Physical Properties of Alkaline Earth Metals

The group 2 of the periodic table consist of 6 elements .These are Beryllium (Be), magnesium(Mg), Calcium (Ca), strontium(Sr),  barium(Ba), radium (Ra).

The name alkaline earth was given since the oxides are alkaline in nature and remain unaffected by heat or fire and exist in Earth’s crust.

Alkaline earth metals are also highly reactive and hence do not occur in the free state but are widely distributed in nature in the combined state as silicates, carbonates, sulphates and phosphate.

Beryllium is found in small quantities as silicate minerals, beryl Be₃Al₂Si₆O₈ and phenacite Be₂SiO₄.

Magnesium is the sixth most abundant element by weight found in the earth’s crust as carbonate ,sulphate and silicate. It also occurs to about 0.13% in sea water as chloride and sulphate.

Calcium is the fifth most abundant element by weight found in the earth crust. It mainly  occurs as CaCO₃ in form of limestone, marble and chalk. Other important minerals of calcium are fluorite or fluorspar CaF₂, gypsum CaSO₄.2 H₂O and anhydrite CaSO₄.

Strontium is mined as  celestite SrSO₄ and Strontianite SrCO₃ and barium is mined as barytes , BaSO₄.

 

(1) Electronic configuration

(2) Atomic radii

Atomic and ionic radii of alkaline earth metals are fairly large though smaller than the corresponding alkali metals and these increases down the group.

Reason: The alkaline earth metals have a higher nuclear charge ,and ,therefore the electrons are attracted more strongly towards the nucleus. As a result ,their atomic and ionic radii are smaller than those of the corresponding alkali metals.

On moving down the group, the atomic and ionic radii increases due to addition of an extra shell of electrons in each succeeding element and the increasing screening effect.

(3) Ionization Enthalpy

The alkaline earth metals have fairly low ionization enthalpies though greater than those of the corresponding elements of group 1 and these decreases down the group.

Reason: The low ionisation enthalpies of the alkaline earth metals is because of their strong tendency to lose electrons due to their smaller nuclear charge and comparatively larger atomic size which results in weaker forces of attraction between the valence electrons and the nucleus. The values of the first ionization enthalpy of elements of Group 2 are greater than those of the elements of group 1 because the atoms of alkaline earth metals have smaller size and higher nuclear charge than those of alkali metals.

On moving down the group, ionization enthalpy values go on decreasing because of the increase in atomic size due to addition of the new shells and screening effect of the electrons in the inner shell which overweigh the effect of increased nuclear charge.

 

The second ionization enthalpies of the elements of group 1 are higher than those of elements of group 2.

Reason:  The second electron in case of alkali metal is to be removed from a cation which has already acquired the stable noble gas configuration whereas in case of alkaline earth metal, the second electron is to be removed from a cation which is yet to acquire the stable noble gas configuration. Therefore, removal of second electron in case of alkaline earth metals requires much less energy than that in case of alkali metals.

The third ionization enthalpy of magnesium will be very high because now the electron has to be removed from the stable noble gas configuration.

(4) Hydration Enthalpy

The hydration enthalpies of alkaline earth metal ions decreases as the size of the metal ion increases down the group i.e. Be²⁺ > Mg²⁺ >Ca²⁺ > Sr²⁺ > Ba²⁺

Reason: Due to smaller size of alkaline earth metal ions as compared to alkali metal ions ,the hydration enthalpies of alkaline earth metal ions are larger than those of alkali metal ions. The compounds of alkaline earth metals are more extensively hydrated than those of alkali metals.

(5) Dipositive Oxidation state (M²⁺)

The alkaline earth metals have two electrons more than the nearest noble gas configuration. Therefore, they can easily lose these two electrons to form divalent cation. Alkaline earth metals uniformly show an oxidation state of +2.

Alkaline earth metal prefer to form divalent ions rather than monovalent ions

(1) The divalent cation of alkaline earth metal acquire stable inert gas configuration.

(2) The existence of divalent ions in the solid state is due to the reason that divalent cations due to smaller size and higher charge form stronger lattices then monovalent cations. As a result , a larger amount of lattice enthalpy is released during the formation of compounds containing M²⁺ ions than in the formation of compounds containing M⁺ ions. It is this greater lattice enthalpy of M²⁺ ions  which more than compensates for the higher second ionization enthalpy thereby making M²⁺ ions more stable than M⁺ ions.

(3) The existence of divalent ions in the aqueous solution is due to greater enthalpy of hydration of the divalent ions which counterbalance the higher value of second ionization enthalpy.

The enthalpy of hydration of MgCl₂ is much higher than that of MgCl. It is this higher enthalpy of hydration which more than compensates for the higher value of second ionisation enthalpy.

 

(6) Electropositive or Metallic Character

The alkaline earth metals are highly electropositive and hence metallic and their electropositive or metallic character increases down the group. They are less electropositive or metallic than the alkali metal.

Reason: Due to low ionisation energies, the alkaline earth metals have a strong tendency to lose both the valence electrons to form dipositive cations. Thus, these elements show strong electropositive or metallic character.

On moving down the group, the atomic radii increases and ionization enthalpy decreases. Consequently the electropositive or metallic character increases.

Since the atoms of the alkaline earth metals have smaller size and higher ionization enthalpies as compared to corresponding alkali metals, their tendency to lose valence electrons is lesser than those of alkali metals.

Alkaline earth metals have less electropositive or metallic character as compared to alkali metals.

Reason: Due to smaller size of the cation and greater number of valence electrons, the metallic bonding in alkaline earth metal is stronger as compared to alkali metal. Because of this reason ,these metals are less soft then alkali metals.

(7) Melting and Boiling Point

The alkaline earth metals have higher melting and boiling point as compared to those of alkali metals . Down the group , there is no regular trend in their melting and boiling point.

Reason : Because of their smaller size and more closed packed crystal lattice as compared to alkali metals, their  melting and boiling points are higher than those of group 1 elements.

(8) Nature of bonds formed

Like alkali metals, alkaline earth metals form ionic compounds which are less ionic than the corresponding alkali metal compounds. The tendency to form ionic compounds increases down the group. The first member , Be, forms covalent compounds. Mg also shows some tendency for covalency. All other elements form ionic compounds.

Reason: They form ionic compounds because they have low ionization enthalpies. Their compounds are less ionic because their ionization enthalpies are higher than those of the corresponding alkali metals. The tendency to form ionic compounds increases down the group because ionization enthalpy decreases. Beryllium ,however, form covalent compounds because it has smaller size and high ionization enthalpy.

 

(9) Density

The alkaline earth metals are denser than the alkali metals due to smaller size and better backing in the crystal lattice. The densities of alkaline earth metal do not show any regular trend with increasing atomic number. The density of these metal first decreases from Be to Ca and then increases from Ca to Ba.

Reason : Because of their smaller size and hence better packing as compared to alkali metals, they are denser than alkali metal. The decrease in density from Be to C may be due to decrease in packing of atoms in their solid lattice.

(10) Flame Colouration

Like alkali metal salts, alkaline earth metal salts also impart a characteristic colour to the flame.

Calcium- brick red

Strontium – Crimson red

Barium- apple green

Radium- crimson

When alkaline earth elements and their compounds are put into a flame, the electrons absorb energy and are excited to higher levels. when they return to their ground state, The absorbed energy is emitted in form of visible light of a particular wavelength.

Beryllium and magnesium atoms are smaller in size and their electrons are strongly held by the nucleus. They need large amount of energy for excitation of electrons to higher energy levels which is not available in the bunsen flame. So they do not impart colour to the flame.