11.Group 1 Elements: The Alkali Metals

1. Physical Properties

The Group 1 elements (Lithium, Sodium, Potassium, Rubidium, Cesium, and Francium) are quite different from typical metals:

  • Softness: They are relatively soft and can be cut with a knife.

  • Density: They have low densities (Lithium, Sodium, and Potassium even float on water).

  • Melting Points: They have relatively low melting and boiling points compared to other metals.

2. Trends as You Go Down the Group

  • Reactivity Increases: The elements become more reactive as you move down the group.

  • Melting and Boiling Points Decrease: Unlike many other groups, these temperatures go down as the atoms get larger.

3. Explaining Reactivity

  • Outer Electron: All Group 1 atoms have one electron in their outermost shell. To become stable, they only need to lose this one electron.

  • Why Reactivity Increases Down the Group: * As you go down, the atom's radius increases (more shells).

    • The outer electron gets further away from the positive nucleus.

    • The attractive force between the nucleus and the outer electron weakens, making it easier to lose the electron.

  • Ion Formation: When they lose their one outer electron, they form a 1+ ion.

4. Compounds and Bonding

  • Ionic Compounds: Because they lose electrons so easily, they almost always form ionic compounds with non-metals.

  • Appearance: These compounds are generally white solids that dissolve in water to form colorless solutions.

  • Ionic Bond: The metal atom donates its electron to a non-metal. The resulting opposite charges attract, forming an electrostatic bond.

5. Chemical Reactions

  • Reaction with Water: They react vigorously to produce a metal hydroxide and hydrogen gas.

    • Example: Sodium + Water → Sodium Hydroxide + Hydrogen

    • Observation: From Potassium downwards, enough energy is released to ignite the hydrogen, producing a lilac flame.

  • Reaction with Chlorine: They react vigorously when heated in chlorine gas to form metal chloride salts (white solids).

    • Example: Sodium + Chlorine → Sodium Chloride

  • Reaction with Oxygen: They react to form metal oxides, though the type can vary:

    • Lithium: Forms Lithium Oxide (Li2O).

    • Sodium: Forms Sodium Oxide (Na2O) or Sodium Peroxide (Na2O2).

    • Potassium: Forms Potassium Peroxide (K2O2) or Potassium Superoxide (KO2).