SPM Chemistry

Physical Properties of Alkali Metals

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Physical Properties of Alkali Metals

NameProton numberElectron arrangement
Lithium32.1
Sodium112.8.1
Potassium192.8.8.1
Rubidium372.8.18.8.1
Caesium552.8.18.18.8.1
Francium872.8.18.32.18.8.1
  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. shiny 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 floats 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 in the number of electron shells.
  3. An atom with more shells is bigger than an atom with fewer shells.

    Boiling Point and Melting Point

    Namemelting pointboiling pointDensity g/cm3
    Lithium180ºC1342ºC0.53
    Sodium98ºC883ºC0.97
    Potassium63ºC759ºC0.86
    Rubidium39ºC688ºC1.48
    Caesium29ºC671ºC1.87
    Francium27ºC677ºC> 1.87
    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 the metallic bond.
    3. The strength of the 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 the 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 compared 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 the water.
    3. Nevertheless, the density increases steadily down the group.
    4. The density of a substance is given by the equation “Density=Mass/Volume”.
    5. Down the group, both the mass and the volume increase, but the increase of mass is faster than the volume, hence the density increases down the group