There are 18 elements in the fourth period of the modern periodic table. Explain.

Monjur Alahi

There are 18 elements in the fourth period of the modern periodic table. Explain.


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Salim Ahmed Kawsar

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  • The fourth period corresponds to the filling of fourth shell, n = 4.
  • Therefore, it begins with filling of 4s subshell. The first two elements of the fourth period are K (Z = 19) : [Ar] 4s1 and Ca (Z = 20) : [Ar] 4s2.
  • According to the aufbau principle, the next higher energy subshell is 3d, which can accommodate up to 10 electrons. Thus, filling of the 3d subshell results in the next 10 elements of the fourth period i.e., from Sc (Z = 21) : [Ar] 4s23d1 to Zn (Z = 30): [Ar] 4s23d10.
  • After filling of 3d subshell, the electrons enter the 4p subshell which can accommodate maximum 6 electrons. Hence, filling of 4p subshell results in the next 6 elements i.e., from Ga (Z = 31): [Ar] 4s23d10 4p1 to Kr (Z = 36): [Ar] 4s2 3d10 4p6.
  • Thus, the elements in fourth period are: 2 elements (with 4s subshell), 10 elements (with 3d subshell) and 6 elements (with 4p subshell).
  • Hence, there are 18 elements in the fourth period of the modem periodic table.

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