The s-Block Elements

Question 197

Explain the significance of magnesium and calcium in biological fluids. 


Significance of magnesium and calcium:
(i) Magnesium is an important constituent of chlorophyll which initiates the process of photosynthesis in green plants.
(ii) Magnesium ions are concentrated more in intracellular than in extracellular fluids in animal bodies. Their presence is also necessary for the activation of phosphate-transfer enzymes. These enzymes take part in the biochemical process (exothermic) occurring in animal body. Mg2+ ions are also involved in carbohydrates metabolism.
(iii) Ca2+ ions are essential for the formation of bones and teeth. The enamel on teeth is a double salt 3Ca3(PO4)2.CaF2.
(iv) Mg2+ and Ca2+ ions are also involved in the transmission of electrical impulses along the nerve fibre and for the contraction of muscles.

Question 198

What are the common physical and chemical features of alkali metals?



Physical properties:
i) metallic character: Alkali metals are highly electropositive in nature and hence, they are typical metals. the metallic character is due to low values of ionisation energies and consequently, they have a tendency to lose the valence electrons.

Low ionisation energy: The first ionisation energies of alkali metals are quite low as compared to the elements of the other groups belonging to the same period. the reason is that atoms of alkali metals are of large sizes. Therefore, the outermost electron is far away from the nucleus and can be easily removed. Within the group, ionisation energies of alkali metals decrease as we move down the group.

Low melting and boiling point: The melting and boiling points of alkali metals are very low because of the large size of their atoms due to which inter-particle forces are very weak in them. The melting and boiling points decrease on going down the group(Lithium to Cesium) as the charge density decreases because of the increase in the size of the monovalent cation.

Low electronegativity: Alkali metals have low values of electronegativity. They have very little tendency to attract the shared pair of electrons towards themselves. The electronegativity values of alkali metals decrease as we move down in the group from Li to Cs.

Soft in nature: All the alkali metals are soft and can be cut with the help of a knife.The softness of alkali metals is due to weak metallic bonding in them as the result of the large size of the atoms. As we move down the group, metallic bonding weakens and therefore, softness increases.

Density: Alkali metals have low density due to the large size of metals atoms.

Oxidation state: The alkali metals exhibit oxidation state of +1 in their compounds and strongly electropositive in character. the electropositive character increases from lithium down to caesium in the group.
Chemical properties:
Decomposition of water: The alkali metals decompose water at the ordinary temperature giving out hydrogen.
2Li+2H2O --> 2LiOH +H2 

2Na +2H2O --> 2NaOH + H2
2K +2H2O --> 2KOH +H2

Reaction with oxygen: The alkali metals readily burn in oxygen or air to form their oxides
4Li +O2 -->2LiO

combination with halogens: The alkali metals burns in halogens forming their halides

2Na +Cl2 --> 2NaCl


Question 199

Discuss the general characteristics and gradation in properties of alkaline earth metals.


(i) The general electronic configuration of alkaline earth metals is [noble gas] ns2.

(ii) The atomic and ionic radii of alkaline earth metals are smaller than that of alkali metals. On moving down the group their atomic and ionic radii increase due to the decrease in their effective nuclear charge.

(iii) These metals lose two electrons to acquire the nearest noble gas configuration. Therefore, their oxidation state is +2.
(iv) Due to their small size, the first ionisation enthalpies of alkaline earth metals are higher than those of the alkali metals. But their second ionisation enthalpies are found to be lower than those of the corresponding alkali metals.
v) Metallic character: They are less electropositive than alkali metals due to higher ionisation enthalpies. Metallic character increases down the group due to a decrease in ionisation enthalpy.
Chemical reactivity:
Reaction with water and air: The alkaline earth metals are less reactive than alkali metals. Be and Mg is kinetically inert of O2 and H2O due to the formation of an oxide layer on its surface. Be does not react with water or steam even at red-hot and does not get oxidised in air below 873K. Powdered be burns on ignition to form BeO and Be3N2
2Be +O2 → 2BeO
3Be +N2  Be3N2
Mg is more electropositive and burns in air with dazzling light forming MgO and Mg3N2.
2Mg +O2 →2MgO
3Mg +N2 →Mg3N2

Ca, Sr and Ba readily react with oxygen to give oxides. Calcium forms oxides whereas Sr and Ba form peroxide. They react with nitrogen to form nitrides.
2Ca +O2→ 2CaO
3Cs +N2 → Ca3N2
Sr +O2 → SrO2
Ba +O2→ BaO2
Mg Reacts with hot water Ca, Ba, Sr react with cold water vigorously.
Mg +H2O(Hot)→ MgO +H2
Ca +2H2O→ Ca(OH)2 +H2
Sr +2H2O → Sr(OH)2 +H2
Ba +2H2O → Ba(OH)2 +H2

Reaction with halogens: Group 2 element react with halogens at increased temperature to from halides.
Be +Cl2+ Heat → BeCl2
Mg +Cl2+ Heat →MgCl2
Ca+Cl2+ Heat →CaCl2
Action with acids: The alkaline earth metals readily react with acids to form salts and liberate H2 gas.
Be +2HCl→ BeCl2+H2
Mg+2HCl→ MgCl2 +H2

Reaction with H2
All metals combine with H2 to form hydrides except Be.
Ca +H → CaH2(Hydrolith)
Mg +H2 → MgH


Question 200

Compare the alkali metals and alkaline earth metals with respect to (i) ionisation enthalpy (ii) basicity of oxides and (iii) solubility of hydroxides.


Alkali metals

Alkaline earth metals

(i) Ionisation enthalpy: These have the lowest ionisation
enthalpies in respective periods. This is because of their large atomic sizes. Also, they lose their only valence electron easily as they attain stable noble gas configuration after losing it.

(i) Ionisation enthalpy: Alkaline earth metals have smaller
atomic size and higher effective nuclear charge as compared to alkali metals. This causes their first ionisation
enthalpies to be higher than that of alkali metals. However, their second ionisation enthalpy is less than the corresponding alkali metals. This is because alkali metals, after losing one electron acquires noble gas
configuration, which is very stable

(ii) Basicity of oxides: The oxides of alkali metals are
very basic in nature. This happens due to the highly
electropositive nature of alkali metals, which makes these
oxides highly ionic. Hence, they readily dissociate in water to give hydroxide ions.

(ii) Basicity of oxides: The oxides of alkaline earth metals are quite basic but not as basic as those of alkali metals. This is because alkaline earth metals are less electropositive than alkali metals.

(iii) The solubility of hydroxides: The hydroxides of alkali metals are more soluble than those of alkaline earth metals.

(iii) The solubility of hydroxides: The hydroxides of alkaline earth metals are less soluble than those of alkali metals. This is due to the high lattice energies of alkaline earth metals. Their higher charge densities (as compared to
alkali metals) account for higher lattice energies.

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