Question

Why 2 orbits of some elements are uncomplet

Answer 1

The orbits of some elements are incomplete because they Sony have sufficient number of electrons to fill in each shell.
The number of electrons to be accommodated in each shell depends on the formula 2n^2
where 'n' is the number of the given shell from the nucleus
That is
K shell _1
L shell_2

The outermost shell should have 8 electrons and at times this wont be possible and the last shell remains incomplete.

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Answer 2

The completion of orbits of different elements is depend on there atomic number or no of electron.it can be determined by 2n^2. Depending on this formula Shell or orbits of different elements is filled.