Group 2 – Alkaline earth metals (ns2)
Be
Beryllium – 1s2, 2s2 or [He]2s2
Mg
Magnesium- [Ne]3s2
Ca
Calcium- [Ar]4s2
Sr
Strontium - [Kr]5s2
Ba
Barium - [Xe]6s2
Ra
Radium - [Rn]7s2
valence electron
configuration is ns2
n = the period number.
Ra is radioactive
1
Extraction of alkaline earth metals
Electrolysis of Molten MgCl2
Variation in properties of the group II
elements and their compounds
•
•
•
•
•
•
Atomic radii
Ionization enthalpies
Hydration enthalpies
Lattice enthalpies
Melting points
Reactions with oxygen, water, and hydrogen
Atomic radii (nm)
Li
0.152 Be
0.112
Na
0.186 Mg
0.160
K
0.231 Ca
0.197
Rb
0.244 Sr
0.215
Cs
0.262 Ba
0.217
Fr
0.270 Ra
0.220
Atomic radius (nm)
Fr
Li
Be
Ra
2. Ionization Enthalpy
Group II 1st I.E/
kJmol-1
Be
900
2nd I.E./ 3rd I.E./
kJmol-1 kJmol-1
1760
14800
Mg
736
1450
7740
Ca
590
1150
4940
Sr
548
1060
4120
Ba
502
966
3390
Ionization Enthalpy
1st I.E.
2000
600
Li
500
Be+
Na
1500
400
K
Rb
Cs
300
2nd IE
Ca+
1000
Ba+
Be
500
Ca
1st IE
Ba
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
8
Hydration Enthalpy/ (∆H hydration)
Alkaline
earth
Meta ion
M2+
H2O
It is the energy liberated at the time of hydration.
M2+(g) + aqueous → M2+(aq) + heat
Hydration Enthalpy/ (∆H hydration /kJmol-1)
-600
-2250
-2000
-300
-1750
-1500
Li+
Na+
K+
Rb+ Cs+
Be2+ Mg2+ Ca2+ Sr2+ Ba2+
Variation in Melting Points
0
C
Be
1250
1000
Ca
Sr
750
Ba
Mg
500
250
Li
Na
10
K
20
Rb
30
40
Cs
50
60
Group II metals usually have a higher melting point
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
Lattice Energy
The enthalpy of formation of the ionic compound
from gaseous ions
L.E. = z+ z- e2
r
Lattice energies of some compounds (kJmol-1)
MO
Mg
Ca
Sr
Ba
-3923
-3517
-3312
-3120
MCO3
-3178
-2986
-2718
-2614
MF2
-2906
-2610
-2459
-2367
Solubility
Solubility depends on the lattice energy of the solid,
and the hydration energy of the ions.
MX2(s)
aqueous
∆H solution
M2+(aq) + 2X-(aq)
Q. 4
Consider Ca(OH)2 and Ba(OH)2.
(i) Which compound has higher lattice energy?
(ii) Which compound has higher hydration energy?
( Only “ > “ sign should be used.
Chemical Properties
Reaction with H2O
Reaction with O2
Reaction with H2
Reaction with halogens
Beryllium Chloride
Bonding.
dimer
Complexes
The factors favoring complex formation are :
Small, highly charged ions with
suitable empty orbitals of low energy
The Group II elements
The Group II elements are metallic solids
and typically form ionic compounds.
Beryllium is exceptional in forming many
covalent compounds.
Bonding in Compounds
The kind of bond formed between the two atoms
depends on the amount of sharing of the electrons
• Kinds of Bonds
Ionic
electron transferred from one
atom to another
generally between a metal
cation and a non metal anion
Covalent
electrons shared between 2
atoms in a bond
Fajans’ rule
Fajans put forward 4 rules which summarize
the factors favoring polarization and hence covalency
1. Covalent character increases with increasing
anion size.
2. Covalent character increases with decreasing
cation size.
3. The covalent character increases
with increasing charge on either ion.
4. Covalent character is greater for cations with
non-noble gas electronic configurations.
Anomalous behavior of Be
1. Because of high charge density and small
atomic size it forms compounds which are
largely covalent.
2. Beryllium does not exhibit co-ordination
number more than four.
3. The oxide and hydroxide of beryllium, unlike
the hydroxides of other elements in this group
are amphoteric in nature