give scientific reason the 3rd group contains only 8 elementd even through the electron capacity of 3rd shell is 18
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of which class and book please tell me if you tell me I can help I your class is higher than me I can't silve
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First of all, the columns (vertical) of the periodic table are called groups and the rows (horizontal) of the periodic table are called periods. I assume by 3rd 'group' you are referring to the third period because the third group which belongs to the transition elements contains 4 elements (Sc,Y,La,Ac) and the 3rd period contains 8 elements (Na,Mg,Al,Si,P,S,Cl,Ar).
The modern periodic table is based on the widely accepted quantum mechanical model of atom not on Bohr atomic model, which the reason why position of several elements are not justified according to it.
Now, let us come to your answer. First of all, note that in quantum mechanical model of atom, electrons are not only particles. In contrast to Bohr atomic model, they are both particles and wave and we can measure only the probability of its existence at a particular point. That is why they are called 'clouds' of probability. There are shells 1,2,3... (which are the principle energy levels and specify the distance of electron 'clouds' from nucleus), subshells s,p,d,f... (which are sub-energy levels and specify the type or shape of these clouds), orbitals(which are the 'clouds' we are talking about). Each orbital contains 2 electrons. Different types of subshells (s,p,d,f) contain different number of orbitals (1,3,5,7), therefore different number of electrons (2,6,10,14).
According to Aufbau Principle, which is the basis of filling various orbitals with electrons takes place according to the attached figure, the filling takes place in the direction of drawn arrows (1s,2s,2p,3s,3p,4s,3d...). Notice that the 4s subshell fills before the 3d (which contains 10 electrons) subshell. Therefore the 3d transition series is placed in the 4th period, after Ca, not after Mg. And therefore, only 8 elements exist in the 3rd period.
The modern periodic table is based on the widely accepted quantum mechanical model of atom not on Bohr atomic model, which the reason why position of several elements are not justified according to it.
Now, let us come to your answer. First of all, note that in quantum mechanical model of atom, electrons are not only particles. In contrast to Bohr atomic model, they are both particles and wave and we can measure only the probability of its existence at a particular point. That is why they are called 'clouds' of probability. There are shells 1,2,3... (which are the principle energy levels and specify the distance of electron 'clouds' from nucleus), subshells s,p,d,f... (which are sub-energy levels and specify the type or shape of these clouds), orbitals(which are the 'clouds' we are talking about). Each orbital contains 2 electrons. Different types of subshells (s,p,d,f) contain different number of orbitals (1,3,5,7), therefore different number of electrons (2,6,10,14).
According to Aufbau Principle, which is the basis of filling various orbitals with electrons takes place according to the attached figure, the filling takes place in the direction of drawn arrows (1s,2s,2p,3s,3p,4s,3d...). Notice that the 4s subshell fills before the 3d (which contains 10 electrons) subshell. Therefore the 3d transition series is placed in the 4th period, after Ca, not after Mg. And therefore, only 8 elements exist in the 3rd period.
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