Chemical properties and spectroscopic data support the view that
in the elements rubidium to xenon, atomic numbers 37-54, the 5s, 4d
5p levels fill up. This is best seen by reference to the modern periodic
table p. (/). Note that at the end of the fifth period the n = 4 quantum
level contains 18 electrons but still has a vacant set of 4/ orbitals.
The detailed electronic configurations for the elements atomic
numbers 55-86 can be obtained from the periodic table and are shown
below in Table 1.5.
Note that the filling of the 4/ orbitals begins after lanthanum
(57) and the 14 elements cerium to lutetium are called the lanthanides
(Chapter 15). The electronic configuration of some of the newly discovered
elements with atomic numbers greater than 95 are uncertain
as the energy levels are close together. Filling of the 5/ orbitals does
begin after actinium (89) and the remaining elements are generally
referred to as actinides (Chapter 15).