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When we complete filling of 4th period by elements in the order of their atomic number, the orbitals 3s² 3p⁶ are full.

So we start the 5th period with 4s orbital. We can add 2 electrons in 4s. Then we fill 6 electrons in 4p orbital.  Further we start filling the 4d orbital which can take 10 electrons.  So in the 4th principal quantum energy stater, we can fill 18 electrons.  Thus 5th period has 18 elements.

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