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B.sc 1st year Book
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Group IA (1) Elements: Li, Na, K, Rb, Cs, Fr (Alkali Metals)

Group IA elements of the periodic table consisting of hydrogen (H), lithium Li ), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and francium (Fr). The elements, lithium to caesium of group IA are generally known as Alkali metals because they form strong alkalies when reacting with water. The last element Uranium is a radioactive element. All the elements of group IA have ns 1 electron in their valence shell or outermost shell.

General characteristics of s-block elements :

  1. All the elements of the IA group are not found in nature in free elemental form because they are most reactive and combine generally with the most electronegative elements readily.
  2. Except for hydrogen, all group members are soft metals and highly malleable and ductile.
  3. They have low binding energy, and low melting and boiling points and are hence readily fused.
  4. They have only one electron in their outermost shell and hence have low ionization energy.
  5. These elements show a constant oxidation number that is +1 in their chemical compounds.
  6. They are the most powerful reducing agents and show the highest value of standard electrode potential ( LI=3.05 volts) in the electrochemical series.
  7. They vigorously react with water and acids to liberate H2.
  8. They have a strong electropositive character and readily form positive ions. The energy required to form aqueous ions is related to the heat of sublimation, ionization and hydration energies.
  9. These elements show characteristic colours when they are exposed to a flame. This property of their characteristic flame is used in their analysis.
  10. They show a close-packed structure in solid state.
  11. All the salts of alkali metals are soluble in water except L2CO3.
  12. They give blue colour when dissolved in ammonia

1- Electronic configuration of group IA Elements :

These elements have an electronic configuration similar to noble gas elements with the addition of one electron in their valence shell thus their valence shell electronic configuration. ns’ e.g…

Table 4.1: Electronic configuration of IA group elements
Element Symbol Atomic No. Electronic Configuration With inert gas core
Lithium Li 3 1s22s1 [He]2s1
Sodium Na 11 1s22s22p63s1 [Ne]3s1
Potassium K 19 1s22s22p63s23p83d104s1 [Ar]4s1
Rubidium Rb 37 1s22s22p63s23p83d104s24p85s1 [Kr]5s1
Caesium Cs 55 1s22s22p63s23p83d104s24p84d104f145s25p85d106s1 [Xe]6s1
Francium Fr 87 1s22s22p63s23p83d104s24p84d104f145s25p85d106s26p87s1 [Rn]7s1

2-Physical state:

All the alkali metals are very soft, ductile, malleable and easily fusible except LL. They show bright lustre when freshly cut and get quickly tarnished on exposure to air.

3- Size of atoms and Ions:

The size of an atom of iron is expressed in terms of its radii. It increases with the increase in the atomic number of an element on passing from top to bottom in a group as shown in table 4.2. Fr is a radioactive element, hence the value is not given in table 4.2.

physical Properties of Alkali Metal, Alkali Metal

The increase in atomic and ionic size is a general trend and is applicable in other groups also. This is due to an increase in the value of the quantum shell to occupy an extra electron in the case of each successive element in the group. The radius of an atom is always greater than its action and it is also a general trend for all the atoms present in different groups.

4- Density:

H is defined as mass per unit volume hence it depends upon the atomic weight and atomic radius of an atom. Atom is spherical and its volume (V=4/3πr power 3) is measured by knowing its radius. The density increases with the increase of atomic number. Atomic weight increases more remarkably as compared to the atomic volume of an element. Li, Na and K are even lighter than water whose density is 1.00gcm. The densities ofL, Na and K are 0.534,0.972 and 0.559g cm3 respectively. Caesium has a density equal to 1.903gmm−3, The density of K is less than Na because of an unusual increase in the size of K with respect to Na.

5- Melting and Boiling Points:

All the elements of group iA are highly electro-positive and this property influences the melting and boiling points of an element. in a group moving from top to bottom, the metallic character increases with the increase of the size of the atom resulting in a decrease In m.p. and b.p. The m.p. and b.p. also depend upon the inter-atomic forces of an element. Larger atoms are closely packed in their crystal lattice with weak inter-atomic forces hence there is a decrease in m.p. and b.p. of the elements on moving from Li to Fr es shown in table 4.2.

6- Ionization energy:

As we know that all the elements of the IA group have the largest size in their respective period and have only one electron in their valence shell hence they show the lowest value of ionization energy. In a group when we proceed from Li to Cs the value of ionization energy decreases. All the elements form M+ ions after losing ns 1 electron quite easily. The value of the second ionization energy of all alkali metals is very high because M+ ion acquired inert gas configuration. Hence after losing one electron the removal of a second electron from the M+ ion to form an M2+ ion is very difficult. Thus, the chemistry of alkali metals is the chemistry of M+ ions which form ionic compounds. Since there is a decrease in ionization energy their metallic character/or electropositive character increases in the same order as shown in table 4.2.

7- Electropositive character or Metalle character:

All the elements having the highest value of electropositive character will show the lowest value of electronegative character. The elements of the IA group are most electropositive and have a tendency to form M+ ions by easy removal of its outermost electron.
e.g  M(g) → M+(g) + e-
The high electropositive nature causes alkali metals to form ionic compounds, These elements can lose their outermost electron even when exposed to light. This effect is called the photoelectric effect. Due to this property, they are used in photoelectric cells specially Kand Cs are used for this purpose.

8- Electronegativity and electron affinity:

These elements are highly electropositive and show very small values of electronegativity and the least tendency to attract electrons towards themselves in their respective periods. The values of electronegativity and electron affinity of IA group elements are shown below:
Metals Li Na K Rb Cs
Electronegativity (Pauling) 1.0 0.9 0.8 0.8 0.7
Electron affinity (oV) −0.618 −0.546 −0.502 −0.486 −0.471

 

Hence, the elements of group IA form ionic compounds with the most electronegative elements such as halogens. It is clear from the above table that the electronegative character and electron affinity of alkali metals decreases on passing from Ll to Cs in a group.

9- Oxidation states:

All the alkali metals show an a+1 oxidation state which is equal to the number of electrons present in their valence shell. All the M+ions have a noble gas: configuration. Their M+ cations are diamagnetic in nature and form colourless compounds. The permanganates and dichromates of these cations are coloured.

10- Hydration energies of M+ cations:

Alkali metal ions are easily hydrated and this property depends on the size of the ion. The smaller the size of a cation greater will be its degree of hydration. Thus, hydration energy decreases from top to bottom in a group.
$$ M_{\left(g\right)}^+\:+\:H_2O\:\frac{hydrated\:cation}{ }\left[M_{\left(aq\right)}\right]^+ $$
 
These hydrated cations whose degree of hydration decreases with the decrease of their hydrated radii and increase in ionic conductance.
Hydrated ions
Li+(aq) >
Na+(aq) >
K+(aq) >
Rb+(aq)
Cs+(aq)
Hrdrated radii (Aº)
3.40 >
2.76 >
2.32 >
2.28 >
2.27
 
Hydration energy is defined as the amount of energy released when 1 g mole of an ion in the gaseous state is dissolved in water to get it hydrated”. It is an exothermic process. The greater hydration energy of lithium is responsible for its greater reducing power as compared to the other elements.

11- Standard electrode potential and reducing property of alkali metals:

All the alkali metals undergo an oxidation reaction in an aqueous solution. They have very high values of standard reduction electrode potential as shown in table 4.2.
M + water → [Maq]+ + e-(oxidation reaction)
M(s)S→ M(g)(IE)→ M+(g) + e- and
M(g)+ + water → [M(aq)]+
All the alkali metals act as strong reducing agents. Amongst all the alkali metals lithium is the most powerful reducing agent whose standard electrode potential value is −3.05 V. Alkali metals liberate H2 gas from water vigorously. They show a high value of hydration energy and can lose electrons very easily.

12- Colour of the flame:

All the alkali metals when exposed to light fibrate their outermost shell electron and show a photoelectric effect. Due to this property, K and Cs are used in photoelectric cells. The easily removable electron of all the alkali metals becomes excited to a higher energy level when put to a flame test. All the excited electrons then return to their original energy level and liberate an extra amount of energy gained by them. The amount of liberated energy is small and appears in the form of tight in the visible region ( 400 nm to 750 nm ) of the spectrum and shows characteristic colour. For the same amount of excitation energy, the energy level of the excited electron in alkali metals follows the order given below:
Li < Na < K < Hb < Cs
This value depends upon the ionization energy of these metals. Amongst all, the energy released in the case of L is the lowest and increases from Li to Cs. As a result of this, the light radiation emitted will be maximum in the case of Ll and decreases from Li to Cs. Lithium produces the crimson red colour of flame whereas others show yellow colour.

13- Reaction with Ammonia:

When ammonia gas is passed over alkali metals, the corresponding metal amides are formed.
2M + 2NH3 → 2MNH2(Amide) + H2↑ (M = Li, Na, K, Rb, Cs)
The alkali metal amides are also formed by evaporating the solutions of alkali metals in anhydrous liquid ammonia

14- Solubility in liquid Ammonia:

All alkali metals give deep blue-coloured solutions when dissolve in anhydrous liquid ammonia. The colour deepens with an increase in the concentration of the solutions. Various unit processes are involved during the ammonolysis of alkali metals.
(i) Ionization of the metals
M → M+ + e-
(ii) Cotaon combines with ammonia molecules and the formation of ammoniated metal cation.
M+ + xNH3 → [M(NH3)x]+ (Ammoniated metal cation)
(iii) Formation of ammoniated electrons by the combination of the free electron with ammonia molecules.
e- + yNH3 → [e(NH3)y]-(Ammoniated electron)
Thus, the overall reaction can be written as : 
M + (x-y)NH3 → [M(NH3)x]+ + [e(NH3)y]-

The important characteristics of alkali metal-ammonia solution are as follows :

(a) Colour:

The blue colour of the alkali metal-ammonia solution is due to the excitation of free electrons to higher energy levels as a result of the absorption of photons in the red region of the spectrum, As the concentration of the alkali metal increases, the metal ion cluster formation takes place and the solution becomes bronze coloured and possesses metallic lustre.

(b) Conductivity:

The high electrical conductivity of the solution is due to the presence of ammoniated electrons as well as ammoniated cations.

(c) Paramagnetism:

The dilute alkali metal-ammonia solutions are paramagnetic due to the presence of free ammoniated electrons.
As the concentration of the solution increases, the ammoniated electrons associate to form electron pairs which results in a decrease in the paramagnetic character.
2[e(NH3)y]- → [e(NH3)y]-.[e(NH3)y]-

(d) Reducing property:

The free ammoniated electrons make the solution a very powerful reducing agent as evidenced by the following reactions:
$$ O^-_2\frac{\left[e\left(NH_3\right)y\right]^-}{ }\:O^-_2 $$
$$ O^-_2\frac{\left[e\left(NH_3\right)y\right]^-}{ }\:O^{2-}_2 $$
R—C≡CH →(Na) in liq. NH3→ R—C≡CNa+ + 1/2H2
The aqueous solution of alkali metal salts dissociates as follows : 

MX(aq) ⇔ M+(aq) + X(aq)
H2O(i) ⇔ H+ + OH 

 
On electrolysing the solution, both cations i.e. M+ and H+ ions move towards the cathode. Since the H+ ion has a greater charge density and smaller size than that of an alkali metal ion, the H+ ion reduces to H2 gas at the cathode more readily than the M+ ion. The evolution of H2 gas decreases the reduction power of alkali metal ions and deposition of pure alkali metal at the cathode becomes difficult.
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