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S-BLOCK ELEMENTS

The s-Block Element

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Group I Elements (Alkali Metals)

They have similar chemical properties. They are soft metals with fixed O.N. = +1 in their compounds.

Electronic configuration

Atomic radius (Å)

Ionic radius

I.E. M.P.

Density E.N. Hhyd of cation

Li He 2s1 1.23 0.60 520 181 0.54 1.0 -519

Na Ne 3s1 1.57 0.95 496 98 0.97 0.9 -406

K Ar 4s1 2.03 1.33 419 63 0.86 0.8 -322

Rb Kr 5s1 2.16 1.48 403 39 1.53 0.8 -293

Cs Xe 6s1 2.35 1.69 375 29 1.87 0.7 -264

Fr Rn 7s1 -- 1.76 -- 27 -- 0.7 --

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Group II Elements (Alkaline Earth Metals)

Mg and Ca are the most abundant elements and Ra is the most scarce element which is unstable and radioactive.

  Electronic configuration

Atomic radius (Å)

Ionic radius

I.E. M.P. Density E.N.

Hhyd of

cation1st 2ndBe He 2s2 0.90 0.31 900 1760 1277 1.85 1.5 -2494

Mg Ne 3s2 1.36 0.65 737 1450 650 1.74 1.2 -1921

Ca Ar 4s2 1.74 0.99 590 1150 838 1.55 1.0 -1577

Sr Kr 5s2 1.91 1.13 548 1060 768 2.60 1.0 -1443

Ba Xe 6s2 1.98 1.35 502 965 714 3.50 0.9 -1305

Ra Rn 7s2 -- 1.40 5.9 979 700 5.00 0.9 --

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Atomic and ionic radius

Atomic radius and ionic radius increaseAn addition of one more shell; ENC decrease

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1 Compare the atomic radius of alkali and alkaline earth metals

Alkali metals > alkaline earth metals becauseIncrease in nuclear charge > increase in shielding effect

ENC increases and electron experience a larger nuclear attraction.

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2 Compare the atomic and ionic radius

Ionic radius < corresponding atomic radiusSame nuclear charge, weaker shielding effectENC increases, stronger nuclear attraction towards electrons.

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Melting and boiling point

M.P. decreasedENC decreased so that nuclear attraction towards electrons ( metallic bond strength)decreased.

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4. Difference between m.p. and b.p. of s-block metals

b.p. > m.p. a lotMost of the metallic bonds remains in the liquid state; nearly all bonds in liquid state have to be broken on vaporization.

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Atomic volume

Atomic volume increasedAtomic size increases,Metallic bond strength decreases.

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5. Compare the atomic volumes of s-block metals

Alkali metals > Alkaline earth metalsAtomic size of alkaline earth metals < alkali metalsMetallic bond strength of alkaline earth metals > alkali metals

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Density

Density increasedAtomic mass increased to a greater extent than atomic volume.

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Alkali metals < Alkaline earth metalsAtomic mass of alkaline earth metals > alkali metalsAtomic volume of alkaline earth metals < alkali metalsDensity = atomic mass / atomic volume

6. Compare the density of s-block metals

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Ionization energy

I.E. decreasedENC decreases due to addition of shells.I.E. of alkaline earth metals > alkali metalsENC increases

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Electron being removed is from the inner shell; the electron thus experience a larger nuclear attraction. Besides, ENC of an ion would be much greater than the corresponding atom as the shielding effect is weaker.

7. Difference between 1st and 2nd I.P. of alkali metals

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Summary

On passing down the group:1. Atomic radius and ionic radius increased

An addition of one more shell; ENC decrease2. I.E. decreased

ENC decreased due to addition of shells.3. M.P. decreased

ENC decreased so that nuclear attraction towards electrons decreased.4. Density increased

atomic mass increased to a greater extent than atomic volume.5. E.A. decreased

ENC decreased so that tendency to accept e- decreased.6. Reducing power and reactivity increased

I.E decrease and reduction potential become more negative.7. Enthalpy of hydration of cation less negative

Electrostatic interaction between the polar water molecules and ions become less as the ionic radius increases.

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Summary of physical properties of Group I and IIA elements

m.p. and b.p. ; density and I.P.

Atomic and ionic radii ; atomic volume

Atomic and ionic radii ; atomic volume

m.p. and b.p. ; density and I.P.

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Variaiton in chemical properties

Owing to the low value of 1st I.P., alkali metals are relatively more easily to form X+, and the resulting compound is quite stable. The sum of 1st and 2nd I.P. of alkaline earth metals is not too low, yet the lattice energy recorded on forming the ionic compounds is large enough for the formation of X2+.

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Try to account for the following redox potentials:

  Eo / V

Li(s) Li+(aq) + e –3.04V

Cs(s) Cs+(aq) + e –2.93V

Rb(s) Rb+(aq) + e –2.93V

K(s) K+(aq) + e –2.92V

Na(s) Na+(aq) + e –2.71V

Reducing power and reactivity of s-block elements

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Reducing power and reactivity of s-block elements

The redox potential:M(s) M+(aq) + e depends on:1. the formation of separate atom from crystal lattice

M(s) M(g)Hsub = heat of sublimation (+ve)

2. the formation of gaseous ion from gaseous atomM(g) M+(g) + eI.E. ionization energy (+ve)

 3. the formation of hydrated ion from gaseous ionM+(g) + aq M+(aq)Hhyd = hydration energy (-ve)

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The overall enthalpy change (H) = Hsub + I.E. + Hhyd

 

H more -ve the greater the redox potential

i.e. the stronger the reducing agent.

Reducing power and reactivity of s-block elements

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From Na to Cs, the reduction potential increased.But Li has greatest reduction potential. 

On passing down the group, both Hatm, I.E. decrease but Hhyd also become less -ve.  

But Hsub and I.E. decrease to a greater extent than the Hhyd, H(overall) is more negative.

Reducing power and reactivity of s-block elements

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Li is an exceptional case, it has the greatest redox potential. It is because the size of Li+ is very very small (it belongs to 2nd period), Hhyd is exceptionally more -ve. Therefore H(overall) is thus more negative.

Reducing power and reactivity of s-block elements

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Variation in chemical propertiesReactions:

1. With air - All tarnish in air (that is, forming a film of oxide on the surface), therefore they are stored in paraffin oil.When burnt in sufficient amount of oxygen :

Kind of oxides Elements which form this type of oxide in adequate supply of air

normal oxide O2-

Li, Mg, Ca, Sr

peroxides Na, Ba

superoxide O2- K, Rb, Cs

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Reaction with air

Dot and cross diagram for oxide O2- ion and peroxide O22- ion :

O – O bond can be easily broken

O O

2-

Size of peroxide ion > size of oxide ion

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Reaction with air

Li+ ion is extremely small, it is not possible for sufficient number of peroxide ions to surround the Li+ ion with causing repulsion between the anions, therefore only normal oxide exists.The larger peroxide and superoxide anions are stabilized by larger cations due to limiting radius ratio.

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Stability of oxide ion > peroxide and superoxide ionBa2+, being the largest ion, has the weakest polarizing power ; electron cloud of the peroxide ion will be distorted by other group IIA [AND also Li+] metal ions and become unstable.Ba2+ is the biggest ion in Group IIA [slightly larger than K+], no severe repulsion would occur between these large peroxide ion when surrounding Ba2+ in the lattice.

9. Why Group IIA elements form normal oxides, except barium ?

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Reaction with air

In case of Li, Mg, Ca, Sr and Ba, the final products will be a mixture of nitrides, carbonates together with the oxides.Only Li in group IA would form Li3N (lithium nitride).

N3- ion is hard to form, why ?6 Li + N2 2 Li3N

Li3N + 3H2O 3LiOH + NH3

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With water - All (except Be) reacts to give out hydrogen.2H2O + 2e- 2OH- + H2 at pH = 7 E = -0.41V

2M + 2H2O 2MOH + 2H2 E = +ve(spontaneous)

• But the vigor of reaction: K > Na > Li although E of Li is greatest. Why? E shows the equilibrium position (i.e. the reaction is

spontaneous or not), E increased implies that equilibrium lies on the product side.

• Rate of reaction must consider the Eact (activation energy). From the information given, the rate of Li is the slowest among the three. That is Eact for Li is the highest, so the rate is relatively slow.

Reaction with water

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The rate of reaction increases on passing down the group. Reactivity of alkali metals towards water is much higher than alkaline earth metals. Magnesium reacts with hot water and steam to give magnesium hydroxide and magnesium oxide respectively.

Reaction with water

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All react vigorously and explosively.2M + 2H+ H2 + 2M+

But reactions between sulphuric acid and Ca, Sr, Ba become less vigorous after the reaction starts due to the formation of insoluble layer of sulphates.

Ca + H2SO4 CaSO4(s) + H2

Reaction with acid

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With non-metal - All combine with X2, S and O2, P or even H2 at suitable temperature.

Li, Mg, Ca, Sr and Ba also combine directly with nitrogen.

Ca + H2 CaH2 calcium hydride

2 K + S K2S potassium sulphide

3 Mg + N2 Mg3N2 magnesium nitride

Reaction with non-metals

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Oxides

All are white crystalline solid, ionic and strongly basic in character. They are hydrolysed by water to form corresponding hydroxides. Degree of hydrolysis increases down the group, since oxide become more ionic.

O2- + H2O 2 OH-

BeO, being exceptional case, is amphoteric :BeO + 2HCl BeCl2 + H2O

BeO + 2OH- + H2O Be(OH)42- beryllate

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Na2O is more basic. Na+ has a weaker

polarizing power than Mg2+ (as the latter one has a higher charge/radius ratio OR charge density), electron in O2- ion is more available to attack hydrogen in water molecule. More hydroxide ion is thus formed.

10. Compare the basic strength of Na2O and MgO

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Hydrides

Formed by heating the element in hydrogen gas (at 400℃ or above) ;Strong reducing agents ;Hydrolysed by water to form hydrogen gas and solution or suspension of hydroxides ;Readiness of hydrolysis increases down the group since the hydride is more ionic ;reaction is more vigorous for alkali metals than for alkaline earth metals

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Chlorides

All are white crystalline solid, soluble in water to form hydrated ion.

NaCl(s) + aq. Na+(aq) + Cl-(aq) Hydrated sodium and chloride ion

No hydrolysis and thus a neutral solution

But for MgCl2, which is partially ionic, hydrolysed by water to give a slightly acidic solution.

MgCl2(s) + 6 H2O Mg(H2O)62+ + 2 Cl-(aq)

Mg(H2O)62+ + H2O Mg(H2O)5(OH)+ + H3O+

Be(H2O)42+ + H2O Be(H2O)3(OH)+ + H3O+

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Thermal stability of other compounds

For a large polarizable anion (e.g. HCO3-,

CO32-,NO3

-, SO42-), the stability depends on the

polarizing power of the cation. If the cation can distort the electron cloud of the anion so much that the bonds (e.g. C-O bond in carbonate) is weakened, the bond will be easily broken on heating to give metallic oxides and gas(es) (CO2 for carbonate).

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Thermal stability

MgCO3 MgO + CO2

MgSO4 MgO + SO3

2 Mg(NO3)2 2 MgO + 2 NO2 + O2

2 NaNO3 2 NaNO2 + O2

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Na2CO3 is thermally stable because polarizing power of Na+ is weaker than

Mg2+ (as the latter one has a higher charge/radius ratio / charge density), electron cloud of the carbonate ion is much

distorted by Mg2+ that the C – O bond is

weakened and thus more easily broken when heated

11. Compare the stability of Na2CO3 and MgCO3

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Polarising power of cation decreases on passing down the group as the size of the cation

become larger. Most group I salts are thermally stable except for those of lithium. While group II salts are relatively less stable to heat. (Note that only lithium carbonate is thermally unstable among group I carbonates.).

Thermal stability

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Thermal stability

Some sodium and potassium salts are decomposed when heated :

2 NaNO3 2 NaNO2 + O2

2 NaHCO3 Na2CO3 + CO2 + H2O

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Solubility of salts in water

All group I compounds are practically soluble.The solubility increases as heat of hydration is more negative than lattice energy.

  Hhyd Hlatt Hhyd - Hlatt

LiI -824 -763 -61NaI -711 -703 -8KI -627 -647 +20

RbI -598 -624 +26CsI -569 -601 +32

LiF -1034 -1039 +6NaF -927 -919 -2KF -837 -817 -20

RbF -808 -779 -29CsF -779 -730 -49

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Lattice energy depends on the sum of the ionic

radii while the hydration energy depend on ionic

radius of the individual ions; both would decrease as

size of ions increases. Hydration energy of a compound is contributed by both the cation and anion.

Solubility of salts in water

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Hydration energy contributed by anion is small, i.e. the hydration energy mainly contributed by the cation. On passing down the group, hydration energy decrease greatly / tremendously. The sum of ionic radii only increase slightly as the size of anion is large, the decrease in lattice energy is small.

Hsoln = Hhyd – LE

Hsoln become less negative (more positive) on passing down the group.

Solubility of salts with large anion (e.g. I-, SO42-,

CO32-) in water

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Solubility of salts with large anion (e.g. I-, SO42-,

CO32-) in water

Compounds Solubility / x 10-3 mol dm-3 Described as

MgSO4 3600 soluble

CaSO4 11 sparingly soluble

SrSO4 0.62 insoluble

BaSO4 0.009 insoluble

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Lattice energy decrease more rapidly than the hydration energy on passing down the group,hydration energy mainly depends on the small anion and would not change much;decrease in lattice energy mainly determines the solubility of the salt.

Hsoln = Hhyd – LE Hsoln become more negative (less positive) on passing down the group.

Solubility of salts with small anion (e.g. F-, OH-) in water

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Solubility of salts with small anion (e.g. F-, OH-) in water

Compounds Solubility / x 10-3 mol dm-3 Described as

Mg(OH)2 0.020 insoluble

Ca(OH)2 1.5 slightly soluble

Sr(OH)2 3.4 soluble

Ba(OH)2 15 soluble

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Compare the solubility of salts of alkali and alkaline earth metals

An increase in charge will increase lattice energy to a greater extent than hydration energy,

salts of alkaline earth metals are generally less soluble than that of alkali metals, anddoubly charged anions give more insoluble compounds.

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General characteristics of s-block elements

Fixed oxidation stateThe only possible positive oxidation state shown by the elements is equal to the total number of electron in the outermost shell. This oxidation state corresponds to the loss of sufficient number of electrons to achieve the octet configuration ns2np6, thus only forms compounds in which they obtain the octet configuration.The loss of more than the valence electron requires too much ionization energy, thus prevents these metals from showing an oxidation number other than the one equal to their group number.

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Ability to form complexesOwing to the lack of underlying (inner) low energy vacant orbital, s-block elements rarely form complexes.

The cations which form stable complexes normally carrying a high charge / radius ratio, resulting in larger electrostatic attraction between the central ions and the ligands.

General characteristics of s-block elements

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Ability to form complexes

Group I metal ions cannot form hydrated ions of definite formula in aqueous solution, though they can by hydrated to certain extent. Lithium ion, which has the smallest size, show certain degree of hydration in crystal of its salts.

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Group II metals ions has higher charge/radius ratio and they have higher tendency to form complexes.Beryllium forms many complex but barium forms very few. e.g. BeF3

- BeF42- Be(H2O)4

2+

The most important complex for Mg is chlorophyll, which has a very complicated structure with fused rings; the Mg atom being at the center of the rings bonded to 4 nitrogen atoms.

Ability to form complexes

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Ca2+ and Mg2+ form stable complex with strong complexing agents. e.g.ethylenediaminetetraacetic acid (EDTA) which has 4 functional oxygen atoms and 2 donor N-atoms per molecule.More discussion ond-block elements.

Ability to form complexes

N CH2CH2 N

CH2C

CH2C

OH

OH

O

O

CCH2

CCH2

HO

HO

O

O

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Abnormality of lithium and its compounds among group IA

1 Lithium carbonate and hydroxide are decomposed by heat.2 Lithium carbonate, hydroxide and fluoride are insoluble in water.3 Lithium forms only normal oxide when reacting with oxygen.4 Lithium forms nitride when heated in air.5 Lithium ion is highly hydrated in water, resulting in lowest mobility.6 Lithium hydroxide is not a strong base.7 Almost all lithium salts are hydrated in its crystal lattice.

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Reason :Exceptional small size of Li+ ion, e.g. forming nitride :

6 Li(s) + N2(g) 2 Li3N(s)

6 Li(g) + 2 N(g) 2 N3-(g) + 6 Li+(g)

Abnormality of lithium and its compounds among group IA

The highly negative LE of Li3N offset the energy required to ionize the nitrogen gas to nitride ion.

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Diagonal relationship between magnesium and lithium

1. Both only form normal oxide.2. Both give nitrides when heating in air.3. Carbonates,sulphates,hydroxides are decomposed by heat to metallic

oxides.4. Carbonates, hydroxides are insoluble in water.Reasons: Effective nuclear charge increases on passing along theperiod but decreases on passing down the group, so Li+

and Mg2+ have similar effective nuclear charge which inturn affecting its polarizing power.

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Flame Test for Metal Ions:

Element Flame Colour Element Flame Colour

Lithium red Calcium brick red

Sodium golden yellow Strontium blood red

Potassium lilac Barium apple green

Rubidium violet    

Caesium violet