Question

Why does the third period contain only 8 elements even though the electron capacity of the third shell is 18?

Answer 1

we know, according to quantum theory of atomic structure.
for n = 1 ⇒only s subshell e.g., 1s
for n = 2 ⇒s and p subshells e.g., 2s , 2p
For n = 3 ⇒ s , p and d subshells e.g., 3s , 3p and 3d
for n = 4⇒ s , p , d and f subshells e.g., 4s , 4p, 4d , 4f

Now, according to Afbau principle ,
* Electronic configuration of any atom are written by increasing order of energy of their orbitals .
* Higher the value of (n + l) , higher will be energy .
This way ,
Order is 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, ................ --------(1)
Now, in case of 3rd period.
we know, number of period shows principal quantum number of valance electrons.
∴ 3rd period means to say principal quantum , n = 3
For above exaplantion , if n = 3 then, 3s , 3p, 3d orbitals .
It means configuration of atom of 3rd period should be 1s²,2s²,2p⁶
,3s²,3p⁶,3d¹⁰ [ here I neglected 4s² ]
Then you can observe number of valance electron = 18
means capacity of electron of 3rd period = 18

But you forgot on things , e.g., Aufbau concept
Without filling 4s we can't jump 3d orbitals as energy of 3d > energy of 4s
So, in configuration of 3rd period we just take upto 3p⁶ orbitals .
E.g., configuration of last element of 3rd period = 1s²,2s²,2p⁶,3s²,3p⁶
e. number of valance electron = 8

Conclusion :- according to Afbau principle , we can't fill 3d orbital without filling 4s . But if we take 4s then it will be 4th period. Thats why third period contain only 8 elements even though the electron capacity of the third shell is 18.

Answer 2

1)In the modern periodic table there are seven horizontal rows called periods. In a period elements are arranged in an increasing order of their atomic numbers.The third row contains 8 elements and the electron capacity of the third shell is 18

2) in the third period while moving from left to right atomic number increases, number of electrons increases in the shell. The number of elements present in the third period is decided on the basis of their electronic configuration and octet rule.

3) agron is the last element of the third period and has a capacity of maximum 18 electrons. Its octet of electrons is completed and as argon belongs to the zero shell contains 18 electron.