In this post, we will come across detail study of the physical and chemical properties of s-block elements. We will also study the preparations, physical and chemical properties; and uses of the different compounds of s-block elements.
The long form of the periodic table is divided into four groups- s, p, d and f blocks.Elements in which the differentiating (last) electron enters into the s sub shell are called s-block elements.
Group 1 includes Lithium (Li), Sodium(Na),Potassium(K),Rubidium(Rb). Caesium(Cs) and Francium(Fr).These elements are called alkali metals as the oxides and hydroxides of these elements are strongly alkaline in nature.The word ‘alkali’ is derived from the Arabic word al-quis which meaning plant ashes. Francium is radioactive in nature.
The electronic configuration these elements are:
| Element | Atomic Number | Electronic configuration |
| Li | 3 | 1s 2 2s 1 |
| Na | 11 | 1s 2 2s 2 2p 6 3s 1 |
| K | 19 | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 |
| Rb | 37 | 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 5s 1 |
| Cs | 55 | 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 5s 2 5p 6 6s 1 |
| Fr | 87 | 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 4f 14 5s 2 5p 6 5d 10 6s 2 6p 6 7s 1 |
From the electronic configuration of an element, its position in the long form of the periodic table can be predicted. If the valence shell electronic configuration of an element is xs 1 or xs 2 then its
Period is x and its group is 1 or 2 respectively. As, the valence shell electronic configuration of Potassium is [Ar]4s 1 . It indicates, potassium lies in Period 4 and group 1.
Rubidium occurs in the minerals leucite, carnallite,pollucite and zinnwaldite.
Some important physical properties of the elements of group 1 are given below:
| Property | Li | Na | K | Rb | Cs |
| Atomic Number | 3 | 11 | 19 | 37 | 55 |
| Atomic Mass | 6.94 | 22.99 | 39.1 | 85.48 | 132.91 |
| Atomic radius(pm) | 134 | 154 | 197 | 211 | 225 |
| Ionic radius(pm) | 60 | 95 | 133 | 148 | 169 |
| Ionisation potential(IP1)(kJ/mol) | 520.1 | 497.2 | 418.7 | 403 | 375.7 |
| Electronegativity | 1.0 | 0.9 | 0.8 | 0.8 | 0.7 |
| Density g/cc | 0.534 | 0.972 | 0.859 | 1.53 | 1.90 |
| Melting point( 0 C) | 180 | 97.8 | 64.0 | 39.0 | 28.5 |
| Boiling point( 0 C) | 1326 | 883 | 756 | 688 | 690 |
| E 0 ox(V) | 3.04 | 2.71 | 2.92 | 2.99 | 2.99 |
Elements of s-block are basically silvery-white (due to presence of mobile electrons), soft (due to weak metallic bond), and light metals. These elements are highly reactive, thus are kept under kerosene oil. They can be easily cut with a knife.
The size of the alkali metals increases down a group as new shells are added. Hence Li
Alkali metals can form M + ion. The ionic radii of these elements increase down the group. The size of the ions is less than the atomic size of the respective element due to- i) removal of the outermost shell ii) increase in the effective nuclear charge. Hence Li +
Down the group the atomic volume increases. Hence Li
Density of the alkali metals increases down the group. The exceptional behaviour of potassium is due to unusual increase in atomic size. Hence Li
The melting and boiling point of the alkali metals gradually decrease down the group. The values of the melting and boiling point are apparently low because the binding energy (Binding energy is the amount of energy required to decompose a molecule, atom, or nucleus into its constituent particles) of the atoms in the crystalline lattice is low. The binding energy is low due to –Presence of the single valence electron, Large size of the atom
The ionisation potential decreases down the group because down a group the no of shell increases, the screening effect also increases, the nuclear pull on the valence shell electrons decreases, the I.P decreases.
The first I.P of the alkali metals are the lowest compared to the elements of other group as-
As the I.P decreases down the group the metallic nature increases
This property of emitting electron easily is utilized in photoelectric effect. (The phenomenon of emitting electrons from the metal surface when light strikes on its surface) K and Cs are used in the photoelectric cells.
The alkali metals due to the low I.P,easily lose the outermost valence electron and for uni-positive ion showing +1 oxidation state.
M + possess noble gas electronic configuration hence apparently large amount of energy is required to remove the next electron hence the second I.P of these elements are fairly large.
M + possess noble gas electronic configuration hence it is diamagnetic (due to absence of unpaired electron)
So also, due the absence of the unpaired electrons compounds of M + is colourless.
Alkali metals are good conductors of heat and electricity due to the presence of loosely bounded valence electron.
The salts of the alkali metals (especially chloride) impart characteristic colour to the Bunsen flame.
| Li | Na | K | Rb | Cs |
| Crimson red | Golden Yellow | Pale violet | Violet | Violet |
Salts of alkali metals are volatile in nature, when heated in Bunsen flame, the outermost valence electron jumps to the higher state by absorbing energy. When the excited electrons return to the ground state, they release the absorbed energy as visible light.
Strong electrostatic force of attraction exist between the ions hence the lattice energy of the salts of alkali metals are quite high. Down the group the lattice energy decreases with the increase in size. Hence, Li>Na>K>Rb>Cs
M + undergoes hydration with water molecule hence is water soluble.The degree of hydration of M + in water decreases down the group (as the size increases). But the ionic conductance of these hydrated ions increases down the group.
It is the energy released in the hydration of the ions. It decreases down the group.
As the ionisation potential decreases down the group, the reducing character as well as the reactivity increases downs the group (Gaseous state). Hence Li
Alkali metals react vigorously with water forming hydroxides and hydrogen.
The reactivity with water increases down the group.
Alkali metals form oxide with oxygen, which tarnish the metal. Thus the metals are stored in kerosene or paraffin oil. Li being light is kept wrapped in paraffin wax. The reactivity with oxygen increases down the group.
Alkali metals form ionic hydride with hydrogen. The reactivity decreases down the group. These hydrides are high melting ionic solids and behave as strong reducing agents. The reducing nature of the hydrides increases down the group.
Alkali metals form halides with halogen reacting violently.
Towards a particular halogen, the reactivity increases down the group. Towards a particular alkali metal the reactivity of the halogens follows F>Cl>Br>I order.
Alkali metals form sulphides and phosphides with sulphur and phosphorous respectively.
Alkali metals are soluble in liquid ammonia forming a blue metal-ammonia solution.
Some properties of metal-ammonia solution are:
Li forms monoxide, Na forms peroxide while K, Rb and Cs forms superoxide.Down the group, the tendency to stabilise the large anions by the large cations through lattice energy increases, thus the tendency to form superoxidesincreases.Peroxides and superoxides are powerful oxidising agents.
Hydroxides are prepared by reacting metals directly with water or dissolving the oxide in water. The hydroxides of the alkali metals are strong base as, due to the low ionization potential of the metals the M-O bond in M-O-H get easily rapture forming M+ and OH- ion. Down the group, the ionization potential of the alkali metals decreases hence the rapture of M-O bond becomes easier. Thus, the basic characteristics of the hydroxides increase. LiOH
These hydroxides are thermally stable, they melts without decomposing.
The hydrides of the alkali metals are high melting ionic solids and behave asstrong reducing agents. The reducing nature of the hydrides increases down the group as the ionisation potential of the alkali metal increases. LiH
All sulfates of alkali metals are water-soluble except lithium sulfate and form double salts with trivalent metals like Al, Fe, and Cr. These double salts are called alum (M2SO4. M2 ’ (SO4)3. 24H2O, where M= Na + , K + , NH4 + and M’ = Al 3+ , Fe 3+ , Cr 3+ )
Sulphates when fused with charcoal, sulphides are formed.
The halides of the alkali metals are prepared by the direct combination of the metal with halogen. It can also be prepared by reacting aqueous halogen acid with carbonates and hydroxides of the alkali metals.
Towards a particular halogen, the reactivity increases down the group-13 elements. Towards a particular alkali metal the reactivity of the halogens follows F>Cl>Br>I order.With the decrease in the size of the cation, its polarising power increases and hence the covalent character increases in the following order: LiCl>NaCl>KCl>RbCl>CsCl.
The unusual variation of the LiCl may be due to its covalent nature.
Lithium is the first element of the alkali metals. It shows anomalous behaviour in many of the cases like:
The anomalous behavior is due the following reasons:
Li resembles Mg due to almost similar atomic and ionic size. (Li=152pm, Mg=160pm, Li + =76pm, Mg 2+ =72pm) Their similarities are:
Chlorides and perchlorates are ethanol soluble
In tropical countries, sodium chloride can be prepared by evaporating sea water. Sea water taken in a large shallow iron tank is evaporated by wind and sun. As the sea water evaporates, the solution gets concentrated and crystal of sodium chloride separates out. The crystals are separated leaving behind the mother liquor called bittern. This mother liquor contains magnesium chloride. The common salts produced by this process possess sulphates and chlorides of magnesium and calcium as impurities.
In laboratory sodium chloride is prepared by reacting sodium hydroxide with hydrochloric acid.
NaOH + HCl → NaCl + H2O
Sodium chloride thus produced is separated out as crystals by evaporation
Pure sodium chloride is prepared by passing pure and dry chlorine gas over heated sodium.
Purification of sodium chloride: Common salt obtained from sea water contains sulphates and chlorides of Ca and Mg as impurities. Thus, a steam of pure hydrogen chloride gas is passed through a saturated solution of common salt, pure sodium chloride separates out due to common ion effect (Cl – ) leaving behind the impurities in the solution. The crystals are collected, washed with pure conc. HCl and then heated in a platinum container to remove hydrogen chloride and water. By this process nearly 100% pure sodium chloride is obtained.
In this process, 20% sodium carbonate solution is heated with excess quick lime at 80-90 0 C. Calcium carbonate being insoluble precipitates out leaving behind sodium hydroxide in the solution. This reversible reaction is favored by increasing the concentration of quick lime and decreasing the concentration of CO3 2- ion. Thus an optimum concentration of 20% sodium carbonate is used.
By this process, a strong alkali (NaOH) is produced from a weak one(Na2CO3). Thus it is called causticising process.
In the recent time sodium hydroxide is prepared by electrolysis of saturated sodium chloride solution.
When electric current is passed through an aqueous solution of sodium chloride, it ionises to form Na + , H + OH – and Cl – ions .
At cathode: H + ion is preferentially discharged at the cathode.
H + + e → H,H + H → H2↑
At anode: Cl – ion is discharged due to high concentration of Cl – ion.
Cl – -e → Cl,Cl + Cl → Cl2 ↑
The spectator ions which remain in the solution combine to form sodium hydroxide.
The main disadvantages of this process are-
Cl2 + 2NaOH → NaCl + NaOCl + H2O
To overcome these disadvantages, three types of cells are frequently used-
It consists of a U-shaped vessel with perforated steel covered with layer of asbestos. The U-shaped steel vessel serves as cathode and the porous asbestos acts as a diaphragm which separates cathode from anode. Saturated solution of sodium chloride is introduced into the U-shaped vessel. A graphite rod is partly immersed in the sodium chloride solution which acts as anode. Steam is blown into the cathode through an inlet tube.
It consists of a large rectangular tank with three compartments separated by slate partition. Mercury is made to flow at the bottom which acts as an intermediate electrode. The outer compartments contain brine solution and fitted with graphite anodes whereas the central compartment contains very dilute NaOH solution and fitted with iron cathode.
Outer compartment:
Sodium produced at the cathode dissolves in mercury forming sodium amalgam and passes into the central compartments.
Central compartment:
It consists of two chambers. The upper one is called primary cell and the lower one is called secondary cell or decomposer. The primary cell consists of a slate made rectangular cell. A thin layer of Hg flows along its slightly sloping floor which mainly serves as a cathode. Graphite and copper rods are dipped in the brine solution kept in it. These rods act as anode.
Sodium amalgam thus produced in the primary cell flows into the lower chamber made of iron which contains water. Here sodium amalgam acts as anode and iron acts as the cathode.
Advantages of the process:
Disadvantages of the process:
Sodium bicarbonate is produced when carbon dioxide is passed through a saturated sodium carbonate solution.Na2CO3+H2O+CO2→2NaHCO3
It is a salt of thiosulphuric acid, H2S2O3, an unstable acid.Pentahydrate sodium thiosulphate (Na2S2O3.5H2O)is also called Hypo.
It is prepared by reacting to a calculated amount of iodine with a mixture solution of sodium sulfide and sodium sulfite. Then the final solution is subjected to crystallization. When crystals of sodium thiosulphate separate out.
This reaction is known as Spring’s reaction.
Sodium sulphite is prepared by passing sulphur dioxide through saturated sodium carbonate solution. Sodium sulphite produced is boiled with sulphur. The reaction mixture is stirred and heated till the alkaline nature of the solution disappears. The final solution is filtered. Excess sulphur is separated by filtration. The filtrate is concentrated to crystalline out sodium thiosulphate crystals.
The waste liquor obtained during the production of sodium sulphide contains sodium carbonate and sodium sulphite in it. This waste liquor is concentrated by evaporation and through it sulphur dioxide gas is passed. The solution produced is subjected to crystallisation when sodium thiosulphate separates out.
Sodium helps in maintaining the body’s fluid balance and blood pressure. It activates angiotensin II, a protein that constricts arteries to further enhance blood pressure. It supports proper nerve functioning by playing a role in action potentials or nerve impulses. During it, sodium rushes out of nerve cells to initiate the electrochemical impulse. Hence, sodium deficiency can cause muscle cramps due to abnormal communication between nerves and muscle fibers. It allows the small intestine to absorb amino acids, glucose, and water from the foods consumed. It also affects kidney function.
The potassium ion (K + ) is the major cation in intracellular (inside cells) fluids (sodium is the main extracellular cation). Potassium is essential for nerve, activating enzymes that are responsible for the oxidation of glucose to produce ATP and heart function.
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