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Characteristics of the Compounds of Alkaline Earth Metals

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  • Last Updated : 02 Dec, 2021

All the elements that exist in nature are arranged in a periodic table after several years of research work, these are placed in groups and rows based on some predefined criteria. Some elements may not follow the criteria but still, they are placed in the same column or group due to their similarities in mass number or similarities in properties. Alkaline earth metals are placed in Group 2, this group has the following elements beryllium, magnesium, calcium, strontium, barium and radium. Beryllium has different properties from the rest of the elements and it follows a diagonal relationship with aluminium.

Characteristics of Alkaline Earth Metals 

  • Radii: Atoms have two types of radii one is Metallic radii and the other is Ionic radii. Atomic and ionic radii both follow the same trend means the atomic size of alkaline earth metals increases down the group. Metallic radii are greater than ionic radii. Both follow the same trend top to down as well as left to right in a periodic table. 

Be< Mg< Ca< Sr< Ba< Ra

  • Ionization Enthalpy: Ionization enthalpy is of two types: Ionization enthalpy (I) and Ionization enthalpy (II). Ionization enthalpy (II) of each element is greater than ionization enthalpy (I), though both enthalpies are higher than ionization enthalpies of alkali metals. Ionization enthalpy(I) and Ionization enthalpy(II) both decreases down the group.

Be> Mg> Ca> Sr> Ba> Ra

  • Electronic Configuration: The electronic configuration of alkaline earth metals can be represented by a noble gas and two valence electrons are placed in s orbital. So the representation is like [Noble Gas] ns2.
  • Standard Potential: Standard potential of alkaline earth metals follows the opposite trend as that of atomic size means the negative standard potential of ions decreases down the group as atomic size increases. 

Be> Mg> Ca> Sr> Ba> Ra

Physical Properties of Alkaline Earth Metals

  1. Alkaline earth metals are shiny in nature.
  2. This category of elements is placed in soft metals as almost all elements of this group are soft.
  3. The melting and boiling point of alkaline earth metals is quite higher than those of alkali metals.
  4. On being exposed to heat electrons get excited and when they came back to their energy levels they show colours.
  5. These metals have free electrons which makes them conductive in nature.
  6. Alkaline earth metals have high ionization energy.
  7. Among this group beryllium and magnesium do not give any colour on being exposed to flame as electrons in these elements are tightly bound.
  8. Alkaline earth metals are denser than alkali metals.
  9. Among alkaline earth metals, the density of radium is the highest. 
  10. These metals are mostly hydrated in nature. 

Chemical Properties of Alkaline Earth Metals

  • Alkaline earth metals react with water to form hydroxides. Among group 2 elements Be and Mg are the only elements that do not react with water because of the protected layer which formed on the surface of these alkaline metals.
  • These also react with air to form different oxides of metals. Among group 2 elements Be and Mg are the only elements that do not react with air also because of the protected layer which formed on the surface of these metals.
  • These elements react with the halogen group which consists of fluorine, chlorine, bromine and iodine to form corresponding halides.

M + X2  → MX2

(where X is a halogen and M is an alkaline earth metal)

  • When heated, alkaline earth metals can react with hydrogen to form corresponding hydrides. Hydride of calcium is also known as Hydrolith. Hydrides of alkaline earth metals are metallic hydrides except for Be and Mg which forms covalent hydrides.

X + H2  → 2XH2

(where X is an alkaline earth metal)

  • These elements have a tendency to react with acids to release hydrogen gas.

X + 2HCl  → XCl2 + H2

  • The nature of alkaline earth metals is reducing in nature.
  • Alkaline earth metals form carbides when they react with carbon.

X + 2C → XC2XC2 

(where X is alkaline earth metal except Be)

  • Carbides of alkaline earth metals are used to produce acetylene gas by their reaction with water.

XC2XC2 + 2H2O → X(OH)2 + C2H2

  • Alkaline earth metals also have the ability to form complexes.
  • The liquid solution of alkaline earth metals with ammonia is paramagnetic in nature.

Uses of Alkaline Earth Metals

  1. Magnesium is mostly used in fireworks due to its burning property.
  2. Magnesium is also used in medicines to make pills used in pregnancy complications.
  3. Radium isotopes are used in nuclear reactors.
  4. Calcium is used in making cement.
  5. Barium is used in making spark plugs for automobiles.
  6. Calcium is used to make calcium carbide which in turn is used to make different types of plastics.
  7. Alkaline earth metals release hydrogen gas when react with acids, this hydrogen gas can be used for other purposes.  
  8. Alkaline earth metals are used in making different types of cells known as Alkaline Fuel Cells (AFC).
  9. Strontium is used in the refining process of zinc and in the manufacturing of magnets.
  10. Strontium is also being used by the paint industry to make special paints that glow in dark.

Two or more elements join to form compounds so alkaline earth metals form ionic compounds as alkaline earth metals are of group 2 and have an oxidation state of +2.

Characteristics of Compounds formed by Alkaline Earth Metals

Oxides

Alkaline earth metals can form corresponding oxides when reacting with oxygen. These oxides are ionic in nature. The structure of these oxides is of rock salt type. Barium is the only element among alkaline earth metals which forms peroxide rather than oxide. These oxides are basic in nature. These oxides except magnesium oxide are ionic in nature but magnesium oxide is covalent in nature.

X + O2  → 2XO 

(where X is an alkaline earth metal)

Hydroxides

The oxides of alkaline earth metals when reacting with water form hydroxides. These hydroxides are also basic in nature and they are thermally stable as well. These hydroxides can be dissolved in water.  

XO + H2O  → X(OH)2  

(where X is an alkaline earth metal)

Halides

Compounds of alkaline earth metals react with the halogen group which consists of fluorine, chlorine, bromine and iodine to form corresponding halides. These halides are ionic in nature except beryllium halide which is covalent in nature. These halides can be dissolved in water except fluorides as they are insoluble in water. These halides can be used for many purposes like as dehydrating agents. Most of these halides occur with water of crystallization. In solid-phase halides cannot occur as individual molecules except Be.

Alkaline earth metals can also form salts as carbonates, sulphates and nitrates.

Carbonates

Alkaline earth metals can form carbonates. These carbonates are of basic nature and will be decomposed to release carbon dioxide when exposed to heat. Carbonates can be formed if hydroxides of alkaline earth metals are made to react with carbon dioxide then carbonates will form along with water. These carbonates cannot be dissolved in water due to their insoluble nature. Carbonates of alkaline earth metals occurs in solid phase and these carbonates are stable to heat. Bicarbonates can be easily formed from carbonates by reacting them with CO2 also these carbonates are ionic in nature.

X(OH)2 + CO2 → XCO3 + H2O

Sulphates

Alkaline earth metals have the ability to form sulphates when react with sulphuric acid. These sulphates are resistant to heat and has white appearance. These sulphates occur in solid phase and can be dissolved in water easily. The trend of solubility of sulphates of alkaline earth metals depends on hydration enthalpy which has the given trend Be> Mg> Ca> Sr> Ba> Ra so the corresponding sulphates also follow the same trend: 

BeSO4 > MgSO4 > CaSO4 > SrSO4 > BaSO4 > RaSO4

Nitrates

Nitrates of alkaline earth metals can be formed by mixing carbonates in nitric acid, other than carbonates, oxides and hydroxides can also be used with nitric acid to form nitrates. Nitric acid to be used should be dilute in nature. Nitrates are soluble in water. These nitrates are unstable to heat and get decomposed on heating to the corresponding oxides of alkaline earth metals. Nitrates when decomposed by heat produces oxygen, metal oxide and nitrogen dioxide which is brown in colour.

2X(NO3)2 → 2XO + 4NO2 + O2

Sample Questions

Question 1: Do Beryllium and magnesium reacts with oxygen or water? Explain.

Answer:

Beryllium and magnesium do not react with water or oxygen as these elements are being protected by a layer of oxide film which forms at their surface. But Beryllium and magnesium reacts with oxygen when these are in powdered form.

Question 2: How alkaline earth metals can be tested?

Answer: 

Alkaline earth metals can be tested by dissolving them in liquid ammonia as in liquid ammonia they give deep blue black colour. Ammoniates are formed when alkaline earth metals are dissolved in solution of ammonia.

Question 3: How BeH2 is formed?

Answer: 

Beryllium does not combine with hydrogen to form hydride so BeH2 cannot be formed directly with hydrogen however, we can prepare BeH2 by the reaction of BeCl2 with LiAlH4.

2BeCl2 + LiAlH4 →  2BeH2 +  LiCl + AlCl3

Question 4: What is the nature of Beryllium hydroxide?

Answer: 

Beryllium hydroxide (Be(OH)2) is amphoteric in nature as it reacts with both acid and base.                            

Be(OH)2 + 2OH → [Be(OH)4] 2-  Beryllate ion  

Be(OH)2 + 2HCl + 2H2O → [Be(OH)4]Cl2

Question 5: Why solubility of hydroxides of alkaline earth metals increases down the group?

Answer: 

As atomic radii increases down the group, the hydration enthalpy decreases because of which solubility of hydroxides of alkaline earth metals increases down the group.


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