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Alkaline earth metals and their physical and chemical properties

Group II element


Atomic and physical properties

Atomic size:

The average distance between a nucleus and valence orbit of an atom or ion  is call atomic or ionic radius.

Trend 

As we move from top to bottom in second group,  atomic or ionic radius increases. 

Reason:  This is because an extra shell of electron is added in each element . The other factor is shielding affect. Due to increase atomic radius, shielding effect also increases. The atomic radii of these elements  are however smaller than those of alkali metals in the same period. This is due to fact that alkaline earth metal have greater nuclear charge with tend to withdraw the electrons more towards the nucleus. Hence the atomic radii of alkaline earth metals are smaller than corresponding alkali metals in the same period.

Ionization energy

It may be defined as ''the minimum amount of energy required remove one mole of valance electrons to from one mole of gaseous uni-posive ions is called ionization energy''.  

X      -------->  X+   +   e-   ionization energy 

Trend

In group:

In the second group,  on going from top bottom, the distance of ns2 electron from the nucleus increases and removal of valance electrons become more and more easy.

 The other factor is the shielding effect. As we move from top to bottom, shielding increases that decreasesthe force of attraction between valance  two electron and nucleus of an atom. 

The  value of ionization ionization energy of radon is slightly higher than barium.

Electronegativity

 Electronegativity is a measure of tendency to attract a shared pair of electrons towards itself.  

Units

No unit for electronegativity.

Trend

In group 

On moving from to bottom in a group of element, electronegativity decrease. 

 Reason:  In a group, atomic size of an atom increase due to the addition of extra shell so attraction between nucleus and valance electron decreases.

Melting and boiling Points 

 Melting and boiling points indicate about the strength of forces present among atoms, ions, molecule within a substance. in case of elements the value of melting and boiling points are according to inter atomic forces.

Trend 

All moving from top to bottom in alkaline earth metal groups, melting point increases. Reason: This is because metallic character of elements increases down the group.  

Therefore, melting and boiling points of alkaline earth metals are greater than alkali metals. The reason is that the group II elements have stronger metallic bond due to presence of two electrons. 

Trend in reactivity with water

The group two elements are more reactive towards water as we go down the group then group one elements.

Be: No reaction with water 

Mg: 

Mg   + H2O  -----------> MgO    +   H2 

Mg     +  H2O (cold( -------------->  Mg(OH)2   +     H2

M : Ca, Sr, Ba

M   +      H2O  ---------------> M(OH)2 +  H2

Reaction with oxygen and nitrogen

Formation of normal oxide 

Alkaline earth metals from normal oxides by direct heating with oxygen are by heating their carbonates.

2M  +   O2 ----------> 2MO

M: Mg, Ca, Sr, Ba

MCO3 -----'> MO   +   CO2   (heat)

Properties of Alkaline earth metals oxides 

I. These oxides are white crystalline solid and highly stable.

ii. BeO and MgO are quiet insoluble and water while other oxides react with water to produce metal hydroxide prelim oxide is amphoteric in nature

Reactions 

BeO   +  2HCl   ------------>BeCl2      +      H2O 

BeO + 2NaOH------------>NaBeO2 +        2H2O 

BeO is covalent due to small size of be2 + while other oxides are ionic all the Brazilian oxide is covalent it has higher melting point then outside of other metal believe oxide is harder than sides of other metal outside

Reason: as briley oxide is a polymeric in nature and each brillium is data hardly coordinated bounded with four other oxygen atoms.


Formation of peroxides

MO    +     O   ----'---->    MO2

Reaction with nitrogen

M   +    N2  -----------> M3N2

Mg   +    N2  -----------> Mg3N2

Ca +     N2 -----------> Ca3N2

Sr   +    N2  -----------> Sr3N2

Ba    +    N2  -----------> Ba3N2

Friend insolubility in Hydroxide sulphate and carbonate

Solubility of hydroxide

Group 2 metal Hydroxide become more soluble as we move from top to bottom in the group.

Be(OH)2 and Mg(OH)2 almost insoluble in water. 

Mg(OH)2 is known as milk of magnesia and used as an antacid.

Ca(OH)2 is slightly soluble in water (1g/L) and also known as lime water.

Ba(OH)2  is enough soluble in water (0.1 mole/L).

Solubility of sulphatej

Group II metal sulphates become less soluble as we move from top to bottom in a group. e.g. BeSO4, MgSO4 are fairly soluble in water. 

CaSO4 is sufficiently soluble in water.

SrSO4 and BaSO4 aren't soluble in water. of carbonates

Solubility of carbonates

Group 2 metal carbonates become less soluble as we go from top to bottom in a group they are soluble and water containing CO2.

CaCO3   +  CO2   +  H2O  ------'------>  Ca(HCO3)2

Friends in thermal stability of carbonate and nitrates of group 2 elements

Group 2 elements carbonates

Effect of heat

All Metal carbonates decompose on heating.

MCO3----------------> MO   + CO2 

M : Ba, Mg, Ca, Sr, Ba

Thermal stability of group II elements increases on moving from top bottom. 

BeCO3 decomposes at 25oC

MgCO3 decomposes at 540 oC

CaCO3 decomposes at 900 oC

SrCO3 decomposes at 1290 oC

BaCO3 decomposes at 1360 oC


Question: Why carbonates of IA group elements are more stable than IIA group elements?

Answer. 

Reason: Cations of  of IIA group are smaller in size  so their charge densities are greater than IA group elements.  The cations of small size and higher charge density have greater ability to polarise carbonate ion and hence facilities the decomposition.

Thus IIA  group elements carbonates are less stable than IA group element carbonates. 

Effect of heat on group second nitrates

All nitrates of  elements undergo thermal decomposition to give metal oxide

M(NO3)2 ------------->MO  +   NO2     +   O2

Be(NO3)2 ------------->MO + NO2 + O2 

Mg(NO3)2 ------------->MO  +   NO2     +   O2


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