicates that the formation of a noble gas
configuration does not necessarily result in an evolution of energy.
Indeed, by reference to Tables 2.1 and 2.4 it can be seen that even
for the reaction between caesium and fluorine, the heat energy
evolved in the formation of the fluoride ion is less than tjie heat
energy required for the formation of the caesium ion. This implies
that the reaction will not proceed spontaneously; in fact it is virtually
explosive. Clearly, therefore, energy terms other than ionisation
energy and electron affinity must be involved, and the most import-
ant is the lattice energy—the energy evolved when the ions produced
arrange themselves into a stable lattice. It can be very large indeed