jueves, 28 de enero de 2010

Alkali Metals

The six elements belonging to group 1 of the periodic table, namely lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr) are called alkali metals. They constitute alkali metals because they readily dissolve in water to form hydroxides, which are strongly alkaline in nature. They also form alkaline oxides. The element francium is radioactive.


Lithium occurs mainly as silicate minerals such as spodumene [LiAl(SiO3)2], lepidolite [(Li,Na,K)2Al2,(SiO3)3 (F,OH)2] etc. It is the 35th most abundant element by weight.
Compounds of sodium and potassium have been known from ancient times. Sodium and potassium are seventh and eighth most abundant elements by weight in the Earth's crust and together make up over 4% of the Earth's crust by weight. NaCl and KCl also occur in large amounts in seawater. Rock salt (NaCl) is the major source of sodium. Potassium occurs mainly as deposits of KCl (sylvite), which is a mixture of KCl and NaCl (sylvinite) and double salt KCl.MgCl2.6H2O (carnallite).
Rubidium and cesium are obtained as a by-product of lithium processing. Francium being radioactive does not occur appreciably in nature.

Electronic configurations

All the alkali metals have one electron in their outermost 's' orbitals preceded by the noble gas configuration. Thus, the general configuration of alkali metals may be written as [Noble gas] ns1 where 'n' represents the valence shell. The electronic configurations of alkali metals are:

The electronic configurations of alkali metals

ElementSymbolAtomic No.Electronic configuration

General Characteristics of Alkali Metals

Atomic and ionic radii

Being the first elements of each period, alkali metals have the largest atomic and ionic radii in their respective periods. As we move within a period, the atomic radius and ionic radius tend to decrease due to increase in the effective nuclear charge. On moving down the group, there is increase in the number of shells and, therefore, atomic and ionic radii increase.
Physical propertyLiNaKRbCs
Atomic radius (pm)152186227248264
Ionic radius (pm)76102138152167

Ionization energies

  • Alkali metals have the lowest ionization energy in each period. Within the group, as we go down, the ionization energies of alkali metals decrease due to their atomic size being the largest in their respective periods. In large atoms the valence electrons are loosely held by the nucleus and are easily lost, leading them to have low ionization energies and acquiring stable noble gas configurations. On moving down the group, the atomic size increases and the number of inner shells also increases, increasing the magnitude of screening effect and consequently, the ionization energy decreases down the group.
  • The second ionizations energies of alkali metals are very high. The removal of an electron from alkali metals causes the formation of monovalent cations having very stable electronic configurations (same as that of noble gases). Therefore, it becomes very difficult to remove the second electron from the stable noble gas configurations, giving very high second ionization energy values (IE2).
Physical propertyLiNaKRbCs
Ionization Energy        I520496419403376
KJ mol -1                     II72984562305126332230

Melting and boiling points

All alkali metals are soft and have low melting and boiling points. As alkali metals have only one valence electron per metal atom, the energy binding the atoms in the crystal lattice of the metal is low. Consequently, the metallic bonds in these metals are not very strong and their melting and boiling points decrease on moving down the group.
Physical propertyLiNaKRbCs
Melting point (K)453.5370.8336.2312.0301.5
Boiling point (K)16201154.41038.5961.0978.0


The densities of alkali metals are low as compared to other metals, with Li, Na and K being even lighter than water (K is lighter than Na). While alkali metals do have close packing of metal atoms in their lattice the large size of their atoms cause them to have low densities. As we move down the group from Li to Cs, even though there is an increase in atomic size, the simultaneous increase in atomic mass compensates more than the increase in atomic size. The densities (mass/volume) of alkali metals thus gradually increase from Li to Cs. However, potassium is lighter than sodium probably due to increase in atomic size of potassium.
Physical propertyLiNaKRbCs
Density (g cm -1)0.530.970.861.531.90

Electropositive or metallic character

The electropositive character of an element is expressed in terms of the tendency of its atom to release electrons:

All the alkali metals are strongly electropositive or metallic in character, since they have low ionization energies and their atoms readily lose the valence electron. As the ionization energies decrease down the family, the electron releasing tendency or electropositive character is expected to increase down the family.
Physical propertyLiNaKRbCs
Eo value (V)- 3.03- 2.71- 2.93 - 2.93- 2.92

Oxidation states

All alkali metals have only one electron in their valence shell. They exhibit an oxidation state of +1 in their compounds and can lose the single valence electron readily to acquire the stable configuration of a noble gas. Thus, they form monovalent ions, M+ (e.g., Li+, Na+, K+, Rb+, Cs+). Since the second ionization energies are very high, they cannot form divalent ions. Thus, alkali metals are univalent and form ionic compounds.

Characteristic flame coloration

As the alkali metals have very low ionization energies, the energy from the flame of a bunsen burner is sufficient to excite the electrons of alkali metals to higher energy levels. The excited state being unstable, these electrons return to their original energy levels, emitting extra energy, which gives characteristic flame colorations. The different colors of the alkali metals can be explained on the basis of amount of energy absorbed for excitation of the valence electron.
Physical propertyLiNaKRbCs
Flame colourcrimson redyellowpale violetvioletbluish

Photoelectric effect

When electromagnetic radiation strikes on the surface of alkali metals, they emit electrons. This is called the photoelectric effect. This occurs as alkali metals have low ionization energies, which allows the electrons to be easily ejected when exposed to light. Among alkali metals, cesium has lowest ionization energy and hence it can show photoelectric effect to the maximum extent.

Nature of the compounds

The compounds of the alkali metals are ionic in nature. Alkali metals form cations readily by losing the valence electrons (due to the low ionization energies and large atomic sizes). They go on to form ionic bonds with the non-metals of the 'p' block.

Lattice energies

The lattice energies of alkali metal salts are very high because strong electrostatic forces of attraction hold up the cations and anions formed.
Lattice energy gives a measure of the forces of attraction between the ions. It is defined as the amount of energy required to break one mole of a crystal into its free ions.

definition of lattice energy

The larger the forces of attraction, the greater will be lattice energy. The lattice energy also depends upon the size of the ion and its charge. For the cation of same valency, ionic solids having the same anion will display a decrease in the lattice energy with increase in size of the cation. This is due to decrease in forces of attraction between them.

Source: http://www.tutorvista.com/content/chemistry/chemistry-iii/s-block-elements/alkali-metals.php