the third period contains only 8 elements even through the electron capacity of the 3rd shell is 18
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Yes, The maximum capacity of the third electron shell is 18 but it will get stable at 8 electrons too. But when the other shells get filled and the resultant no of electrons becomes 18 it gets added up and settles in the third electron shell.
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Yes, you are right. The capacity of electrons in third shell is 18.
However, if we go into quantum theory of an atom, we come across sub-shells, orbitals, spin of electrons, and all the stuff which I won’t bore you with, except for one rule which applies to this question, the Aufbau Principle, or the n+l rule.
The 3rd shell has three sub-shells, 3s, 3p, 3d.
The 4th shell has 4 sub-shells, 4s, 4p, 4d, 4f.
The Sequence of the filling of various sub-shells may also be determined from (n+l) rule. The electron comes in the vacant sub-shell with lowest value of (n+l). If the value of (n+l) is same, then electron enters the sub-shell having lowest value of n. Aufbau said that electrons enter into orbitals with lowest energies first.
(n=no. of shell, l= sub-shell no.)
Values of l for s,p,d,f subshells are 0,1,2 & 3 respectively. Their capacities for accommodating electrons are 2,6,10,14 electrons respectively.
Hence, we can conclude that 3d (3+2=5) has a higher value compared to 4s (4+0=4). Therefore, instead of going to the 3d orbital, the electron prefers to be accommodated in 4s, where energy is lesser.
The universe always prefers the path of least energy.
Here comes your answer.
Period number is given by the last shell which an electron enters into. The electrons fill up 3s and 3p subshells (total electrons = 2+6=8, hence 8 elements), and then they start filling 4s before going back to 3d. As said before, 3d accommodates 10 electrons, hence the deficiency in your question.
However, if we go into quantum theory of an atom, we come across sub-shells, orbitals, spin of electrons, and all the stuff which I won’t bore you with, except for one rule which applies to this question, the Aufbau Principle, or the n+l rule.
The 3rd shell has three sub-shells, 3s, 3p, 3d.
The 4th shell has 4 sub-shells, 4s, 4p, 4d, 4f.
The Sequence of the filling of various sub-shells may also be determined from (n+l) rule. The electron comes in the vacant sub-shell with lowest value of (n+l). If the value of (n+l) is same, then electron enters the sub-shell having lowest value of n. Aufbau said that electrons enter into orbitals with lowest energies first.
(n=no. of shell, l= sub-shell no.)
Values of l for s,p,d,f subshells are 0,1,2 & 3 respectively. Their capacities for accommodating electrons are 2,6,10,14 electrons respectively.
Hence, we can conclude that 3d (3+2=5) has a higher value compared to 4s (4+0=4). Therefore, instead of going to the 3d orbital, the electron prefers to be accommodated in 4s, where energy is lesser.
The universe always prefers the path of least energy.
Here comes your answer.
Period number is given by the last shell which an electron enters into. The electrons fill up 3s and 3p subshells (total electrons = 2+6=8, hence 8 elements), and then they start filling 4s before going back to 3d. As said before, 3d accommodates 10 electrons, hence the deficiency in your question.
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