http://www.ck12.org Chapter 5. Electrons in Atoms
the 4sorbital contains one or more electrons. Therefore, the atoms of elements 21-30 have two valence electrons,
except for chromium and copper, which each have only one valence electron. Because thedandfsublevels always
fill behindssublevels of a higher principal energy level, they can never be valence electrons. The maximum possible
number of valence electrons in an atom is eight, which results from a full set ofsandporbitals.
The fifth period consists of the elements rubidium (Rb) through xenon (Xe). The pattern of sublevel filling is the
same as in the fourth period: 5sfollowed by 4dfollowed by 5p. Several more exceptional configurations occur, as
listed below (Table5.6).
TABLE5.6: Electron Configurations of Fifth-Period Elements
Element Name Symbol Atomic Number Electron Configuration
Rubidium Rb 37 [Kr] 5 s^1
Strontium Sr 38 [Kr] 5 s^2
Yttrium Y 39 [Kr] 4 d^15 s^2
Zirconium Zr 40 [Kr] 4 d^25 s^2
Niobium Nb 41 [Kr] 4 d^45 s^1
Molybdenum Mo 42 [Kr] 4 d^45 s^1
Technetium Tc 43 [Kr] 4 d^55 s^2
Ruthenium Ru 44 [Kr] 4 d^75 s^1
Rhodium Rh 45 [Kr] 4 d^85 s^1
Palladium Pd 46 [Kr] 4 d^10
Silver Ag 47 [Kr] 4 d^105 s^1
Cadmium Cd 48 [Kr] 4 d^105 s^2
Indium In 49 [Kr] 4 d^105 s^25 p^1
Tin Sn 50 [Kr] 4 d^105 s^25 p^2
Antimony Sb 51 [Kr] 4 d^105 s^25 p^3
Tellurium Te 52 [Kr] 4 d^105 s^25 p^4
Iodine I 53 [Kr] 4 d^105 s^25 p^5
Xenon Xe 54 [Kr] 4 d^105 s^25 p^6Sixth and Seventh Period Elements
The sixth period contains 32 elements and begins with cesium (Cs) filling the 6ssublevel. The 6ssublevel is followed
by 4f, then 5d, and finally 6p. Refer to the table below (Table5.7) for the electron configurations of the sixth period
elements, noting again the large number of exceptions.
TABLE5.7: Electron Configurations of Sixth-Period Elements
Element Name Symbol Atomic Number Electron Configuration
Cesium Cs 55 [Xe] 6 s^1
Barium Ba 56 [Xe] 6 s^2
Lanthanum La 57 [Xe] 5 d^16 s^2
Cerium Ce 58 [Xe] 4 f^15 d^16 s^2
Praseodymium Pr 59 [Xe] 4 f^36 s^2
Neodymium Nd 60 [Xe] 4 f^46 s^2
Promethium Pm 61 [Xe] 4 f^56 s^2
Samarium Sm 62 [Xe] 4 f^66 s^2
Europium Eu 63 [Xe] 4 f^76 s^2