Hydrogen's Dual Chemical Nature

The question asks to identify an element sharing properties with both alkali metals and halogens. This dual behavior stems from its unique electron configuration and position in the periodic table.

Analyzing Hydrogen's Properties

Hydrogen (H) has the atomic number 1 and its electron configuration is $1s^1$. This single valence electron dictates its chemical behavior:

• Similarity to Alkali Metals: Like alkali metals (Group 1), which have one valence electron, hydrogen can lose its electron to form a cation, $H^+$. This resembles the $M^+$ ion formation typical of alkali metals.
• Similarity to Halogens: Like halogens (Group 17), which have seven valence electrons and need one more to complete their octet, hydrogen can gain an electron to form an anion, $H^-$ (hydride ion). This resembles the $X^-$ ion formation typical of halogens.
• Unique Behavior: Hydrogen can also achieve a stable electron configuration by sharing its electron, forming covalent bonds (e.g., in $H_2$ or $H_2O$), a characteristic not typically seen in alkali metals or halogens in their elemental or common ionic forms.

Evaluating Other Options

The other options do not exhibit properties of both alkali metals and halogens:

• Neon (Ne) & Helium (He): These are noble gases (Group 18). They have stable electron configurations and are chemically inert, neither readily losing nor gaining electrons.
• Lithium (Li): This is an alkali metal (Group 1). It readily loses its single valence electron to form a $Li^+$ ion but does not typically form a negative ion like halogens.

Therefore, Hydrogen is the element that uniquely behaves like both alkali metals and halogens.