Chapter 10 : The s-Block Elements
The group 1 elements comprise Lithium, Sodium, Potassium, Rubidium, Caesium, Francium.
Topics covered in this snack-sized chapter:
All the alkali metals have one valence electron, ns1
outside the noble gas core.
10.1.1 Electronic Configuration
The alkali metal atoms have the largest sizes in a particular period of the periodic table.
10.1.3 Atomic and Ionic Radii
The ionization enthalpies of the alkali metals are considerably low and decrease down the group from Li to Cs.
10.1.2 Ionization Enthalpy
The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes.
10.1.4 Hydration Enthalpy
All the alkali metals are silvery white, soft and light metals. Because of the large size.
10.1.5 Physical Properties
Reactivity towards water: The alkali metals react with water to form hydroxide and dihydrogen.
10.1.6 Chemical Properties
Solutions in liquid ammonia: The alkali metals dissolve in liquid ammonia giving deep blue solutions.
Lithium metal is used to make useful alloys.
It is used in thermonuclear reactions.
Lithium is also used to make electrochemical cells.
General characteristics of some of their compounds are:
10.2 General Characteristics of the Compounds of the alkali Metals arrow_upward
On combustion in excess of air, lithium forms mainly the oxide, Li2
O (plus some peroxide Li2
), sodium forms the peroxide, Na2
(and some superoxide NaO2
) whilst potassium, rubidium and caesium form the superoxides, MO2
10.2.1 Oxides and Hydroxides
The alkali metal halides, MX, (X=F, Cl, Br, I) are all high melting, colourless crystalline solids.
The anomalous behaviour of lithium is due to the:
- Exceptionally small size of its atom and ion, and
Lithium is much harder. Its m.p. and b.p. are higher than the other alkali metals.
Lithium is least reactive but the strongest reducing agent among all the alkali metals. On combustion in air it forms mainly monoxide, Li2
O and the nitride, Li3
N unlike other alkali metals.
10.3.1 Points of Difference between Lithium and other Alkali Metals
Both lithium and magnesium are harder and lighter than other elements in the respective groups.
Lithium and magnesium react slowly with water. Their oxides and hydroxides are much less soluble and their hydroxides decompose on heating. Both form a nitride, Li3
N and Mg3
, by direct combination with nitrogen.
The oxides, Li2
O and MgO do not combine with excess oxygen to give any superoxide.
Industrially important compounds of sodium include sodium carbonate, sodium hydroxide, sodium chloride and sodium bicarbonate.
10.3.2 Points of Similarities between Lithium and Magnesium
Sodium Carbonate (washing soda) Na2
Solvay process: Passing CO2
to a concentrated solution of sodium chloride saturated with ammonia, where ammonium carbonate followed by ammonium hydrogen carbonate.
Sodium hydrogen carbonate crystal separates. To give sodium carbonate.
It is used in water softening, Laundering and cleaning.
Sodium Chloride, NaCl
The most abundant source of sodium chloride is sea water which contain 2.7 to 2.9% by mass of the salt.
Crude sodium chloride, generally obtained by crystallisation of brine, contains sodium sulphate, calcium sulphate, calcium chloride and magnesium chloride as impurities.
It is used as common salt or table salt for domestic purpose.
Sodium Hydroxide (Caustic Soda), NaOH
Sodium hydroxide is generally prepared commercially by the electrolysis of sodium chloride in Castner-kellner cell.
It is used in the manufacture of soap.
Sodium Hydrogencarbonate(Baking Soda), NaHCO3
Sodium hydrogen carbonate is made by saturating a solution of sodium carbonate with carbon dioxide.
It is used in fire extinguishers.
Sodium ions are found primarily on the outside of cells.
Potassium ions are the most abundant cations within cell fluids.
Example, in blood plasma, sodium is present to the extent of 143 mmolL–1, whereas the potassium level is only 5 mmolL–1 within the red blood cells.
The group 2 elements comprise beryllium, magnesium, calcium, strontium, barium and radium.
10.5 Biological Importance of Sodium and Potassium arrow_upward
Their general electronic configuration may be represented as [noble gas] ns2
10.6.1 Electronic Configuration
The atomic and ionic radii of the alkaline earth metals are smaller than those of the corresponding alkali metals in the same periods.
10.6.2 Atomic and Ionic Radii
The alkaline earth metals have low ionization enthalpies due to fairly large size of the atoms.
10.6.3 Ionization Enthalpies
The hydration enthalpies of alkaline earth metal ions decrease with increase in ionic size down the group.
10.6.4 Hydration Enthalpies
The alkaline earth metals, in general, are silvery white, lustrous and relatively soft but harder than the alkali metals. Beryllium and magnesium appear to be somewhat greyish.
10.6.5 Physical Properties
Reactivity towards acids: The alkaline earth metals readily react with acids liberating dihydrogen.
10.6.6 Chemical Properties
M + 2HCl → MCl2
Beryllium is used in the manufacture of alloys.
Magnesium-aluminium alloys being light in mass are used in air-craft construction.
General Character of Alkaline earth metals are:
10.7 General Characteristics of Compounds of the Alkaline Earth Metals arrow_upward
Oxides of Hydroxides: The alkaline earth metals burn in oxygen to form the monoxide.
Salts of Oxoacids
- Carbonates: carbonates of alkaline earth metals are insoluble in water and can be precipitated by addition of a sodium of a soluble salt of these metals.
- Sulphates: The sulphates of the alkaline earth metals are all white solids and stable to heat. BeSO4
, and MgSO4
are readily soluble in water.
Beryllium has small atomic and ionic sizes and thus does not compare well with other member of the group.
It does not exhibit coordination number more than four as in its valence shell there are only four orbitals.
- Nitrates: The nitrates are made by dissolution of the carbonates in dilute nitric acid. Magnesium nitrate crystallises with six molecules of water.
Like aluminum, beryllium is not readily attacked by acids because of the presence of an oxide film on the surface of the metal.
Beryllium hydroxide dissolves in excess of alkali to give a beryllate ion, [Be(OH)4
just as aluminum hydroxide gives aluminate ion, [Al(OH)4
10.8.1 Diagonal Relationship between Beryllium and Aluminum
Beryllium and aluminium ions have strong tendency to form complexes, Be
Important compounds of calcium are calcium oxide, calcium hydroxide, calcium sulphate and cement.
Calcium Oxides or Quick lime, CaO
It is prepared on a commercial scale by heating limestone (CaCO3
) in a rotary kiln at 1070-1270 K.
It is used in the manufacture of sodium carbonate from caustic soda.
Calcium Hydroxide(Slaked lime), Ca(OH)2
Calcium hydroxide is prepared by adding water to quick lime, CaO.
Carbon dioxide is passed through lime water it turns milky due to the formation of calcium carbonate.
It is used in the preparation of mortar, a building material.
Calcium Sulphate (Plaster of Paris), CaSO4
It is obtained when gypsum, CaSO4
O, is heated on 393K.
Cement is an important building material.
The largest use of Plaster of Paris is in the building industry as well as plasters.
Cement is an important building material.
Cement is a product obtained by combining a material rich in lime, CaO with other material such as clay which contains silica, SiO2
along with the oxides of aluminium, iron and magnesium.
It is used in concrete and reinforced concrete, in plastering and in the construction of bridges, dams and buildings.
An adult body contains about 25 g of Mg and 1200 g of Ca compared with only 5 g of iron and 0.06 g of copper.
About 99% of body calcium is present in bones and teeth.
All enzymes that utilize ATP in phosphate transfer require magnesium as the cofactor.
10.10 Biological Importance of Magnesium and Calcium arrow_upward