Page 70 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 4] ELECTRONIC CONFIGURATION OF THE ATOM 59

3p
3s

2p 2p
2s 2s

1s 1s
Ne atom Mg atom
Fig. 4-6. Electronic configurations of Ne and Mg

The electronic configurations of the first 20 elements are given in Table 4-6. The buildup principle is crudely
analogous to filling a vessel that has a layered base with marbles (see Fig. 4-7). The available spaces are filled
from the bottom up.
In atoms with partially filled p, d,or f subshells, the electrons stay unpaired as much as possible. This
effect is called Hund’s rule of maximum multiplicity. Thus the configurations of the carbon and nitrogen atoms
are as follows:

2p 2p
2s 2s

1s 1s

C atom N atom

Table 4-6 Detailed Electronic Configuration of the First
20 Elements
2
2
6
H 1s 1 1 Na 1s 2s 2p 3s 1 11
6
2
2
He 1s 2 2 Mg 1s 2s 2p 3s 2 12
2
2
2
2
6
Li 1s 2s 1 3 Al 1s 2s 2p 3s 3p 1 13
2
2
6
2
2
Be 1s 2s 2 4 Si 1s 2s 2p 3s 3p 2 14
2
2
6
2
2
2
B 1s 2s 2p 1 5 P 1s 2s 2p 3s 3p 3 15
2
2
2
2
6
2
C 1s 2s 2p 2 6 S 1s 2s 2p 3s 3p 4 16
2
6
2
2
2
2
N 1s 2s 2p 3 7 Cl 1s 2s 2p 3s 3p 5 17
2
2
2
2
6
2
O 1s 2s 2p 4 8 Ar 1s 2s 2p 3s 3p 6 18
2
6
6
2
2
2
2
F 1s 2s 2p 5 9 K 1s 2s 2p 3s 3p 4s 1 19
2
6
6
2
2
2
2
Ne 1s 2s 2p 6 10 Ca 1s 2s 2p 3s 3p 4s 2 20
In turns out that fully filled or half-filled subshells have greater stability than subshells having some other numbers
of electrons. One effect of this added stability is the fact that some elements do not follow the n + l rule exactly.
For example, copper would be expected to have a configuration
2
2
6
2
2
6
n + l configuration for Cu 1s 2s 2p 3s 3p 4s 3d 9
1
2
6
6
2
2
Actual configuration for Cu 1s 2s 2p 3s 3p 4s 3d 10
The actual configuration has two subshells of enhanced stability (filled 3d and half-filled 4s) in contrast to one
subshell (filled 4s) of the expected configuration. (There are also some elements whose configurations do not
follow the n + l rule and which are not so easily explained.)