Group I Properties

General Characteristics of Group I (Alkali Metals)

• Group I elements: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs), Francium (Fr)
• Location in Periodic Table: Far-left column, Group 1, s-block
• Valence electron configuration: All have 1 electron in the outermost shell (ns¹ configuration)
• Nature: All are metals, with similar chemical properties due to the same valence electron configuration
• Tendency: Highly reactive, form ionic compounds by losing their single outer electron to form M⁺ ions (where M is the alkali metal)

Physical Properties

1. Relatively Soft Metals

• Can be cut with a knife (softness increases down the group)
• Freshly cut surfaces are shiny, but tarnish quickly in air due to oxidation
• Example:
  • Lithium: Hardest of the three (Li, Na, K) but still softer than most metals
  • Sodium: Softer than lithium, cuts smoothly
  • Potassium: Very soft, can be cut easily with less pressure

2. Melting Point Trend – Decreases Down the Group

• Reason:
  • Down the group, atomic radius increases, metallic bonds weaken because the delocalised electrons are further from the positive nucleus, reducing attraction
• Data trend (°C):
  • Lithium: 180°C
  • Sodium: 98°C
  • Potassium: 63°C
• Conclusion: Metals become easier to melt as we move down the group

3. Density Trend – Generally Increases Down the Group

• Reason:
  • Atomic mass increases faster than the increase in volume for most of the group, leading to greater density
• Exceptions:
  • Sodium is less dense than lithium (due to unusual crystal structure)
• Data trend (g/cm³):
  • Lithium: 0.53
  • Sodium: 0.97
  • Potassium: 0.86
• Observation: All alkali metals have densities less than water (except Rb, Cs, and Fr), so they float when placed in water

Chemical Properties

1. Reactivity Increases Down the Group

• Reason:
  • Outer electron is further from the nucleus → weaker attraction → easier to lose → faster reactions
• Example reactions with water:
  • Lithium: Floats, fizzes slowly, produces hydrogen gas, forms lithium hydroxide
    Equation: 2Li + 2H₂O → 2LiOH + H₂
  • Sodium: Floats, melts into a ball due to heat, fizzes more vigorously
    Equation: 2Na + 2H₂O → 2NaOH + H₂
  • Potassium: Very vigorous, lilac flame observed, may explode in water
    Equation: 2K + 2H₂O → 2KOH + H₂

2. Reactions with Oxygen

• All alkali metals tarnish in air due to rapid oxidation
• Example:
  • Lithium forms lithium oxide (Li₂O)
  • Sodium forms sodium oxide (Na₂O) and sodium peroxide (Na₂O₂)
  • Potassium forms potassium superoxide (KO₂)

3. Reactions with Halogens

• React vigorously to form white ionic halide salts
• Example:
  • 2Na + Cl₂ → 2NaCl (white solid salt)
  • K + Br₂ → KBr

Prediction of Properties for Other Group I Elements

• Rubidium (Rb):
  • Softer than potassium
  • Melting point lower than potassium (~39°C)
  • Reacts violently with water, producing enough heat to ignite hydrogen gas
• Caesium (Cs):
  • Melting point ~29°C (can melt in warm hands)
  • Reacts explosively with water
• Francium (Fr):
  • Radioactive and extremely rare
  • Predicted to be the most reactive of all alkali metals

Summary Table – Trends in Group I