Physical Chemistry Third Edition

19.6 Atoms with More Than Two Electrons 811

Exercise 19.4

Give the ground configuration of the elements (a) S, (b) Ta, (c) Hg.

Table 19.1 lists the known exceptions to the diagonal rule through element 103.

Some of the prominent exceptions are Cr, Cu, Mo, Ag, and Au. In each of these cases,

there is adsubshell having 5 or 10 electrons instead of having 4 or 9 electrons. These

exceptions to the rule apparently correspond to the idea that a half-filled or filledpor

dsubshell is more stable than otherwise expected. If a half-filled subshell has a single

electron in each space orbital the electrons can have a fairly low energy of repulsion,

because the orbital regions of thepanddorbitals are well separated from each other

in space. However, other factors might be more important.^24 Nickel has sometimes

been assigned a configuration with a single 4selectron, and most of the elements in

the second transition series from element 39 (Y) to element 48 (Cd) have sometimes

been assigned configurations with a single 5selectron.

There has been some discussion about whether lanthanum and actinium are excep-

tions to the diagonal mnemonic rule. Lanthanum has sometimes been assigned a 5d

electron in the ground level, and actinium has been assigned a 6delectron. For this

reason, these elements are sometimes placed with the transition metals in the periodic

table. However, from an analysis of spectroscopic observations, it appears that lan-

thanum has a 4felectron in its ground state and that actinium has a 5felectron, as

Table 19.1 Exceptions to the Diagonal

Mnemonic Rule

Atomic No. Symbol Ground Configuration

24 Cr [Ar](3d)^5 (4s)^1

29 Cu [Ar](3d)^10 (4s)^1

41 Nb [Kr](4d)^4 (5s)^1

42 Mo [Kr](4d)^5 (5s)^1

44 Ru [Kr](4d)^7 (5s)^1

45 Rh [Kr](4d)^8 (5s)^1

46 Pd [Kr](4d)^10

47 Ag [Kr](4d)^10 (5s)^1

64 Gd [Xe](4f)^7 (5d)^1 (6s)^2

65 Tb [Xe](4f)^8 (5d)^1 (6s)^2

66 Dy [Xe](4f)^9 (5d)^1 (6s)^2

67 Ho [Xe](4f)^10 (5d)^1 (6s)^2

68 Er [Xe](4f)^11 (5d)^1 (6s)^2

78 Pt [Xe](4f)^14 (5d)^9 (6s)^1

79 Au [Xe](4f)^14 (5d)^10 (6s)^1

90 Th [Rn](6d)^2 (7s)^1

91 Pa [Rn](5f)^2 (6d)^1 (7s)^2

92 U [Rn](5f)^3 (6d)^1 (7s)^2

93 Np [Rn](5f)^4 (6d)^1 (7s)^2

94 Cm [Rn](5f)^7 (6d)^1 (7s)^2

103 Lr [Rn](5f)^14 (6d)^1 (7s)^2

(^24) M. P. Melrose and E. R. Scerri,op. cit.(note 23).