Alkali Metals: General characteristics, Physical Properties

Alkali Metals

Position of alkali metals in the periodic table

Alkali metals occupy the group I of the periodic table. Elements lithium, sodium, potassium, rubiduim, caesium and francium constitute alkali metals. They are named so from the Arabic word `Alquili' meaning `plant ashes'. Ashes of plants are composed mainly of sodium and potassium carbonates.

General characteristics

1.    The alkali metals are shiny white and soft.

2.    They can be readily cut with a knife.

3.    They are extremely reactive metals and form strong alkaline oxides and hydroxides.

4.   The last metal of this group, francium is radioactive.

5.    Since the alkali metals are extremely reactive they occur only as compounds in nature.

6.    All the alkali metals exhibit an oxidation state of +1. This is because the metals can easily lose their single outermost electron.

7.    The alkali metals give characteristic colour in bunsen flame. The colours given by Li, Na and K are crimson red, yellow, lilac respectively. This is because when the alkali metal or any of its compounds are heated in a bunsen flame, the ns' electron gets excited to higher energy levels and while returning to their ground state the excitation energy absorbed by them is released as light in the visible region.

Table  Electronic configuration of alkalimetals

Element      Symbol       Atomic        Electronic

                   number       configuration

Lithium       Li      3        [Helium] 2s1

Sodium       Na     11      [Neon] 3s1

Potassium   K       19      [Argon]4s1

Rubidium    Rb     37      [Krypton]5s1

Caesium      Cs     55      [Xenon]6s1

Francium    Fr      87      [Radon]7s1

Gradation in Physical Properties

1.    Density: In general, these elements have high density due to the close packing of atoms in their metallic crystals. Lithium has low density due to the low atomic weight of the atom. Density of the elements increases on moving down the group due to the increase in the mass of the atoms with increasing atomic number. However, K is lighter than Na probably due to an unusual increase in atomic size.

2.     Atomic volume: Atomic volume increases on moving down the group from Li to Cs. Hence there is an increase in atomic and ionic radii in the same order.

3.     Melting and boiling points : All alkali metals have low melting and boiling point due to the weak bonding in the crystal lattice. The weak interatomic bonds are attributed to their large atonic radii and to the presence of one valence electron. With the increase in the size of the metal atoms, the repulsion of the non-bonding electron gets increased and therefore melting and boiling points decreases on moving down the group from Li to Cs.

4.     Ionization energy : The first ionization energies of alkali metals are relatively low and decreases on moving down from Li to Cs.

                                                               ^i.      M_{(g) -- >}  M⁺_(g) + 1e^-

As the atomic radius gets increased on moving down the group, the outer electron gets farther and farther away from the nucleus and therefore ionization energy decreases.

The second ionization energies of alkali metals are fairly high. This implies that the loss of the second electron is quite difficult, because it has to be pulled out from the noble gas core.

5.     Electropositive character : As alkali metals have low ionization energies, they have a great tendency to lose electrons forming unipositive ions. Therefore they

M  -- >     M⁺ + 1e^-

have strong electropositive character. Electropositive character increases as we go down the group. The alkali metals are so highly electropositive that they emit electrons when irradiated with light. This effect is known as photoelectric effect. Due to this property, Cs and K are used in photoelectric cells.

6.    Oxidation state: All the alkali metals have only one electron in their outermost valence shall. As the penultimate shell being complete, these elements lose one electron to get the stable configuration of the nearest inert gas. Thus, they are monovalent elements showing an oxidation state of +1.

7.     Reducing properties : As alkali metals have low ionization energy, they lose their valence electrons readily and thus bring about reduction reaction. Therefore these elements behave as good reducing agents.