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Question

How would you justify the presence of 18 elements in the 5th Period of the periodic table?

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Solution

Simple explanation: 5 th period in the periodic table has s-block , p-block& d-block elements… hence max. no. of electrons accomodated by s, p & d sublevels = no. of elements in that period = 18 (2+ 6+10) Detailed explanation: For 5th period, principal quantum number, n = 5 and values of azimuthal quantum number, l = 0, 1, 2, 3 Now, filling of orbitals in sub-shells takes place in order of increasing energy of the sub-shell.Since, energy of 4f sub-shell is higher than 5s sub-shell, the sub-shells available for filling are 5s, 4d and 5p Total number of orbitals = 1 for 5s + 5 for 4d + 3 for 5p = 1+ 5 + 3 = 9 Therefore, total number of electrons that can be accommodated in 9 orbitals = 9 x 2 = 18 Hence, the number of elements in 5th period is 18.

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