bei48482_FM

246 Chapter Seven

Transition Elements

The origin of the transition elements lies in the tighter binding of selectrons than

dor felectrons in complex atoms, discussed in the previous section (see Fig. 7.8).

The first element to exhibit this effect is potassium, whose outermost electron is in

a 4sinstead of a 3dsubstate. The difference in binding energy between 3dand 4s

electrons is not very great, as the configurations of chromium and copper show. In

both these elements an additional 3delectron is present at the expense of a vacancy

in the 4ssubshell.

The order in which electron subshells tend to be filled, together with the maximum

occupancy of each subshell, is usually as follows:

1 s^22 s^22 p^63 s^23 p^64 s^23 d^104 p^65 s^2

4 d^105 p^66 s^24 f^145 d^106 p^67 s^26 d^105 f^14

Figure 7.12 illustrates this sequence. The remarkable similarities in chemical behavior

among the lanthanides and actinides are easy to understand on the basis of this se-

quence. All the lanthanides have the same 5s^25 p^66 s^2 configurations but have incom-

plete 4fsubshells. The addition of 4felectrons has almost no effect on the chemical

properties of the lanthanide elements, which are determined by the outer electrons.

Similarly, all the actinides have 6s^26 p^67 s^2 configurations and differ only in the num-

bers of their 5fand 6delectrons.

These irregularities in the binding energies of atomic electrons are also responsible

for the lack of completely full outer shells in the heavier inert gases. Helium (Z2)

and neon (Z10) contain closed Kand Lshells, respectively, but argon (Z18) has

only 8 electrons in its Mshell, corresponding to closed 3sand 3psubshells. The rea-

son the 3dsubshell is not filled next is that 4selectrons have higher binding energies

s

s

s

s

s

s

s

p

p

p

p

p

p

d

d

d

d

d

f

f

f

n = 1234567

E

Figure 7.12The sequence of quantum states in an atom. Not to scale.

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