Assertion: The third period contains only 8 electrons and not 18 like 4th period.
Reason: In III perriod filling starts from `3s^(1)` and complete at `3p^(6)` whereas in IV period it starts from `4s^(1)` annd complete after `3d^(10)` and `4s^(2)`.

Assertion (A): The first period in periodic table contain only two elements. Reason (R) : 'K' subshell could be occupied by two electrons only, 1s^1 and 1s^2 .

Why does an electron enter 4s orbital but not 3d after completely filling 3p orbital?

Numerous forms of the periodic table have been devised from time to time. A modern version which is most convenient and widely used is the long or extended from of periodic table. The aufbau principle and the electronic configuration of atoms provide a theoretical foundation for the periodic classification. The horizontal rows are called periods. There are altogether seven periods. The first period consists of 2 elements. The subsequent periods consists of 8, 8,18, 18 and 32 elements respectively. The seventh period is incomplete and like the sixth period would have maximum of 32 elements. Elements having similar outer electronic configurations in their atoms are grouped in vertical columns. These are referred to as groups or families. According to the recommendations of IUPAC, the groups are numbered 1 to 18 replacing the older notation of groups 0, IA, IIA, ....VIIA, VIII, IB.....VIIB. Each successive period in the periodic table is associated with the filling up next higher principal energy level following aufbau sequence. The number of elements in each period is twice the number of atomic orbitals available in the energy level that is being filled. All the elements are classified into four blocks, i.e., s-block, p-block, d-block, and f-block depending on the type of atomic orbitals that are being filled with electrons. Elements A, B, C, D and E have the following electronic configuration: (A) 1s^2, 2s^2 2p^1 (B) 1s^2, 2s^2 2p^6 , 3s^2 3p^1 (C ) 1s^2, 2s^2 2p^6 , 3s^2 3p^3 (D) 1s^2 , 2s^2 2p^6 , 3s^2 3p^5 (E) 1s^2 , 2s^2 2p^6 , 3s^2 3p^6 Which among these will belong to same group in the periodic table ?

Determine the position of an element in long form of periodic table if its electronic configuration is [""_(18)Ar]3d^(10)4s^(2) .

Determine the position of an element in long form of periodic table if its electronic configuration is [_18Ar]3d^10 4s^2 .

Numerous forms of the periodic table have been revised from time to time. A modern verion, which is most convenient and widely used is the long or extended from the periodic table. The aufbau principle (electrons are filled in the progressive order of their increasing energy i.e., by n + l rule) and the electronic configuration of atom provides a theoretical foundation for the periodic classification. The horizontal rows are called periods. There are altogether seven periods. The first period consists of 2 elements. The subsequent periods consist of 8, 8, 18 and 32 elements respectively. The seventh maximum of 32 elements. Elements having similar outer electronic configurations in their atoms are grouped in vertical columns. These are referred to as groups or famillies. According to the recommendation of IUPAC, the groups are numbered 1 to 18 replacing the older notation of groups 0, IA, IIA ..... VIIA, VIII, IB, ...... VII B. Each successive period in the periodic table is associated with the filling up of next higher principal energy level following aufbua sequence. The number of elements in each period is twice the number of atomic orbitals avaliable in the energy level that is being filled. All the elements are classified into four blocks, i.e., s-block, p-block, d-block and f-block depending on the type of atomic orbitals that are being filled with the last electron of the element. Elements a,b,c,d and e have the following electronic configurations. (a) 1s^(2),2s^(2)2p^(1) (b) 1s^(2),2s^(2)2p^)(6),3s^(2)3p^(1) (c) 1s^(2),2s^(2)2p^(6),3s^(2)3p^(1) (d) 1s^(2),2s^(2)2p^(6),3s^(2)3p^(5) (e) 1s^(2), 2s^(2)2p^(6),3s^(2)3p^(5) Which among these will belong to the same group in periodic table?

Suneetha (A) : Third period contain 18 elements, because it has 3 subshells namely 3s, 3p and 3d. Sreelatha (B) : Third period has 8 elements, because while electrons are being filled into the shell '3d’ gets. electrons only after '4s' is filled.