Chemistry of Alkaline Earth Metals

Class 11 Chemistry Chapter 10 s-block elements



Alkaline earth metals are also known as Group II elements. These elements are Beryllium, Magnesium, Calcium, Strontium, Barium and Radium. The name alkaline earth suggest that these metals form alkalies with water and are found in earth’s crust. In this article let’s discuss in detail about alkaline earth metals.

Electronic Configuration of Alkaline Earth Metals

The general electronic configuration of group II elements is given by [Noble Gas]ns2. Two electrons from the last shell are loosely bounded to the nucleus hence can be easily donated. Thus, valence of group II elements is 2. By donating electrons, these elements form mostly ionic compounds.

Radius of Alkaline Earth Metals

The atomic radii of alkaline earth metals are the larger than any other element in the corresponding period, however, smaller than alkali metals. As we move down the group, atomic radii increases. The ionic radii are smaller than atomic radii of respective element. However, ionic radii of element also increases as we go down the group.

Ionization Enthalpy

Group II elements have low ionisation enthalpies. It go on decreasing down the group from Be to Ra. It is because the nuclear charge on the outermost shell electron decreases with increase in size. However, it is observed that, first ionization enthalpies of group II elements are higher than group I elements. The reason behind is smaller size of group II elements than group I elements. But the second ionization enthalpy of group II elements is lower than group I elements. This is because group I elements already attained stable electronic configuration. Thus it is difficult to remove one more electron as compared to group II elements.

Physical Properties of Alkaline Earth Metals

Alkaline earth metals are silvery white, lustrous and soft. These metals are harder than group I alkali metals. Beryllium and magnesium have greyish appearance. Moreover, they have smaller size as compared to alkali metals. Density of alkaline earth metals is higher than alkali metals. Atomic size goes on increasing as the size increases. Alkaline earth metals have higher melting and boiling points, than alkali metals because of smaller size. Due to low ionisation enthalpies, group II metals are strongly electropositive in nature. Electropositivity increases down the group from Be to Ba. alkaline earth metals when burnt in presence of oxygen produce flame color. Calcium, Strontium and Barium produce flame of brick red, crimson and apple green colors. In Beryllium and Magnesium flame is not coloured, as electrons are too strongly attached to nucleus.

Chemical Properties of Alkaline Earth Metals




Alkaline earth metals are less reactive as compared to alkali metals. As we move down the group with increase in atomic size, the chemical reactivity increases.

  • Reactivity Towards Air: Alkali earth metals burn in presence of oxygen thereby, forming oxides. Beryllium, magnesium, calcium form monoxide. Strontium, Barium forms peroxides. Beryllium and Magnesium undergo oxidation forms oxide layer on the surface, this prevents further oxidation of metals. Powdered beryllium burns brilliantly on ignition in air. Magnesium is more electropositive than beryllium. It burns with excessive bright light.
                             ∆
            2Mg + O2 → 2MgO

                            ∆
            Ba + O2 →BaO2

                            ∆
            3Be + N2 → Be3N2

  • Reaction with water: Alkaline earth metals react with water to form corresponding hydroxide and led to evolution of hydrogen gas. Be does not react even with boiling water, Mg react with boiling water, Ca, Sr, Ba have high reactivity with cold water. Order of reactivity of these elements with water is Mg less than Ca less than Sr less than Ba.
             Mg + 2H2O → Mg2+ + 2OH + H2(↑)

  • Reactivity towards dihydrogen: At high temperatures, Alkaline earth metals except Be, combine directly with hydrogen, so as to form metal hydride of MH2 type. These are ionic solids having high melting point.
                           ∆
            M+ H2 → M+H2 (where M=Mg, Ca, Sr, or Ba)



  • Reactivity towards Halogen: All alkaline earth metals react with the halogen at high temperature so as to form ionic crystalline halides with general formula MX2.
                           ∆
            M + X2 → MX2

  • Reactivity Towards Acids: Alkaline earth metals on treatment with acids liberate hydrogen gas.
                           ∆
             M + 2HCl → MCl2 + H2 (Where M → Alkaline earth metals)
  • Reducing nature: Alkaline earth metals are strong reducing agents like alkali metals. But the reducing power is less than that of alkali metals. In case of Beryllium, Beryllium have less negative electrode potential in comparison to the other group members. Thus small size has hydration enthalpy and relatively large atomization enthalpy.

Uses of Alkaline Earth Metals

  • Uses of Beryllium: Beryllium is used for making alloys. Copper – Beryllium alloys are used for manufacturing high strength springs. Beryllium metal is also used to make X-ray tube windows.
  • Uses of Magnesium: It is also used for making alloys. Magnesium-aluminium alloys are lightweight so these are used in making aircraft parts. Magnesium powder and ribbon is used in flash powders, bulbs, incendiary bombs and signals. Milk of magnesia i.e. aqueous solution of magnesium hydroxide is used as antacid in medicine. Magnesium carbonate is present in our toothpaste.
  • Uses of Calcium and Barium: Calcium is helpful during extraction of metals from their oxides particularly when it is difficult to reduce with carbon. Calcium and barium metals show increase in reactivity with oxygen and nitrogen with increase in temperature. Thus these are useful in removing air from vacuum tubes.
  • Uses of Radium: Radium salts are used in radiotherapy, For example, in the treatment of cancer.



Some General Characteristics of Compound of Alkaline Earth Metals

  • All alkaline earth metals undergo oxidation to form metal oxides. For example: BeO, MgO, CaO, SrO, BaO, RaO, etc. Except BeO all other oxides of alkaline earth metals forms rock salt. BeO is amphoteric in nature while other oxide is basic in nature. These oxides are sparingly soluble in water.
  • As we move down the group thermal solubility and basic character increases with increase in atomic number. Hydroxide of alkaline earth metal is less basic when compared to alkali metals.
  • Alkaline earth metals combine with halogens at high temperatures forming their halides. Beryllium halides are covalent in nature and these are soluble in organic solvents, while halides of other group II elements are ionic, hence, these are soluble in polar solvents.
  • Alkaline earth metals forms salts of oxoacids, which include carbonates, sulphates and nitrates. Carbonates formed by alkaline earth metals are insoluble in water however, precipitate is formed when sodium or ammonium carbonate solution is added. Sulphates of alkaline earth metals are white solids. BeSO4 and MgSO4 are readily soluble in water. However, decrease in solubility is seen from CaSO4 to BaSO4. Alkaline earth metals form nitrates when their carbonates are dissolved in nitric acid. Also, nitrates decompose on heating to form oxides.
             2M(NO3)2 → 2MO + 4NO2 + O2 where (M = Be, Mg, Ca, Sr, Ba)

Keywords: Hydration enthalpy, Radiotherapy, Ionization enthalpy, Atomic radii, Ionic radii, Oxoacids, Electronic Configuration, Alkali Metals.

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Comments

  1. Raunak Sethi

    I spent large time to find something like this

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