Each period in the periodic table starts with a s-subshell of the new shell and ends with filling of

p-subshell of the same shell

The correct answer is option 2. p-subshell of the same shell.

In the periodic table, elements are arranged in periods (rows) and groups (columns) based on their electronic configurations and properties. The structure of each period and the sequence of subshell fillings can be understood by examining the electronic configurations of elements.

Electronic Configuration Basics:

Electrons in an atom are arranged in shells (energy levels) and subshells (s, p, d, f). The principal quantum number (n) denotes the main energy level or shell. Subshells are characterized by different orbital shapes and are designated as s (spherical), p (dumbbell-shaped), d (cloverleaf-shaped), and f (complex-shaped).

Period Structure:

Each period in the periodic table corresponds to the filling of electrons in a new principal energy level (shell). The period number corresponds to the principal quantum number (n) of the outermost shell of the elements in that period. For example, the first period (Period 1) corresponds to elements with electrons filling the 1st shell (n=1), the second period (Period 2) corresponds to elements with electrons filling the 2nd shell (n=2), and so on.

Subshell Fillings Within a Period:

Start of the Period (Left side): Each new period begins with the filling of the s-subshell of the new shell.

Period 1 starts with 1s: Hydrogen (H) and Helium (He) have their outermost electrons in the 1s subshell.

Period 2 starts with 2s: Lithium (Li) through Neon (Ne) have their outermost electrons filling the 2s subshell.

Middle of the Period: After the s-subshell is filled, the p-subshell of that same shell starts to fill.

Period 2 continues with 2p: Boron (B) through Neon (Ne) have their outermost electrons filling the 2p subshell after the 2s subshell is filled.

End of the Period (Right side): The period ends with the completion of the filling of the p-subshell of that particular shell.

Period 2 ends with 2p: Neon (Ne) completes the 2p subshell filling.

Explanation of Options:

(1) Same subshell: This is not correct because each subshell (s, p, d) within a period belongs to the same principal energy level (shell), but they are filled in sequence (s first, then p, then d for transition metals).

(2) p-subshell of the same shell: This is correct. Each period ends with the filling of the p-subshell of the shell corresponding to that period. For example, Period 2 ends with the filling of the 2p subshell.

(3) p-subshell of the next shell: This is incorrect because a new period begins with the filling of the s-subshell of the new shell, not the p-subshell of the next shell.

(4) d-subshell of the same shell: This is not correct as d-subshells are typically filled in transition metals, which occur in the middle of the periodic table, not at the end of periods.

Therefore, the filling sequence within a period starts with the s-subshell and ends with the p-subshell of the same shell, making option (2) the correct explanation of how each period in the periodic table is structured in terms of subshell fillings.