Q.The electronic configuration of some elements are given below:
a. 1s²
b. 1s² 2s² 2p⁶
In which group and period of the periodic table they are placed?

Step 1a. Analyse $1s^2$. Total 2 electrons, outermost shell n=1 (so period 1); the 1s subshell is completely filled (a complete duplet) — this is the noble-gas end-of-period configuration, placing it in group 18. This is Helium.

Step 2a. Answer: period 1, group 18.

Step 1b. Analyse $1s^22s^22p^6$. Total 10 electrons, outermost shell n=2 (so period 2); the 2s and 2p subshells together are completely filled (a complete octet) — again the noble-gas end-of-period configuration, placing it in group 18. This is Neon.

Step 2b. Answer: period 2, group 18.

Reading period from the highest n present and group from whether the valence shell shows a complete duplet/octet (section 7.3.3, Problem 7.3 method)

• Placing $1s^2$ in group 2 by analogy with $ns^2$ (misapplying the s-block pattern) rather than recognising it as He's complete-duplet, group-18 case explained explicitly in Problem 7.3.
• Reading the period as the total electron count rather than the highest principal quantum number n present.