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Properties of Alkali Metals

A.    Electronic Configuration.

The atoms of alkali metals in the ground state consist of one electron in s-orbital outside a noble gas core. This similarity in the electronic configuration of all these elements is reflected in the similarities of their physical and chemical properties discussed below.

B.    Physical Properties.

Important physical properties of alkali metals are set out in. We shall confine our discussion to only first five members of the family of alkali metals as very little is known about the last member, namely, francium which is a radioactive element.

A.    Correlation between atomic structure and physical properties is discussed below. Attempt has been made to explain the physical properties of alkali metals on the basis of two facts, namely,
(a)    loose binding of s-electrons; and
(b)    size of the alkali metal atoms and ions.

(a)    Loose Binding of s-electrons

In alkali metals the noble gas core shields the s-electrons form the direct attraction of the charge of the atomic nucleus. Therefore, the s-electron is very loosely held. They consequences of loose binding of s-electrons are:

(i)    Ionization Energy

The first ionization energy of all these elements is low. This is the main factor which enables these metals to form positive ions readily and thus behave as electropositive elements.

M(g)  →   M(g)+ + e-

The second ionization energy is fairly high which implies that the loss of the second electron is quite difficult. This is due to the fact that the second electron has to be pulled out form the noble gas core. Hence, if can be concluded that the chemistry of these elements is essentially of unipositive ions.

Trend in the Group. On moving down the group, the ionization energy decrees because the s-electrons are getting away form the nucleus due to the addition of new shells. Therefore, their attraction for the nucleus decreases and the electrons can be removed by expending less energy. Hence, their electropositive character (i.e., the ability to lose an electron) increases.

(ii)    Reducing Property

The tendency of alkali metals to act as strong reducing agents is obvious form the fact that these metals liberate H2 from water and acids

2M + 2H2O   →   2MOH + H2
 2M + 2HCI    →    2MCI + H2

This property may be attributed to their strong tendency to lose their outermost (ns( ) electron. Reducing power is measured in terms of the standard electrode redacting potentials (M++ e <==> M , E0). These elements show very high negative E0 values which suggests that they are extremely powerful reducing agents and since lithium metal has the highest value (E0 = -3.04V) therefore it is the strongest reducing agent among the alkali metals. 

(iii)    Photoelectric Effect

The low energy photons (i.e., light) can eject the loosely held s-electron from the surface of these metals. Therefore, these metals especially cesium are used in photoelectric cells which are sensitive to blue light.

(iv)    Electronegativity

The values of electronegativity for the alkali metals are very low. This is because these elements have a tendency to lose rather than gain an electron.

Trend in the Group. The value of electronegativity decreases from lithium to cesium indicating decreasing tendency of the elements to hold their s-electron.

(v)    Colouration to the Flame

These elements give characteristic coloration to the flame. Their salts, particularly chlorides, when heated in a Bunsed burner flame on platinum wire, dissociate into atoms (and not ions). The outermost electron is excited to higher energy states. An electron which is in a higher energy state must lose its excess energy to the surroundings and very back to original energy level. This excess energy is emitted as light. This emitted light will correspond to a definite energy jump in the atom and is characteristic of the atom in question. The emitted light for each alkali metal atom will correspond to definite energy jumps corresponding to some definite frequency of the visible part of the spectrum. For this reason, alkali metal compounds impart characteristic colour to the flames, see.

(vi)    Electrical Conductivity

It has been established that in metals, it is the valence electrons that hold the individual atoms together in the crystal. In fact, metals are often described as “islands” of shielded nuclei in a “sea” of valence electrons. Among the alkali metals, this “sea” is very diffuse and the binding in the solid is relatively weak. Therefore the valence electrons move freely form one metal ion to another without much difficulty with the result that these metals have high electrical conductivity.

(vii)    Ionic Compound Formation

Since these elements are highly electropositive, they react readily with highly electronegative elements by the transfer of their s-electron. Therefore, the atoms in their compounds so formed have large electronegativity differences resulting in the formation of ionic bonds.

However, these elements also show covalent bonding in certain cases. For example, the vapours of alkali metals contain some diatomic molecules such a Na2,Cs2 which are covalently bonded. The strength of the covalent bond in diatomic molecules decreases down the group. Another proof of their ability to form covalent compounds is furnished by the formation of organo-metallic compounds such as CH3,Li,C2H5Li, C6H5CH2Na.

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