PERIODIC CLASSIFICATION OF ELEMENTS-Why are atomic numbers 27-36 in the fourth period of the Modern Periodic Table, even though only elements with the same number of electronic 'shells' are arranged in the same period(atomic numbers 19-26 have 4 shells and the rest in period 4 have 5 shells)?

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Solution

The fourth period contains 18 elements, beginning with potassium and ending with krypton. As a rule, period 4 elements fill their 4s shells first, then their 3d and 4p shells, in that order

I think you are mistaken..atomic numbers 27-36 have only 4 shells..

For understanding that,write the subshell wise electronic configuration..

This gives the following order forfilling the orbitals: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

Atomic number 27:

1s2 2s2 2p6 3s2 3p6 4s2 3d7

Contains only 4 shells

Atomic number 36:

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6

Also contains 4 shells...

Hope you understood..thank you

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Question 4
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