(solved)Question 3.27 of NCERT Class XI Chemistry Chapter 3

$\displaystyle \underline{\underline{\text{(a) 5 electrons in outer shell}}}$

Point 1: by aufbau diagram [see the video], (n - 1)d is filled only after ns is filled. Hence, d cannot be the outermost sub-shell. Similarly, (n - 2)f is filled only after (n - 1) d is filled. Hence f cannot be the outermost sub-shell. Therefore, only s or p can be outermost sub-shell. But s cannot have more than 2 electrons. Hence, we have to look for $\displaystyle np^5$ outermost sub-shell. Any element of G17 has the outermost configuration p⁵.

Point 2: we observe that $\displaystyle \text{[He]}2s^22p^5$ is the configuration of florine Z = 9

$\displaystyle \therefore F (\text{Z = 9} ) \underline{Ans} $

$\displaystyle \underline{\underline{\text{(b) element to lose 2 electrons}}}$

The outermost config should be [Noble Gas]ns^2 so that a noble gas configuration is achieved after losing 2 electrons!

Hence $\displaystyle \text{[He]}2s^2 \text{ or Z = 4, Be } \underline{Ans}$

$\displaystyle \underline{\underline{\text{(c) element to gain 2 electrons}}}$

Chalcogens G16 are 2 electrons away from Group 18 noble gases.

Hence $\displaystyle \text{O (Z = 8)} \underline{Ans}$

$\displaystyle \underline{\underline{\text{(d) non/metal, gas, liquid}}}$

Group I has hydrogen H as non-metal-cum-gas, Na as solid-cum-metal, Cs is a liquid at 28 degree C.

Another answer is Group 17 - Ts (tennessine) is a metal, Bromine a liquid, Chlorine a gas-cum-nonmetal.

Note: Ts was discovered later in 2010, so the probable answer at the time of the publication of NCERT book seems to be Group 1