Physical Properties of Alkali Metals

Name Proton number Electron arrangement melting point boiling point Density g/cm3
Lithium 3 2.1 180ºC 1342ºC 0.53
Sodium 11 2.8.1 98ºC 883ºC 0.97
Potassium 19 2.8.8.1 63ºC 759ºC 0.86
Rubidium 37 2.8.18.8.1 39ºC 688ºC 1.48
Caesium 55 2.8.18.18.8.1 29ºC 671ºC 1.87
Francium 87 2.8.18.32.18.8.1 27ºC 677ºC > 1.87
  1. All Group 1 metal exist as solid at room temperature and hence have all the typical metallic properties, such as:
    1. good conductors of heat
    2. good conductors of electricity,
    3. high boiling points,
    4. shinny surface (but rapidly tarnished by air oxidation).
  2. Nevertheless, Group 1 metals also show some non-typical metallic properties, such as:
    1. low melting points,
    2. low density (first three float on water),
    3. very soft (easily squashed, extremely malleable, can be cut by a knife).

Important trends down the group:

  1. size of atoms increases
  2. the melting point and boiling point decrease
  3. the density increases.
  4. the hardness decreases.

Size of Atom

  1. Down the group, the size of atom increases.
  2. This is due to the increase of number of electron shells.
  3. Atom with more shells is bigger than atom with less shells.

Boiling Point and Melting Point

  1. The melting point and boiling point generally decrease down the group.
  2. All the atoms of Group 1 metals are bonded together by a force called metallic bond.
  3. The strength of metallic bond depends on the distance between the atoms. The closer the atoms, the stronger the bond.
  4. Down the group, the size of the atoms increases, causing the distance of the atoms increases.
  5. As the distance between the atoms increases, the metallic bond between the atoms decreases.
  6. Therefore, less energy is needed to overcome the metallic bond during melting process.
  7. Consequently, the melting point of Group 1 metal decreases down the group.

Density

  1. The densities of Group 1 metals are low compare with the other metals.
  2. The densities of the first 3 elements (Lithium, Sodium and Potassium) are lower than water. Thus, they can float on the surface of water.
  3. Nevertheless, the density increases steadily down the group.
  4. Density of a substance is given by the equation "Density=Mass/Volume".
  5. Down the group, both the mass and the volume increase, but increase of mass is faster than the volume, hence the density increases down the group


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