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Tuesday, January 1, 2019


NAMING OF 1st GROUP: The group 1st containing Li, Na, K, Rb, Cs & Fr , Francium is radioactive and has a very short life ( t1/2=  21 minutes ), therefore very little is known about it. They are commonly called alkali metals because form hydroxide when react with water.
RELATIVE ABUNDANCE:  Na and K are 6th and 7th most abundance elements in earth crust respectively and both Na, K constitutes 4% of total earth crust.
                                        Na > K > Rb > Li > Cs
STRUCTURE OF ALKALI METALS: At normal temperature all the alkali metals are adopt BBC (Body Centred cubic) type lattice with coordination number (8) but at low temperature Li adopt HCP with coordination number (12).
The general electronic configuration of alkali metals may be represented by [noble gas]     nswhere n = 2 to 7

Except Francium (Fr) all the alkali metals are soft, malleable and metallic lusture when they are freshly cut due to oscillation of loosely binded electrons.  Down the group softness increases due to the decreasing of cohesive energy hence Li is the hardest element while Cs is the softest element in first group.
All the alkali elements are silvery white solid. The silvery luster of alkali metals is due to the presence of highly mobile electrons of the metallic lattice. There being only a single electron per atom,
These are highly malleable and ductile. the metallic bonding is not so strong. As the result, the metals are soft in nature. However, the softness increases with increase in atomic number due to continuous decrease in metallic bond strength on account of an increase in atomic size.
COHESIVE ENERGY: Cohesive energy is just reverse of atomization energy, magnitude is same but sign is different. The (energy) force by which atoms or ions are bind together in solid state called cohesive energy.
The atoms of alkali metals have the largest size in their respective periods. The atomic radii increase on moving down the group among the alkali metals.
                                  Li < Na < K < Rb < Cs < Fr
REASON: On moving down the group a new shell is progressively added. Although, the nuclear charge also increases down the group but the effect of addition of new shells is more predominant due to increasing screening effect of inner filled shell on the valence s-electrons. Hence the atomic size increases in a group.
IONIC RADIUS: Alkali metals change into positively charged ions by losing their valence electron. The size of cation is smaller than parent atom of alkali metals. However, within the group the ionic radii increase with increases in atomic number.
                                     Li+ < Na+ < K+ < Rb+ < Cs+ < Fr+
HYDRATED RADIUS: The alkali metal ions get extensively hydrated in aqueous solutions. Smaller the ion more is the extent or degree of hydration. Thus, the ionic radii in aqueous solution follow the order
                                     Li+ > Na+ > K+ > Rb+ > Cs+ > Fr+
The charge density on Li+ is higher in comparison to other alkali metals due to which it is extensively hydrated.
 The first ionization energy of the alkali metals are the lowest as compared to the elements in the other group. The ionization energy of alkali metals decreases down the group.
REASON: The size of alkali metals is largest in their respective period. So the outermost electron experiences less force of attraction from the nucleus and hence can be easily removed.
The value of ionization energy decreases down the group because the size of metal increases due to the addition of new shell along with increase in the magnitude of screening effect.
The alkali metals show +1 oxidation state. The alkali metals can easily loose their valence electron and change into uni-positive ions
REASON: Due to low ionization energy, the alkali metals can easily lose their valence electron and gain stable noble gas configuration. But the alkali metals cannot form M+2
ions as the magnitude of second ionization energy is very high.
The alkali metals have low values of reduction potential and therefore have a strong tendency to lose electrons and act as good reducing agents. The reducing character increases from sodium to caesium. However lithium is the strongest reducing agent.
                               Li > Na > K > Rb > Cs > Fr
REASON: The alkali metals have low value of ionization energy which decreases down the group and so can easily lose their valence electron and thus act as good reducing agents.  The reducing character of any metal is best measured in terms of its electrode potential which among other things depends upon its
(1) Heat of vaporization
 (2) Ionization energy and
 (3) Heat of hydration.
Since Li+ ion has the smaller size, its heat of hydration has the highest value. Therefore, among the alkali metals Li has the highest negative electrode potential (E0 cell=3.05 volts) and hence is the strongest reducing agent.
On account of their low ionization energies, these metals have a strong tendency to lose their valence electrons and thus change into positive ions. Consequently, alkali metals are strongly electropositive or metallic in character. As this tendency for losing electrons increases down the group, the electropositive character increases.
                                   Li < Na < K < Rb < Cs
(7) COLOUR: The compounds of alkali metals are typically white
(8) MAGNETIC BEHAVIOR: The compounds of alkali metals are diamagnetic. Superoxides of alkali metals are, however, paramagnetic.
(9) HYDRATION: Most of alkali metal salts dissolve in water. In solution alkali metal ions are hydrated. Since Li+ ion is smallest in size it is most heavily hydrated. Salts of lithium such as LiF, Li2CO3, and Li3PO4 are insoluble in water.
The melting and boiling points of alkali metals are very low because the intermetallic bonds in them are quite weak. And this decreases with increase in atomic number with increases in atomic size. 
The densities of alkali metals are quite low as compared to other metals. Li, Na and K are even lighter than water. The density increases from Li to Cs.
REASON: Due to their large size, the atoms of alkali metals are less closely packed. Consequently have low density. On going down the group, both the atomic size and atomic mass increase but the increase in atomic mass compensates the bigger atomic size. As a result, the density of alkali metals increases from Li to Cs. Potassium is however lighter than sodium. It is probably due to an unusal increase in atomic size of potassium.
All the alkali metals form ionic (electrovalent) compounds. The ionic character increases from Li to Cs because the alkali metals have low value of ionization energies which decreases down the group and hence tendency to give electron increases to form electropositive ion.
The alkali metals are good conductors of heat and electricity. This is due to the presence of loosely held valence electrons which are free to move throughout the metal structure.
Ionic mobility of ion is inversely proportional to size of hydrated ion
Size of the hydrated ion is = Li+(aq) > Na+(aq) > K+(aq) > Rb+(aq) > Cs+(aq)
Order of ionic mobility        = Li+(aq) < Na+(aq) < K+(aq) < Rb+(aq) < Cs+(aq)
Alkali metals (except Li) exhibit photoelectric effect (A phenomenon of emission of electrons from the surface of metal when light falls on them). The ability to exhibit photoelectric effect is due to low value of ionization energy of alkali metals. Li does not emit photoelectrons due to high value of ionization energy.  Generally K, Rb, Cs used photoelectric cell (mainly Cs).
The alkali metals and their salts impart a characteristic colour to flame
REASON: On heating an alkali metal or its salt (especially chlorides due to its more volatile nature in a flame), the electrons are excited easily to higher energy levels because of absorption of energy. When these electrons return to their ground states, they emit extra energy in form of radiations which fall in the visible region thereby imparting a characteristic colour to the flame.
ILLUSTRATIVE EXAMPLES (1): Why are Group 1 elements called alkali metals?
SOLUTION: The Group 1 elements are called alkali metals because they form water soluble hydroxides.
ILLUSTRATIVE EXAMPLE (2): What is the most reactive alkali metal and why?
SOLUTION: The most reactive alkali metal is cesium due to its lowest first ionization enthalpy and lowest electronegativity.
ILLUSTRATIVE EXAMPLE (3): The alkali metals have low densities. Explain. ?
SOLUTION: The alkali metals have low densities due to their large atomic sizes. In fact, Li, Na and K are even lighter than water.
ILLUSTRATIVE EXAMPLE (4): Write three general characteristics of the elements of s-block of the periodic table which distinguish them from the elements of the other blocks.
SOLUTION: The three general characteristics are:
            (1)       The compounds of the s-block elements are mainly ionic.
            (2)       The valency is equal to the group number.
            (3)       Due to low ionization energy, s-block elements are good reducing agents.
ILLUSTRATIVE EXAMPLE (5) which alkali metal is most abundant in earth’s crust?
SOLUTION: Sodium is the most abundant alkali metal in the earth’s crust.
ILLUSTRATIVE EXAMPLE (6): Why is the density of potassium less than sodium?
SOLUTION: This is due to abnormal increase in the atomic size of potassium.
ILLUSTRATIVE EXAMPLE (7): Why is lithium the strongest reducing agent in the periodic table?
SOLUTION: The Eo value (reduction potential) depends on the three factors i.e. sublimation, ionization and hydration enthalpies. With the small size of its ion lithium has the lightest hydration enthalpy which accounts for its high negative Eo value and its reducing power
ILLUSTRATIVE EXAMPLE (8): Name the metal which floats on water without any apparent reaction with it.
ILLUSTRATIVE EXAMPLE (9): Which is softer – Na or K and why?
SOLUTION: Potassium is softer than sodium due to weak metallic bonding because of the large size of K atoms. 
ILLUSTRATIVE EXAMPLE (10): What makes sodium highly reactive?
SOLUTION: Low ionization enthalpy, strongly electropositive nature, tendency to attain noble gas configuration by the loss of one valence electron makes sodium highly reactive.
ILLUSTRATIVE EXAMPLE (11): Alkali metals impart colour to Bunsen flame due to
            (A) The presence of one electron in their outermost orbital
            (B) Low ionization energies                      
            (C) Their softness
            (D) Their reducing nature
ILLUSTRATIVE EXAMPLE (12): The metallic lustre exhibited by sodium is explained by
            (A) Diffusion of sodium ions         
            (B)Oscillation of loose electrons
            (C) Excitation of free protons
            (D) Existence of body–centered cubic lattice
SOLUTION:  (B)