How would you justify the presence of 18 elements in the fifth of period of periodic table?

Answer 1

In the fifth period of the Periodic Table, the atoms of the elements in the first two groups are adding 1 and 2 electrons, respectively, to their highest energy 5s sublevel. Starting in group 3/IIIB and going through group 12/IIB, the atoms of those elements are adding electrons to their highest energy 4d sublevel. Since the d sublevel can contain a maxium of 10 electrons, there are 10 elements whose atoms are filling the 4d sublevel. Once the 4d sublevel is filled, the next higher energy sublevel is the 5p sublevel. Starting with the group 13/IIIA elements, the 5p sublevel is being filled. Since a p sublevel can contain a maximum of 6 electrons, there are six elements whose atoms are filling the 5p sublevel.

5s sublevel filling: 2 elements

4d sublevel filling: 10 elements

5p sublevel filling: 6 elements

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Total: 18 elements

For a printable periodic table that includes electron configurations, go to the following link: http://www.nist.gov/pml/data/periodic.cfm

Answer 2

The presence of 18 elements in the fifth period of the periodic table is due to the filling of 4f sublevel, which accommodates a maximum of 14 electrons, and the filling of 5d sublevel, which accommodates up to 10 electrons. This accounts for the total of 18 elements in the fifth period.