Page 72 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 4] ELECTRONIC CONFIGURATION OF THE ATOM 61

enter the f subshell in the n − 2 shell—the second shell below the valence shell. These elements are the inner
transition elements.

1s 1s
2s 2p
3s
3p
4s 3d
4p
5s 4d 5p
6s 5d
6p
7s 6d
Main groups Transition groups Main groups

4f
5f
Inner transition groups
Fig. 4-8. Periodic table as an aid to assigning electronic configurations

An effective way to determine the detailed electronic configuration of any element is to use the periodic
table to determine which subshell to fill next. Each s subshell holds a maximum of 2 electrons; each p subshell
holds a maximum of 6 electrons; each d subshell holds a maximum of 10 electrons; and each f subshell holds
a maximum of 14 electrons (Table 4-5). These numbers match the numbers of elements in a given period in the
various blocks. To get the electronic configuration, start at hydrogen (atomic number = 1) and continue in order
of atomic number, using the periodic table of Fig. 4-8.

EXAMPLE 4.12. Using the periodic table, determine the detailed electronic configuration of magnesium.
Ans. Starting at hydrogen, we put two electrons into the 1s subshell, then two more electrons into the 2s subshell. We
continue (at atomic number 5) with the 2p subshell, and enter six electrons there, corresponding to the six elements
(elements 5 to 10, inclusive) in that p block of the periodic table. We have two more electrons to put into the 3s
subshell, which is next. Thus, we always start at hydrogen, and we end at the element required. The number of
electrons that we add to each subshell is equal to the number of elements in the block of the periodic table. In
this case, we added electrons from hydrogen to magnesium, following the atomic numbers in order, and we got a
6
2
2
2
configuration 1s 2s 2p 3s .
EXAMPLE 4.13. Write the detailed electronic configurations for K, S, and Y.
2
2
2
6
6
Ans. K 1s 2s 2p 3s 3p 4s 1
6
2
2
2
S 1s 2s 2p 3s 3p 4
2
6
2
10
2
6
2
6
2
Y 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 1
In each case, the superscripts total to the atomic number of the element.
EXAMPLE 4.14. Determine the detailed electronic configuration of Gd, atomic number 64.
6
6
10
2
10
2
2
6
6
1
2
2
2
Ans. Gd 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 5d 4f 7
We note that the 5d subshell started before the 4f subshell, but only one electron entered that shell before the 4f
subshell started. Indeed, the periodic table predicts this correct configuration for Gd better than the n +l rule or other
common memory aids.
Instead of writing out the entire electronic configuration of an atom, especially an atom with many electrons,
we sometimes abbreviate the configuration by using the configuration of the previous noble gas and represent
the rest of the electrons explicitly. For example, the full configuration of cobalt can be given as
2
2
6
2
6
2
Co 1s 2s 2p 3s 3p 4s 3d 7