Q.Explain the following:
Hydrogen shows similarity with alkali metals as well as halogens.

Step 1. Resemblance to alkali metals. Hydrogen has one valence electron (1s¹), just like the alkali metals' ns¹. Both hydrogen and the alkali metals react with a halogen X2 to form a compound of general formula HX / MX respectively (H2 + Cl2 → 2HCl; 2Na + Cl2 → 2NaCl) — both are monovalent, electropositive with respect to the halogen.

Step 2. Resemblance to halogens. Halogens have configuration ns²np⁵, one electron short of the next noble gas; hydrogen's 1s¹ is likewise one electron short of helium's 1s². So, like a halogen gaining an electron to form X⁻, hydrogen can gain an electron to form the hydride ion H⁻ (e.g. in NaH), showing halogen-like behaviour.

Step 3. Conclusion. Because hydrogen can either lose its one electron (alkali-metal-like) or gain one electron (halogen-like), it genuinely straddles both families, which is why it is placed separately, above group 1, rather than fully inside either group.

Comparing hydrogen's 1s¹ configuration against both the ns¹ (group 1) and ns²np⁵ (group 17) patterns

• Only mentioning the alkali-metal resemblance and forgetting the halogen-like electron-gaining behaviour (or vice versa).
• Forgetting that H⁺ (unlike Na⁺) cannot exist freely, which is precisely why hydrogen isn't simply grouped with the alkali metals despite the resemblance.