Question

Fourth period has 18 and not 32 elements? Explain

Answer 1

When n = 4, we find that in the orbitals 3d, 4s and 3p which are present in the outermost shells have energies in increasing order — i.e. 3p<4s<3d.

Total number of orbitals = 9 (1 from 4s, 3 from 3p & 5 from 3d orbital),

So, maximum number of electrons found in the three orbitals is 9X2 = 18.

Therefore, 4th period should have 18 elements.

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Answer 2

Explanation:

After the filling of 4s orbital, instead of 4p orbitals, there starts the filling 3d orbitals. This is in keeping with the fact that 3d orbitals have less energies than 4p orbitals. Thus 10 elements are built up by the filling of 3d orbitals. ... Hence the fourth period consists up of 18 and not 8 elements.

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