Trends in Group 2 Elements (Alkaline Earth Metals)
Trends in Group 2 Elements (Alkaline Earth Metals)
Trends in Group 2 Elements (Alkaline Earth Metals)
GROUP 2
ELEMENTS
(ALKALINE
EARTH
METALS)
THE SYLLABUS
8.1 Characteristic Properties
Metallic character
Low electronegativity
Formation of basic oxides and hydroxides
Fixed Oxidation state in their compounds
Weak tendency to form complexes
Flame colours of salts flame test
THE SYLLABUS
8.2 Variation in properties of the sblock
elements and their
compounds
Variations in atomic radii, ionisation enthalpies, hydration
enthalpies and melting points.
OBJECTIVES
ALKALINE EARTH
METALS
Alkaline Earth
Metals includes
Beryllium,
Magnesium,
Calcium,
Strontium and
Barium
Group II elements:
silvery in colour
harder and higher boiling and melting points than
Group I counterparts
stronger metallic bond due to 2e are contributed to
form bond and smaller atomic sizes
show different crystal structures
Calcium
Group II elements:
Beryllium
Strontium
Magnesium
Barium
Radium
0.112
0.160
0.197
0.215
0.217
0.220
0.031
0.065
0.099
0.113
0.135
0.140
h
h
f
f
b
1278
648.8
839
769
729
697
Boiling
point
(C)
Density
(g cm3)
Abundance
on earth
(%)
2477
1100
1480
1380
1640
1140
1.85
1.75
1.55
2.54
3.60
5.0
0.000 28
2.33
4.15
0.038
0.042
Trace
ATOMIC RADIUS
The atomic radius of an element is a measure of its atoms size
ATOMIC RADIUS
Be
Mg
ATOMIC RADIUS
Lets look at Beryllium and Magnesium
Be
Mg
1S22s2
1s22s22p63s2
ATOMIC RADIUS
The only factor which is going to affect the size
of the atom is therefore the number of layers of
inner electrons which have to be fitted in around
the atom. Obviously, the more layers of
electrons you have, the more space they will
take up - electrons repel each other. That means
that the atoms are bound to get bigger as you
go down the Group
In each case, the two outer electrons feel a net
pull of 2+ from the nucleus. The positive charge
on the nucleus is cut down by the negativeness
of the inner electrons.
This is equally true for all the other atoms in
Group 2. This is as a result that the number of
IONIZATION ENERGY
IONIZATION ENERGY
Ionisation energy is governed by
the charge on the nucleus,
the amount of screening by the inner electrons,
the distance between the outer electrons and the
nucleus.
As you go down the Group, the increase in nuclear
charge is exactly offset by the increase in the
number of inner electrons. Just as when we were
talking about atomic radius further up this page, in
each of the elements in this Group, the outer
electrons feel a net attraction of 2+ from the
centre.
However, as you go down the Group, the distance
between the nucleus and the outer electrons
Ionization Enthalpy
Variation in Properties of the s-Block Elements (SB p.54)
ELECTRONEGATIVITY
ELECTRONEGATIVITY
ELECTRONEGATIVITY
Imagine a bond between a magnesium atom and a
chlorine atom. Think of it to start with as a covalent
bond - a pair of shared electrons. The electron pair will
be dragged towards the chlorine end because there is
a much greater net pull from the chlorine nucleus than
from the magnesium one.
ELECTRONEGATIVITY
The electron pair ends up so close to the
chlorine that there is essentially a transfer of
an electron to the chlorine - ions are formed.
The large pull from the chlorine nucleus is why
chlorine is much more electronegative than
magnesium is.
ELECTRONEGATIVITY
Now compare this with the beryllium-chlorine bond.
The net pull from each end of the bond is the same as before, but you
have to remember that the beryllium atom is smaller than a magnesium
atom. That means that the electron pair is going to be closer to the net
2+ charge from the beryllium end, and so more strongly attracted to it.
In this case, the electron pair doesn't get attracted close enough to the
chlorine for an ionic bond to be formed. Because of its small size,
beryllium forms covalent bonds, not ionic ones. The attraction between
the beryllium nucleus and a bonding pair is always too great for ions to be
formed.
As the metal atoms get bigger, any bonding pair gets further and
further away from the metal nucleus, and so is less strongly attracted
towards it. In other words, as you go down the Group, the elements
become less electronegative.
As you go down the Group, the bonds formed between these elements
and other things such as chlorine become more and more ionic. The
bonding pair is increasingly attracted away from the Group 2 element
towards the chlorine (or whatever).
BOILING POINT
There is no
regular trend in
the boiling point
as you can see
there is a drastic
decrease with
magnesium then
an increase in
boiling point to
calcium,
strontium and
barium.
Ca(OH)2(aq) + H2(g)
Sr(s) + 2H2O(l)
Sr(OH)2(aq) + H2(g)
25
5.3 Group 2
Magnesium oxide
2Mg (s) + O2 (g)
2MgO (s)
a. in the air
b. in oxygen
5.3 Group 2
5.3 Group 2
Reaction with water
Mg (s) + 2H2O (l)
slowly
steam
rapidly
Beryllium does not react directly with water all. The rest of the Group
II metals react with increasing rapidity on descending the group.
5.3 Group 2
Oxide reaction with water
MgO (s) + H2O (l)
Mg(OH)2 (aq)
Partially soluble
In the saturated solution, pH(Mg(OH) 2) = 10
The rest of the Group II oxides react with increasing rapidity on
descending the group.
5.3 Group 2
Reaction with acids
Mg (s) + H2SO4(aq)
5.3 Group 2
Thermal stability describes how easily or otherwise a compound will
decompose on heating. Increased thermal stability means a higher
temperature is needed to decompose the compound.
Group II
Carbonates,
CO32
Nitrates,
NO3
M(NO3)2 MO +
2NO2 + 1/2O2
Same pattern but higher
temperatures needed for
decomposition
NO2 + O2-
32
33
34
41.2 Characteristic Properties of the s-Block Elements (SB p.46, notes p. 14)
Group II Elements
2BeO(s)
2Mg(s) + O2(g)
2MgO(s)
2Ca(s) + O2(g)
2CaO(s)
2Ba(s) + O2(g)
2BaO(s)
2BaO(s) + O2(g)
2BaO2(s)
35
MgO(s) + CO2(g)
Ca(OH)2(s)
2+
Mg
Mg2+
36
O H
O H
O
O
C O
CaO(s) + H2O(g)
MgO
MgO
H2O
CO2
Group II
element
Be
Mg
Ca
Sr
Ba
Normal
oxide
BeO
MgO
CaO
SrO
BaO
Peroxide
SrO2
BaO2
Superoxide
Reason:
High charge density high polarizing power
serious distortion on electron cloud of the
peroxide ion
37
Ca(OH)2(aq)
(weakly alkaline)
BeO is amphoteric
BeO(s) + 2H+(aq)
hot
Be2+(aq) + H2O(l)
[Be(OH)4]2(aq)
38
BaO2(s) + 2H2O(l)
Ba(OH)2(aq) + H2O2(aq)
Normal oxides:
e.g. CaO(s) + 2HCl(aq)
39
CaCl2(aq) + H2O(l)
Peroxides:
e.g. Na2O2(s) + 2HCl(aq)
2NaCl(aq) + H2O2(aq)
Superoxides:
e.g. 2KO2(s) + 2HCl(aq)
40
Na2Be(OH)4(aq)
ii. Hydroxides
Group II:
All decompose on heating forming metal
oxides and water.
41
Melting point/
MgO
2852
CaO
2614
SrO
2430
BaO
1918
refractory
material
As M2+ cationic size increases down the Group, the ionic bonds
become weaker, hence, less energy is needed to break the
bonds and a low melting point is expected.
CaCO3(limestone)
+ CO2
Ca(OH)2(slaked lime)
+ H2O
CaO(lime)
temporary hardness
permanent hardness
Chemical
Formula
Chemical Reaction
Mg(OH)2
Mg(OH)2 + 2H+
Mg2+ + 2H2O
CaCO3
CaCO3 + 2H+
Ca2+ + H2O + CO2 (g)
NaHCO3
NaHCO3 + H+
Na+ + H2O + CO2 (g)
Al(OH)3
NaAl(OH)2CO3
NaAl(OH)2CO3 + 4H+
Na++ Al3++ 3H2O +
CO2(g)
Na+ (aq)
NaCl (s)
Hsolu
Na+(aq) + Cl(aq)
Hlatt =
Hhyd =
772 kJmol-1
776 kJmol-1
Na+(g) + Cl(g)
49
Questions
Describe and explain how the thermal stability of the carbonates
of the group 2 elements varies down the group. Write and
Equation for the decomposition of a carbonate of a named Group
2 elements
With reference to lattice energy and hydration energy account
for the variation of the sulfates of group 2 elements
Explain the variation I thermal decomposition of nitrates of group
2 elements. Write an equation for the decomposition of nitrate of
a named group 2 element