atomic structure

Group II-A Elements Alkaline Earth Metals Occurrence, Physical and Chemical Properties, Reactivity and Compounds – Chemistry

Group II-A Elements

Elements of Group I I-A are called alkaline earth metals. This is due to the reason that they are earth-making alkaline metals. Their oxides and hydroxides are alkaline.

  Alkaline Earth Metals ( Group II A) are as follow:

  Be (Beryllium)

 Mg (Magnesium)

 Ca (Calcium)

 Sr  (Strontium)

 Ba (Barium)

 Ra (Radium)

Alkaline earth metals are the most reactive metals in the periodic table after alkali metals (Group IA).

Group II A elements having a valence shell configuration of ns2 (full filled orbitals).

Alkaline earth metals (Group II A elements) loses 2 electrons to form a cation with charge +2 and also oxidation state of +2.

Occurrence: 

Magnesium:

It is very abundant in rocks of the earth’s crust. It is found in seawater and springs water. Its minerals are Dolomite (MgCO3.CaCO3), Magnesite (MgCO3), Epsom salt (MgSO4.7H2O), Soapstone (Talc) [Mg2(Si2O5)2.Mg(OH)2], and Asbestos [CaMg3(SiO3)4].

Calcium:

It is very abundant in rocks of the earth’s crust. It is present in seashells. Its important minerals are Calcite or Limestone or Marble (CaCO3), Gypsum (CaSO4.2H2O).

General Physical Properties of Alkaline earth metals

 (group II-A):

(i)  Atomic radii: Their atomic radii increase down the group due to the addition of more and more shells.

(ii)  Ionization Energies: Their ionization energies decrease down the group. However, Ra shows slightly higher ionization energy than of Ba. 

(iii)  Electronegativity: Their electronegativities are very low. Their electronegativities decrease down the group. 

(iv) Melting boiling points: Their melting and boiling points have no regular trend. 

(v)  Flame test: They give characteristic colors to Bunsen burner flame. When alkaline earth metals are heated in the flame their electrons jump to higher orbits. Then on dropping back the electrons emit visible light of different colors.

Element Mg Ca Sr Ba

Color of flame Bright white Brick red Crimson red Green

Chemical Properties:

Alkaline earth metals are reactive but less reactive than alkali metals.

Reaction with Water:

Alkaline earth metals react slowly with water. Their reactivity with water increases down the group. Be do not react with water, Mg reacts with boiling water or steam and produces magnesium oxide and liberates hydrogen gas. 

 Mg(s) + H2O(g) → MgO(s) + H2

While others (Ca, Sr, Ba) can react with cold water and produce metal hydroxides along with hydrogen gas.

 Ca(s) + 2H2O(g) → Ca(OH)2(aq) + H2

Reaction with Oxygen:

               All they react with oxygen except Be and produce normal oxides. Be do not react due to having a strong protective layer of BeO, however powdered Be may react with oxygen.                                                  

2Ca(s) + O2(g) → 2CaO(s)

2Sr(s) + O2(g) → 2SrO(s)

Sr and Ba on heating in air produce peroxides. But this ability is not shown by Be, Mg and Ca .

Heat  Sr(s) + O2(g) → SrO2(s)

  • Solubility of oxides increases down the group.
  • Oxides of group II-A are basic.
  • CaO is called quick lime. Its reaction with water produces Ca(OH)2 and the reaction is called slaking. Solid Ca(OH)2 is called slaked lime while an aqueous solution of Ca(OH)2 is called lime water.  

Reaction with Nitrogen:

               All they react with the nitrogen of the air on heating and produce their nitride. These nitrides are ionic and solid. However, nitride of Be is covalent.

  Heat      3Mg(s) +N2(g) → Mg3N(s)

Compounds of Alkaline Earth Metals (Group II-A):

Hydroxides Solubility:

Solubility of hydroxides of alkaline earth metals increases down the group. It means their solubility products increase down the group.

(1) Be(OH)2 , Ksp = 1.6 Í1026 (2) Mg(OH)2 , Ksp = 8.6 Í1012 (3) Ca(OH)2 , Ksp = 1.3 Í104

(4) Sr(OH)2 , Ksp = 3.2 Í104 (5) Ba(OH)2 , Ksp = 5.4 Í103

Explanation:

Solubility depends upon two factors Lattice energy and Heat of Hydration.

  1. Lattice energy: Higher is lattice energy lower will be the solubility.
  2. Heat of hydration: Higher is the heat of hydration higher will be solubility.

Lattice energy decreases down the group so solubility should increase down the group. The heat of hydration also decreases down the group so solubility should decrease down the group. The two factors are acting oppositely. Lattice energy decreases gradually down the group. On the other hand heat of hydration decreases little down the group due to the smaller size of hydroxyl ion, therefore, the dominating factor is lattice energy. Hence solubility of hydroxide increases down the group.

  • Mg(OH)2 is less soluble in water produce suspension in water. The suspension of Mg(OH)2 is called milk of magnesia. It is used for the treatment of acidity of the stomach.
  • Solid Ca(OH)2 is called slaked lime while an aqueous solution of Ca(OH)2 is called lime water. It is used for qualitative and quantitative analysis of CO2.
  1. Sulphates:

Solubility:

Solubility of sulphates of alkaline earth metals decreases down the group. It means their solubility products decrease down the group. CaSO4 is sufficiently soluble in water while BaSO4 is almost insoluble in water. 

 Explanation:

Solubility depends upon two factors Lattice energy and Heat of Hydration.

  1. Lattice energy: Higher is lattice energy lower will be the solubility.
  2. Heat of hydration: Higher is the heat of hydration higher will be solubility.

Lattice energy decreases down the group so solubility should increase down the group. Heat of hydration also decreases down the group so solubility should decrease down the group. The two factors are acting oppositely. Lattice energy decreases gradually down the group, therefore, its effect is not significant. On the other hand heat of hydration decreases rapidly down the group, therefore, it is the dominant factor and thus solubility of sulphates decreases down the group.

  1. Carbonates:

Solubility:

 All the carbonates of alkaline earth metals are insoluble in water. However, they are soluble in acids.

Thermal Stability:

               Thermal stability of carbonates of alkaline earth metals increases down the group and it is evident from their temperature of decomposition.

 Explanation:

Two factors are responsible for the stability of the carbonates.

(i) Smaller is the metal ion more is the lattice energy of the resulting metal oxide and hence higher is the stability of the oxide and the carbonate forming this oxide will be less stable. Thus going down the group stability of oxide decreases so the stability of their carbonates increases.

  1. A cation with high charge density causes significant polarization of anion. This creates some covalent character in the compound that assists in its thermal decomposition. Thus, as the size of the cation increases down the group, its charge density decreases accordingly. Therefore its polarizing power also decreases in the same order. This means that the covalent character of carbonates decreases down the group. Hence there is a corresponding increase in thermal stability of carbonates going down the group.

The thermal decomposition of carbonates produces metal oxide and carbon dioxide.

   CaCO3(s) → CaO(s) + CO2(g)

  1. Nitrates

Thermal Stability:

 Thermal stability of nitrates of alkaline earth metals increases down the group.

  Explanation:

Two factors are responsible for the stability of nitrates.

  1. Smaller is the metal ion more is the lattice energy of the resulting metal oxide and hence higher is the stability of the oxide and the nitrate forming this oxide will be less stable. Thus going down the group stability of oxide decreases so the stability of their nitrate increases.
  2. A cation with high charge density causes significant polarization of anion. This creates some covalent character in the compound that assists in its thermal decomposition. Thus, as the size of the cation increases down the group, its charge density decreases accordingly. Therefore its polarizing power also decreases in the same order. This means that the covalent character of nitrates decreases down the group. Hence there is a corresponding increase in thermal stability of nitrates going down the group.

Thermal decomposition of nitrates produces metal oxide, nitrogen dioxide and oxygen.

   2Ca(NO3)2(s) → 2CaO(s) + 4NO2(g) + O2(g)

Also Read !!

Group 1 Elements alkali Metals Occurrence, Physical and Chemical Properties, Reactivity and Compounds

Trends in the physical properties of period 3 elements

Trends in the chemical properties of period 3 elements

Anomalous Behavior of Lithium

Electron affinity trends

Ionization Energy Trends

Electronegativity – Definition, Periodic Trends ,Examples , Importance ,Electronegativity Difference

Ionic Bond

Covalent Bond

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