NEET Chemistry Notes s-Block Elements – Biological Importance of Sodium and Potassium

NEET Chemistry Notes s-Block Elements  – Biological Importance of Sodium and Potassium

Biological Importance of Sodium and Potassium

Biological Importance of Sodium and Potassium

• Na⁺ ions are found mainly in extracellular region (outside the cell) and play an important role in the transmission of nerve signals. They also regulate the flow of water across cell membrarfes and in transport of sugars and amino acid into the cells.
•  Prolonged sweating results in sodium ion loss in sweat, thus, it is important that Na⁺ ions are replaced through proper diet.
•  Potassium ions are the most abundant cations within cell fluids where they activate many enzymes, participate in oxidation of glucose to produce ATP (adenosine triphosphate).
•  Potassium ions along with sodium ions are responsible for transmission of nerve signals. The functional features of nerve cells depend upon the sodium potassium ion gradient that is established in the cell. Their ionic gradients are maintained by sodium-potassium pumps that operate across the cell membranes.

Group 2 Elements:

Alkaline Earth Metals

Be, Mg, Ca, Sr, Ba and Ra also belong to s-block and are called “alkaline earth metals”.
Properties of Alkaline Earth Metals
These are also the elements of s-block. These are less metallichs ‘compared to alkali metals. These are called alkaline earth metals because their oxides are basic in nature and these are found mostly in the earth’s crust.

Physical Properties

• The alkaline earth metals also have low ionisation enthalpies but these are higher than that of alkali metals.
• Like alkali metal ions, the hydration enthalpies of alkaline metal ions decrease with increase in ionic size on moving down the group.
  
  The hydration enthalpies of alkaline earth metal ions are larger than those of alkali metal ions.
• The melting and boiling points of these metals are higher than the corresponding alkali metals due to smaller size.
•  Except Be and Mg (due to their small size and high IE), all
  other alkaline earth metals give flame test like calcium, strontium and barium impart characteristic brick red, crimson and apple green colours, respectively to the flame.

Chemical Properties

Reaction with Air

These are slowly oxidised on exposure to air forming oxides. These oxides are white crystalline solid and basic in nature. However, the oxide of Be is amphoteric in nature due to the smaller size of Be²⁺ ion. As a result of smaller size, this ion has polarising power
resulting to appearance of covalent character in the compounds of this ion. The basic strength of oxides increases down the group.

 Reaction with Water

Be shows no reaction with water (cold as well as hot in the form of steam); Mg does not react with cold water,
i. e. it reacts with hot water (steam) only while rest of the members react with cold water as

The basic character and solubility of hydroxides increases down the group.

Reaction with Hydrogen

Be does not react directly with hydrogen. Rest of the elements react with hydrogen to form ionic hydrides of MH₂ type. The hydride of Be is prepared by reducing its chloride as ,

BeH₂ and MgH₂ are covalent in nature while other hydrides are ionic in nature.

 Reaction with Halogens

All alkaline earth metals react with halogens at elevated temperatures to form halides of MX₂ type. Among the halides, BeX₂ and MgX₂ have covalent character in them due to the higher polarising powers or more covalent character of their ions. These halides are readily soluble in water and their solubility decreases down the group due to decrease in the hydration energy.
On moving down the group, ionic character increases, thus their melting point and conductivity increases from MgX₂ or BaX₂ These are good conductors in molten state.

Anomalous Behaviour of Beryllium

Beryllium due to its exceptionally small, size (like Al) and high polarising power (like Al) shows similarities with aluminium, in its propertied. These properties are BeO, like Al₂0₃ , is amphoteric and covalent while oxides of other alkaline earth elements are ionic and basic in nature.
Both BeCl2 and like A1₂0₃ are soluble in organic solvents because of covalent nature and both have a bridged polymeric structure.
Beryllium hydroxide dissolves in excess of alkali to give a beryllate ion, [Be(OH)₄]²_ just as aluminium hydroxide gives aluminate ion, [Al(OH)₄]
Beryllium and aluminium ions have strong tendency to form complexes,BeF ₄, AlFg.
Carbides of Be are covalent and react with water to produce methane gas whereas carbides of other members are ionic and produce acetylene with water.

Industrial Use of Lime and Limestone

Lime (Quick Lime), CaO

It is used in the manufacture of

• CaOCl₂,Ca(OH)₂, CaC₂, , glass and cement, etc.
• It is used as basic lining in the furnaces.
• It is used in the purification of sugar and in water softening.

Limestone, CaC03

It is used in the manufacture of quick lime, slaked lime, cement, glass and washing soda, etc.
It is used as a flux in smelting of iron and lead ores.

USES

It is used in making toys, decorative materials and casts for statues. It i§ used in medical applications for setting fractured bonps in the right position and in dentistry.
It is used for making smooth surfaces.

Bleaching Powder, CaOCl₂

It is used as a bleaching agent as well as an oxidising agent.
It oxidises

H₂S into S, Fe^z+ into Fe³⁺,NH₃ into N₂ andKN0₂ into KN0₃.

Cement

It is an important building material. Cement is a product obtained by combining a material such as clay which contains silica, Si0₂  alongwith the oxides of aluminium, iron and megnesium. The average composition of portland cement is Cao – 50-60%; Si0₂_ 20-25%; A1₂0₃– 5-109%; MgO -2-3%; Fe₂0₃-l-2% and S0₃-1-20%.           ‘

Biological Importance of Mg and Ga

Mg²⁺ ions are present inside the animal cells while Ca²⁺ ions are in the body fluids, i.e. in the extracellular region, in such the same way as K⁺ are inside the cell and Na ⁺ ions outside the cell. All enzymes that utilize ATP in phosphate transfer require Mg²⁺ as cofactor. In green plants, magnesium is present in chlorophyll.
Ca²⁺ and Mg²⁺ are also essential for the transmission of impulses along nerve fibres.
In bones and teeth, Ca is present as apatite, Ca₃(P0₄)₂ and in enamel on teeth as fluorapatite, 2Ca₃(P0₄)₂.CaF₂.Ca²⁺ ions play an important role in blood clotting and are required to trigger the contraction of muscles. Ca²⁺ ions also regulate the heart beats.

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