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Members of the s-Block

Elements
IA IIA
Li

Be

Na

Mg

Ca

Rb

Sr

IA Alkali metals

Cs

Ba

Fr

Ra

IIA Alkaline Earth

metals

Chapter summary
Characteristic properties of the s-block
elements
Variation in properties of the s-block
elements
Variation in properties of the s-block
compounds
Uses of compounds of the s-block elements

Characteristic properties of sblock elements

Metallic character
Low electronegativity
Basic oxides, hydroxides
Ionic bond with fixed oxidation states
Characteristic flame colours
Weak tendency to form complex

Metallic character
High tendency to lose
e- to form positive ions
Metallic character
increases down both
groups

Electronegativity
Low nuclear
attraction for
outer electrons
Highly
electropositive
Small
electronegativity

Group I

Group II

Li

1.0

Be 1.5

Na

0.9

Mg 1.2

0.8

Ca 1.0

Rb

0.8

Sr

Cs

0.7

Ba 0.9

Fr

0.7

Ra 0.9

1.0

Basic oxides, hydroxides

Oxide

Hydroxides

Oxide

Hydroxides

Li2O

LiOH

BeO

Be(OH)2

Na2O,
NaOH
Na2O2
K2O2, KO2 KOH

MgO

Mg(OH)2

CaO

Ca(OH)2

SrO

Sr(OH)2

Rb2O2,
RbO2
Cs2O2,
CsO2

BaO, Ba2O2 Ba(OH)2

RbOH
CsOH

Oxides, Peroxide, Superoxide

Reaction with water:

Oxide: O2- + H2O 2OHPeroxide: O22- + 2H2O H2O2 + 2OHSuperoxide: 2O2- + 2H2O 2OH- + H2O2 + O2
.. .. 2:O:O:
.. ..

Peroxide ion

. .:O:.O:
.. ..
Super oxide

Li does not form

peroxide or super oxide
Li2O2 Li2O + O2

Hydroxides
Group I
hydroxides

Li

Na

Rb

Cs

All are soluble, base strength

increase.
Group II
hydroxide

Be

Mg

Ca

Sr

Ba

Solubility increase, from

Amphoteric to basic, base strength
increase

Predominantly ionic with fixed

oxidation state
Group I: Most electropositive metals.
Low first I.E. and extremely high second I.E.
Form predominantly ionic compounds with
non-metals by losing one electron.
Fixed oxidation state of +1.
Group II: Electropositive metals.
Low first and second I.E. but very high third
I.E.. Have a fixed oxidation state of +2.
Be and Mg compounds possess some degree
of covalent character.

Characteristic flame colours

Na+ Cl- (g) Na (g) + Cl (g)
Na(g) Na* (g)
[Ne]3s1 [Ne]3p1
Na*(g) Na(g) + h (589nm, yellow)

Flame test
Li
Na
K
Rb
Cs

HCl(aq)

deep red Ca brick red

yellow
Sr blood red
lilac
Ba apple green
bluish red
blue

sample

Weak tendency to form

complex
Complex formation is a common feature of d-block
element. e.g. Co(NH3)63+

s-block metal ions have

no low energy vacant
orbital available for
bonding with lone pairs
of surrounding ligands,
they rarely form
complexes.

:NH3
H3N:
H3N:

:NH3

Co
:NH3

:NH3

Variation in properties of
elements

Atomic radii
Ionization enthalpies
Hydration enthalpies
Melting points
Reactions with oxygen, water, hydrogen
and chlorine

Atomic radii (nm)

Li

0.152 Be

0.112

Na

0.186 Mg

0.160

0.231 Ca

0.197

Rb

0.244 Sr

0.215

Cs

0.262 Ba

0.217

Fr

0.270 Ra

0.220

Fr
Li
Be

Ra

Ionization Enthapy
Group I 1st I.E.

2nd I.E.

Li

519

7300

Na

494

4560

418

3070

Be

900

1760

14800

Rb

402

2370

Mg

736

1450

7740

Cs

376

2420

Ca

590

1150

4940

Sr

548

1060

4120

Ba

502

966

3390

Group II 1st I.E. 2nd I.E. 3rd I.E.

Ionization Enthalpy
1st I.E.
2000

600

Li
500
400
300

Be+
Na
K

1500

Rb

Cs

2nd IE

Ca+

1000

Ba+

Be
500

Ca
1st IE

Ba

Ionization Enthalpy
Group I
1. Have generally low 1st I.E. as it is well shielded
from the nucleus by inner shells.

2. Removal of a 2nd electron is much more difficult

because it involves the removal of inner shell
electron.
3. I.E. decreases as the group is descended.
As atomic radius increases, the outer e is further
away from the well-shielded nucleus.

Ionization Enthalpy
Group II
1. Have low 1st and 2nd IE.
2. Removal of the 3rd electron is much more difficult
as it involves the loss of an inner shell electron.
3. IE decrease as the group is descended.
4. IE of the group II is generally higher than group I.

Hydration Enthalpy
M+(g) + aqueous M+(aq) + heat
-600

M+

-300

Li+ Na+ K+

Rb+ Cs+

Hydration Enthalpy
-2250
-600
-2000
-1750
-300
-1500

Li+ Na+ K+

Rb+ Cs+

Be2+ Mg2+ Ca2+ Sr2+ Ba2+

Hydration Enthalpy
General trends:
1. On going down both groups, hydration enthalpy
decreases.
(As the ions get larger, the charge density of the
ions decreases, the electrostatic attraction between
ions and water molecules gets smaller.)
2. Group 2 ions have hydration enthalpies higher
than group 1.
( Group 2 cations are doubly charged and have
smaller sizes)

Variation in Melting Points

1250

Be

1000
Ca

Sr

750

Ba
Mg

500

250

Li
Na
10

K
20

Rb
30

40

Cs
50

60

Variation in Melting Points

Strength of metallic bond depends on:
1. Ionic radius
2. Number of e- contributed to the electron sea per atom
3. Crystal lattice structure

Note: The exceptionally high m.p. of calcium

is due to contribution of d-orbital participation
of metallic bonding.

Variation in Melting Points

Group I
Li
Na
K
Rb
Cs

Structure Group II
B.C.C.
Be
B.C.C.
Mg
B.C.C.
B.C.C.
B.C.C.

Ca
Sr
Ba

Structure
H.C.P.
H.C.P.
C.C.P.
C.C.P.
B.C.C.

Reactions with oxygen

S-block elements are strong reducing agents.
Their reducing power increases down both groups.
(As the atomic size increases, it becomes easier to
remove the outermost electron)
S-block elements reacts readily with oxygen.
Except Be and Mg, they have to be stored under
liquid paraffin to prevent contact with the atmosphere.

Reactions with oxygen

Normal
Oxide
Structure

Formed by

.. 2:O:
..
Li and
Group II

Peroxide

.. .. 2:O-O:
.. ..
Na and Ba

Superoxide
. .:O:.O:
.. ..
K, Rb, Cs

Reaction with water

M(s) M+(aq) + eH2O(l) + e- OH-(aq) + H2(g)
Li
Na
K
Rb
Cs

-3.05 volt
-2.71
-2.93
-2.99
-3.20

Be -1.85 volt
Mg -2.38
Ca -2.87
Sr -2.89
Ba -2.90

Energetic vs. Kinetic Factor

Reaction with hydrogen

All the s-block elements except Be react directly with
hydrogen.
2Na(s) + H2(g) 2NaH(s)
Ca(s) + H2(g) CaH2(s)
The reactivity increases down the group.
Only BeH2 and MgH2 are covalent, others are ionic.

Reaction with chlorine

All the s-block metals react directly with chlorine
to produce chloride.
All group I chlorides are ionic.

BeCl2 is essentially covalent, with comparatively low

m.p.
The lower members in group II form essentially ionic
chlorides, with Mg having intermediate properties.

Check point 39-3

Although lithium has highly negative Eo, it only
reacts slowly with water. This illustrates the importance
of the role of kinetic factors in determining the rate
of a chemical reaction.

Lithium has a higher m.p., this increases the activation

energy required for dissolution in aqueous solution.
It does not melt during the reaction as Na and K do, and
thus it has a smaller area of contact with water.

Variation in properties of the

compounds

Reactions of oxides and hydroxides

Reactions of chlorides
Reactions of hydrides
Relative thermal stability of carbonates and
hydroxides
Relative solubility of sulphate(VI) and
hydroxde

Reactions of oxides and

hydroxides
1. All group I oxides reacts with water to form
hydroxides
Oxide: O2- + H2O 2OHPeroxide: O22- + 2H2O H2O2 + 2OHSuperoxide: 2O2- + 2H2O 2OH- + H2O2 + O2
2. All group I oxides/hydroxides are basic and the
basicity increases down the group.

Reactions of oxides and

hydroxides
3. Group II oxides/hydroxides are generally less basic
than Group I. Beryllium oxide/hydroxide are
amphoteric.

Reactions of chlorides
1. All group I chlorides are ionic and readily
soluble in water. No hydrolysis occurs.
2. Group II chlorides show some degree of covalent
character.
Beryllium chloride is covalent and hydrolysis to
form Be(OH)2(s) and HCl(aq).
Magnesium chloride is intermediate, it dissolves and
hydrolysis slightly.
Other group II chlorides just dissolve without
hydrolysis.

Reactions of hydrides
They all react readily with water to give the
metal hydroxide and hydrogen due to the
strong basic property of the hydride ion, H:H:-(s)+ H2O(l) H2(g)+ OH-(aq)
Hydride ions are also good reducing agent.
They can be used to prepare complex hydrides
such as LiAlH4 and NaBH4 which are used to
reduce C=O in organic chemistry.

Thermal Stability
Thermal stability refers to decomposition of the
compound on heating. Increased thermal stability
means a higher temperature is needed to decompose
the compound.

Thermal Stability of
carbonates
Li2CO3 Li2O + CO2 ( at 700oC)
All other group I carbonates are stable at ~800oC
BeCO3 BeO + CO2
MgCO3 MgO + CO2
CaCO3 CaO + CO2
SrCO3 SrO + CO2
BaCO3 BaO + CO2

( at 100oC)
( at 540oC)
( at 900oC)
( at 1290oC)
( at 1360oC)

Thermal Stability of
hydroxides
All group I hydroxides are stable except LiOH
at Bunsen temperature.
Be(OH)2(s) BeO(s) + H2O(g)
Mg(OH)2(s) MgO(s) + H2O(g)
Ca(OH)2(s) CaO(s) + H2O(g)
Sr(OH)2(s) SrO(s) + H2O(g)
Ba(OH)2(s) BaO(s) + H2O(g)

H = +54 kJ/mol
H = +81 kJ/mol
H = +109 kJ/mol
H = +127 kJ/mol
H = +146 kJ/mol

Thermal stability
1. Carbonates and hydroxides of Group I metals
are as a whole more stable than those of Group II.
2. Thermal stability increases on descending the group.
3. Lithium often follow the pattern of Group II rather
than Group I.
This is an example of the diagonal relationship.

Explanation of Thermal
Stability
1. Charge of the ions
2. Size of the ions
3. Compounds are more stable if the charge increases
and size decreases.
4. For compounds with large polarizable anions, thermal
stability is affected by the polarizing power of the
cations.

Explanation of Thermal
Stability
+

Decreasing
polarizing
power

Increasing
stability

Explanation of Thermal
Stability
O

Mg2+

Mg2+ O2- + CO2

:O C
O:-:O

-:O

Mg2+

Mg2+ O2- + H2O

Explanation of Thermal
Stability

MgCO3

MgO
BaO
MgO

BaCO3

BaO

Relative solubility of Group II

hydroxides
Compound Solubility / mol per 100g
water
Mg(OH)2

0.020 x 10-3

Ca(OH)2

1.5 x 10-3

Sr(OH)2

3.4 x 10-3

Ba(OH)2

15 x 10-3

Solubility of hydroxides
increases down the group.

Solubility of Group II sulphates

Compound Solubility / mol per 100g
water
MgSO4

3600 x 10-4

CaSO4

11 x 10-4

SrSO4

0.62 x 10-4

BaSO4

0.009 x 10-4

Solubility of sulphates
increases up the group.

Explanation of solubility
aqueous
M+(aq) + X-(aq)

MX(s)
H solution

H hydration

-H lattice
M+(g) + X-(g)

H solution =

-H lattice + H hydration

Explanation of solubility
1. Group I compounds are more soluble than Group II
because the metal ions have smaller charges and
larger sizes. H lattice is smaller, and H solution is
more exothermic.

H solution =

-H lattice + H hydration

Explanation of solubility
H solution =

-H lattice + H hydration

2. For Group II sulphates, the cations are much smaller

than the anions. The changing in size of cations does
not cause a significant change in H lattice (proportional
to 1/(r+ + r-).
22SO
SO
However, the changing in size of4 cations does cause 4
H hydration (proportional to 1/r+ and 1/r-) to become less
exothermic, and the solubility decreases when
descending the Group.
MgSO4

SrSO4

Explanation of solubility
H solution =

-H lattice + H hydration

3. For the smaller size anions, OH-.

Down the Group, less enthalpy is required to
break the lattice as the size of cation increases.
However the change in H solution is comparatively
smaller due to the large value of 1/r- .
As a result, H solution becomes more exothermic
and the solubility increases down the Group.
Mg(OH)2

Sr(OH)2

Uses of s-block compounds

Sodium carbonate
Manufacture of glass
Water softening
Paper industry

Sodium hydrocarbonate
Baking powder
Soft drink

Uses of s-block compounds

Sodium hydroxide
Manufacture of soaps, dyes, paper and drugs
To make rayon and important chemicals

Magnesium hydroxide
Milk of magnesia, an antacid

Calcium hydroxide
To neutralize acids in waste water treatment

Strontium compound
Fireworks, persistent intense red flame

thank