Short Answer

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

• They have one electron in their outermost shell (ns¹) which makes them highly reactive and metallic.
• Alkali metals are soft, shiny, and good conductors of heat and electricity. They readily form compounds with non-metals, especially halogens, to produce salts.

Detailed Explanation :

General Characteristics of Alkali Metals

Alkali metals are located in Group 1 of the periodic table. Their outer electron configuration is ns¹, meaning they have one valence electron in the outermost s-orbital. This single electron is loosely held, which gives them unique chemical properties.

Physical Properties

1. Softness:
   • Alkali metals are soft and can be cut easily with a knife.
   • Softness increases down the group: Li < Na < K < Rb < Cs.
2. Shiny Metallic Luster:
   • Freshly cut surfaces are silvery and shiny.
   • The luster fades quickly due to reaction with air forming oxides.
3. Low Density:
   • Lithium, sodium, and potassium are light metals; some can float on water.
   • Density increases down the group, but potassium is less dense than sodium due to atomic structure.
4. Melting and Boiling Points:
   • Relatively low compared to other metals, decreasing down the group.
   • Example: Li (180°C), Cs (28°C).
5. Conductivity:
   • Good conductors of heat and electricity because of mobile valence electrons.

Chemical Properties

1. Reactivity:
   • Highly reactive due to the single valence electron.
   • Reactivity increases down the group as the outer electron is farther from the nucleus and easier to remove.
2. Reaction with Water:
   • Form alkaline hydroxides and release hydrogen gas.
   • Example: 2Na + 2H₂O → 2NaOH + H₂
3. Reaction with Oxygen:
   • Form oxides, peroxides, or superoxides depending on the metal and reaction conditions.
   • Example: 2K + O₂ → K₂O
4. Reaction with Halogens:
   • Form ionic salts.
   • Example: 2Na + Cl₂ → 2NaCl
5. Reaction with Acids:
   • React vigorously to form salts and hydrogen gas.
   • Example: 2K + 2HCl → 2KCl + H₂

Trends in Alkali Metals

1. Atomic and Ionic Size:
   • Atomic radius increases down the group due to added electron shells.
2. Ionization Energy:
   • Ionization energy decreases down the group as the outer electron is farther from the nucleus → easier to remove.
3. Electronegativity:
   • Decreases down the group because the ability to attract electrons weakens.
4. Reactivity:
   • Increases down the group → cesium is more reactive than lithium.

Occurrence and Uses

1. Occurrence:
   • Highly reactive → do not occur free in nature, found in compounds like salts.
   • Example: NaCl (table salt), KCl, Li₂CO₃.
2. Uses:
   • Sodium: Street lamps, chemical industry.
   • Potassium: Fertilizers.
   • Lithium: Batteries, psychiatric drugs.
   • Cesium: Atomic clocks.

Conclusion

Alkali metals are Group 1 elements with one valence electron, making them highly reactive and metallic. They show trends in atomic size, ionization energy, reactivity, and physical properties down the group. Their chemical properties, including reactions with water, oxygen, halogens, and acids, make them important in industrial, chemical, and technological applications.