Introduction To Group II Alkaline Earths Metals

1. ©HOPTON
AN INTRODUCTION TO
GROUP II
Alkaline earths
• By
• Dr. A. S. Dighade
• J.D.Patil Sanguldkar Mahavidyalay, Daryapur

2. INTRODUCTION
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GROUP II (Alkaline Earths)
©HOPTON

3. CONTENTS
• General properties
• Trends in electronic configuration
• Trends in atomic and ionic radius
• Trends in melting point
• Trends in ionisation energy
• Reaction with oxygen and water
• Oxides and hydroxides
• Carbonates
• Sulfates
GROUP II
©HOPTON

4. GROUP PROPERTIES
GENERAL • metals
• all have the electronic configuration ... ns2
TRENDS • melting point
• electronic configuration
• electronegativity
• atomic size
• ionic size
©HOPTON

5. THE s-BLOCK ELEMENTS
Elements in Group I (alkali metals) and Group II (alkaline earths) are known as
s-block elements because their valence (bonding) electrons are in s orbitals.
Be
Gp I
Mg
Ca
Sr
Ba
Rn
Li
Na
K
Rb
Cs
Fr
Gp II
ALKALINE EARTHS
ALKALI METALS
1s2 2s2
Francium and radium are both
short-lived radioactive elements
… 5s2
… 6s2
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s23p64s2
1s2 2s1
… 5s1
… 6s1
1s2 2s2 2p6 3s1
1s2 2s2 2p6 3s23p64s1
©HOPTON

6. GROUP TRENDS
As the nuclear charge increases, the electrons go into shells further
from the nucleus.
The extra distance of the outer shell from the nucleus affects…
Atomic radius Ionic radius
Ionisation energy Melting point
Chemical reactivity
Be
1s2 2s2
Mg
…3s2
Ca
… 4s2
Sr
… 5s2
2,2 2,8,2 2,8,8,2 2,8,18,8,2
New e/c
Old e/c
ELECTRONIC CONFIGURATION
4 12 20 38
Atomic Number
Ba
… 6s2
2,8,18,18,8,2
56
©HOPTON

7. GROUP TRENDS
ATOMIC RADIUS INCREASES down Group
• the greater the atomic number
the more electrons there are;
these go into shells increasingly
further from the nucleus
ATOMIC & IONIC RADIUS
Be Mg Ca Sr
0.106 0.140 0.174 0.191
Atomic radius / nm
Ba
0.198
2,2 2,8,2 2,8,8,2 2,8,18,8,2
Electronic config. 2,8,18,18,8,2
• atoms of Group II are smaller than
the equivalent Group I atom
the extra proton exerts a greater
attraction on the electrons
1s2 2s2 2p6 3s2 1s2 2s2 2p6 3s23p64s2
12 protons
1s2 2s2 2p6 3s2
11 protons
1s2 2s2 2p6 3s1
©HOPTON

8. GROUP TRENDS
ATOMIC & IONIC RADIUS
Be Mg Ca Sr
0.106 0.140 0.174 0.191
Atomic radius / nm
Ba
0.198
2,2 2,8,2 2,8,8,2 2,8,18,8,2
Electronic config. 2,8,18,18,8,2
Be2+ Mg2+ Ca2+ Sr2+
0.030 0.064 0.094 0.110
Ionic radius / nm
Ba2+
0.134
2 2,8 2,8,8 2,8,18,8
Electronic config. 2,8,18,18,8
IONIC RADIUS INCREASES down Group
• ions are smaller than atoms – on removing the outer shell
electrons, the remaining electrons are now in fewer shells
1s2 2s2 2p6 3s2 1s2 2s2 2p6 3s23p64s2
1s2 2s2 2p6 1s2 2s2 2p6 3s23p6
©HOPTON

9. GROUP TRENDS
DECREASES down Group
• each atom contributes two electrons to the delocalised cloud
• metallic bonding gets weaker due to increased size of ion
• Group I metals have lower melting points than the equivalent Group II
metal because each metal only contributes one electron to the cloud
NOTE Magnesium doesn’t fit the trend because crystalline
structure can also affect the melting point of a metal
MELTING POINT
Be Mg Ca Sr
2,2 2,8,2 2,8,8,2 2,8,18,8,2
Electronic config.
1283 650 850 770
Melting point / ºC
Ba
2,8,18,18,8,2
710
Larger ions mean
that the electron
cloud doesn’t bind
them as strongly
©HOPTON

10. FIRST IONISATION ENERGY
DECREASES down the Group
Despite the increasing nuclear charge the values decrease due to the
extra shielding provided by additional filled inner energy levels
Be Mg Ca Sr
899 738 590 550
1st I.E. / kJ mol-1
Ba
500
1800 1500 1100 1100 1000
14849 7733 4912 4120 3390
2nd I.E. / kJ mol-1
3rd I.E. / kJ mol-1
BERYLLIUM
There are 4 protons pulling
on the outer shell electrons
1st I.E. = 899 kJ mol-1
12+
4+
MAGNESIUM
There are now 12 protons
pulling on the outer shell
electrons. However, the extra
filled inner shell shield the
nucleus from the outer shell
electrons. The effective nuclear
charge is less and the
electrons are easier to remove.
1st I.E. = 738 kJ mol-1
©HOPTON
©HOPTON

11. Successive Ionisation Energy values get larger
Be Mg Ca Sr
899 738 590 550
1st I.E. / kJ mol-1
Ba
500
1800 1500 1100 1100 1000
14849 7733 4912 4120 3390
2nd I.E. / kJ mol-1
3rd I.E. / kJ mol-1
12+
1st I.E. = 738 kJ mol-1
12+ 12+
2nd I.E. = 1500 kJ mol-1
There are now 12 protons and
only 11 electrons. The
increased ratio of protons to
electrons means that it is
harder to pull an electron out.
3rd I.E. = 7733 kJ mol-1
There is a big jump in IE because
the electron being removed is
from a shell nearer the nucleus;
there is less shielding.
SUCCESSIVE IONISATION ENERGIES
©HOPTON

12. CHEMICAL PROPERTIES OF THE ELEMENTS
Reactivity increases down the Group due to the ease of cation formation
OXYGEN react with increasing vigour down the group
Mg burns readily with a bright white flame
0 0 +2 -2
2Mg(s) + O2(g) —> 2MgO(s)
Ba burns readily with an apple-green flame
2Ba(s) + O2(g) —> 2BaO(s)
In both cases…
the metal is oxidised Oxidation No. increases from 0 to +2
oxygen is reduced Oxidation No. decreases from 0 to -2
Mg —> Mg2+ + 2e¯
O + 2e¯ —> O2-
©HOPTON

13. CHEMICAL PROPERTIES OF THE ELEMENTS
Reactivity increases down the Group due to the ease of cation formation
WATER react with increasing vigour down the group
Mg reacts very slowly with cold water
Mg(s) + 2H2O(l) —> Mg(OH)2(aq) + H2(g)
but reacts quickly with steam
Mg(s) + H2O(g) —> MgO(s) + H2(g)
Ba reacts vigorously with cold water
Ba(s) + 2H2O(l) —> Ba(OH)2(aq) + H2(g)
©HOPTON

14. OXIDES OF GROUP II
Bonding • ionic solids; EXCEPT BeO which has covalent character
• BeO (beryllium oxide) MgO (magnesium oxide)
CaO (calcium oxide) SrO (strontium oxide)
BaO (barium oxide)
Reaction
with water
React with water to produce the hydroxide (not Be)
e.g. CaO(s) + H2O(l) —> Ca(OH)2(s)
©HOPTON
BeO MgO CaO SrO
NONE reacts reacts reacts
Reactivity with water
BaO
reacts
Insoluble Sparingly
soluble
Slightly
soluble
Quite
soluble
Very
soluble
- 9-10
Solubility of hydroxide
M(OH)2 in water
pH of 0.1M solution 12.5 13.1
13.0
10.4

15. HYDROXIDES OF GROUP II
Properties basic strength also increases down group
• this is because the solubility increases
• the metal ions get larger so charge density decreases
• get a lower attraction between the OH¯ ions and larger 2+ ions
• the ions will split away from each other more easily
• there will be a greater concentration of OH¯ ions in water
Lower charge density of the larger Ca2+
ion means that it doesn’t hold onto the
OH¯ ions as strongly. More OH¯ get
released into the water. It is more soluble
and the solution has a larger pH.
©HOPTON
Be(OH)2 Mg(OH)2 Ca(OH)2 Sr(OH)2 Ba(OH)2
Insoluble Sparingly
soluble
Slightly
soluble
Quite
soluble
Very
soluble
- 9-10
Solubility
in water
pH of 0.1M solution 12.5 13.1
13.0
10.4

16. HYDROXIDES OF GROUP II
Uses
Ca(OH)2 used in agriculture to neutralise acid soils
Ca(OH)2(s) + 2H+ (aq) —> Ca2+(aq) + 2H2O(l)
Mg(OH)2 used in toothpaste and indigestion tablets as an antacid
Mg(OH)2(s) + 2H+ (aq) —> Mg2+(aq) + 2H2O(l)
Both the above are weak alkalis and not as caustic as sodium hydroxide
©HOPTON

17. CARBONATES OF GROUP II
Properties
• insoluble in water
• undergo thermal decomposition to oxide and carbon dioxide
e.g. MgCO3(s) —> MgO(s) + CO2(g)
• the ease of decomposition decreases down the group
MgCO3 CaCO3 SrCO3 BaCO3
1.5 x 10-4 1.3 x 10-5 7.4 x 10-6 9.1 x 10-6
980
Solubility g/100cm3 of water
Decomposition temperature / ºC 400 1280 1360
One might think that the greater charge density of the smaller Mg2+ would mean that it
would hold onto the CO3
2- ion more and the ions would be more difficult to separate.
The driving force must be the formation of the oxide. The smaller ion with its greater
charge density holds onto the O2- ion to make a more stable compound.
EASIER HARDER
©HOPTON

18. MgSO4 CaSO4 SrSO4 BaSO4
3.6 x 10-1 1.1 x 10-3 6.2 x 10-5 9.0 x 10-7
Solubility g/100cm3 of water
GROUP TRENDS
SULFATES
SOLUBILITY DECREASES down the Group
• as the cation gets larger it has a lower charge density
• it becomes less attracted to the polar water molecules
USE barium sulfate’s insolubility is used as a test for sulfates
Greater charge density of Mg2+ ion
means that it is more attracted to water
so the ionic lattice breaks up more easily
Lower charge density of larger Ca2+ means that it
is less attracted to water so the ionic lattice
breaks up less easily – IT IS LESS SOLUBLE
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19. ©HOPTON
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