Question

the third period contains only 8 elements even though the electronic capacity is 18
give scientific reason

Answer 1

Heya

In the third period only those elements go in whose balance shell is 'M' shell. Or those elements which have only three shells.

The capacity of m shell, as you said, is 18

But after 8 electrons go in to the third shell, the next one goes to the 4th shell.

4s orbital is of lower energy than of 3d. Electrons should first fill the orbitals of lower energy.

After 8 electrons go into 3rd shell, the next two electrons go into the 4th shell. The rest 10 electrons get back to the 3rd shell.

So, only 8 elements have m shell as their valance shell. Third period has only 8 elements in it.

The rest 10 elements, have 4th shell as their valance one.

Hope it helps!

Answer 2

Answer:

Actually 3rd shell should contains 18 elements but there are 10 electrons which are found to have high energy equivalent to energy of 4th shell. So these electrons are not counted in the 3rd shell. Hence, only 8 electrons are in 3 rd shell resulting 2, 8, 8, 18, electronic configuration.

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