Short Answer

Alkali metals are highly reactive because they have one electron in their outermost shell.

• This single valence electron is loosely held due to low ionization energy, making it easy to lose and form positive ions.
• Reactivity increases down the group as the atomic size increases and the outer electron is farther from the nucleus, as seen in lithium (Li) to cesium (Cs).

Detailed Explanation :

Electronic Configuration and Reactivity

Alkali metals belong to Group 1 of the periodic table, including lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

• Their general electronic configuration is ns¹, meaning there is one valence electron in the outermost s-orbital.
• The single valence electron experiences weak attraction from the nucleus, especially in heavier atoms, making it easily lost in chemical reactions.

Factors Responsible for High Reactivity

1. Low Ionization Energy:
   • Alkali metals have the lowest ionization energies in their respective periods.
   • Low ionization energy allows the outer electron to escape easily, forming M⁺ ions, which are more stable.
2. Large Atomic Size:
   • Atomic radius increases down the group, so the valence electron is farther from the nucleus.
   • Less nuclear pull → electron is more easily removed, increasing reactivity.
3. Electropositivity:
   • Alkali metals are highly electropositive, meaning they readily lose electrons to form ionic compounds.
   • Example: Na + Cl → NaCl
4. Single Electron in Outer Shell:
   • Only one electron needs to be lost to achieve the stable noble gas configuration, making the process energetically favorable.
5. Shielding Effect:
   • Inner electrons shield the valence electron from nuclear charge.
   • Increased shielding down the group → valence electron is easier to remove → higher reactivity.

Trends in Reactivity

1. Down the Group:
   • Reactivity increases from Li → Cs due to increasing atomic size and decreased ionization energy.
2. Across the Period:
   • Alkali metals show similar reactivity across periods because each has one valence electron, but reactivity is generally lower in lighter metals.

Reactions Showing High Reactivity

1. Reaction with Water:
   • Alkali metals react vigorously with water to form alkaline hydroxides and hydrogen gas.
   • Example: 2K + 2H₂O → 2KOH + H₂
2. Reaction with Oxygen:
   • Form oxides, peroxides, or superoxides depending on the metal.
   • Example: 4Na + O₂ → 2Na₂O
3. Reaction with Halogens:
   • Form ionic salts with halogens, such as NaCl or KBr.
4. Reaction with Acids:
   • React vigorously to form salts and release hydrogen gas.
   • Example: 2Na + 2HCl → 2NaCl + H₂

Safety and Significance

• Alkali metals are stored under oil to prevent reaction with air or moisture.
• Their high reactivity makes them useful in chemical synthesis and industrial applications, like sodium in organic chemistry reactions.

Conclusion

Alkali metals are highly reactive because they have one loosely held valence electron, low ionization energy, large atomic size, and strong electropositivity. Their reactivity increases down the group due to easier electron loss. These properties make alkali metals extremely reactive with water, oxygen, halogens, and acids, which is important for chemical reactions, industrial processes, and understanding periodic trends.