Group 2 Elements - Alkaline Earth Metals

GROUP 2 ELEMENTS - ALKALINE EARTH METALS

 

 The group 2 elements comprise beryllium, magnesium, calcium, strontium, barium and radium. They follow alkali metals in the periodic table. These (except beryllium) are known as alkaline earth metals. The first element beryllium differs from the rest of the members and shows diagonal relationship to aluminium.

 

Electronic Configuration of alkaline earth metals

These elements have two electrons in the s -orbital of the valence shell. Their general electronic configuration may be represented as [noble gas] ns2. Like alkali metals, the compounds of these elements are also predominantly ionic.



 

Atomic and Ionic Radii of alkaline earth metals

The atomic and ionic radii of the alkaline earth metals are smaller than those of the corresponding alkali metals in the same periods. This is due to the increased nuclear charge in these elements. Within the group, the atomic and ionic radii increase with the increase in atomic number.

 

Ionization Enthalpy of alkaline earth metals

The alkaline earth metals have low ionization enthalpies due to fairly large size of the atoms. Since the atomic size increases down the group, their ionization enthalpy decreases. The first ionisation enthalpies of the alkaline earth metals are higher than those of the corresponding Group 1 metals. This is due to their small size as compared to the corresponding alkali metals. It is interesting to note that the second ionisation enthalpies of the alkaline earth metals are smaller than those of the corresponding alkali metals.

 

Hydration Enthalpy of alkaline earth metals

Like alkali metal ions, the hydration enthalpies of alkaline earth metal ions decrease with increase in ionic size down the group.

Be2+> Mg2+ > Ca2+ > Sr2+ > Ba2+

The hydration enthalpies of alkaline earth metal ions are larger than those of alkali metal ions. Thus, compounds of alkaline earth metals are more extensively hydrated than those of alkali metals, e.g., MgCl2 and CaCl2 exist as MgCl2.6H2O and CaCl2· 6H2O while NaCl and KCl do not form such hydrates.

 

Physical Properties of alkaline earth metals

 The alkaline earth metals, in general, are silvery white, lustrous and relatively soft but harder than the alkali metals.

Beryllium and magnesium appear to be somewhat greyish. 

Calcium, strontium and barium impart characteristic brick red, crimson and apple green colours respectively to the flame.

In flame the electrons are excited to higher energy levels and when they drop back to the ground state, energy is emitted in the form of visible light. The electrons in beryllium and magnesium are too strongly bound to get excited by flame. Hence, these elements do not impart any colour to the flame.

The flame test for Ca, Sr and Ba is helpful in their detection in qualitative analysis and estimation by flame photometry. The alkaline earth metals like those of alkali metals have high electrical and thermal conductivities which are typical characteristics of metals.

 

 

Chemical Properties of alkaline earth metals

 

Reactivity of alkaline earth metals towards air

 

The affinity towards oxygen increases down the group. Thus. Be. Mg and Ca when heated with O2 form monoxides while Sr. Ba and Ra form peroxides.

Reactivity of alkaline earth metals towards water

Group-2 elements are less reactive with water as compared to alkali metals.

M + 2H2O → M(OH)2 + H2 (where, M = Mg, Ca, Sr or Ba)

Be does not react even with boiling water and Ba react vigorously even with cold water. Thus. increasing order of reactivity with water is

Mg < Ca < Sr < Ba

 

Reactivity of alkaline earth metals towards halogens

All the elements of group-2 combine with halogens at high temperature, forming their corresponding halides (MX2).

 

Beryllium halides (BeF2, BeCI2, etc) are covalent, hygroscopic and fume in air due to hydrolysis, BeC12 exists as a dimer, The halides of other alkaline earth metals are fairly ionic and this character increases as the size of the metal increases.

The halides are soluble in water and their solubility decreases in the order

MgX2 > CaX2 > SrX2 > BaX2

 

Reactivity of alkaline earth metals  towards dihydrogen

All the elements except beryllium combine with hydrogen upon heating to form their hydrides, MH2.

BeH2, however, can be prepared by the reaction of BeCl2 with LiAlH4.

 

Reducing nature of alkaline earth metals

All the alkaline earth metals are strong reducing agents because of their lower electrode potentials but these are weaker than the corresponding alkali metals.

As we move down the group from Be to Ra, the reducing character increases due to decrease in ionisation enthalpy.

 

Solutions in liquid ammonia of alkaline earth metals

Like alkali metals, the alkaline earth metals dissolve in liquid ammonia to give deep blue black solutions forming ammoniated ions.

 

From these solutions, the ammoniates, [M(NH3)6]2+ can be recovered.

 

Uses of alkaline earth metals

·        Beryllium is used in the manufacture of alloys.

·        Copper-beryllium alloys are used in the preparation of high strength springs. Metallic beryllium is used for making windows of X-ray tubes.

·        Magnesium forms alloys with aluminium, zinc, manganese and tin.

·        Magnesium-aluminium alloys being light in mass are used in air-craft construction.

·        Magnesium (powder and ribbon) is used in flash powders and bulbs, incendiary bombs and signals. A suspension of magnesium hydroxide in water (called milk of magnesia) is used as antacid in medicine. Magnesium carbonate is an ingredient of toothpaste.

·        Calcium is used in the extraction of metals from oxides which are difficult to reduce with carbon.

·        Calcium and barium metals, owing to their reactivity with oxygen and nitrogen at elevated temperatures, have often been used to remove air from vacuum tubes. Radium salts are used in radiotherapy, for example, in the treatment of cancer.