GROUP 2
(ALKALI EARTH
METALS)
REACTIONS
Group 2 metals (use pages 178-179 )
How would you represent the first, second and third
ionisation energies of a general metal?
What does the first ionisation energy depend on?
For a group two metal why is it impossible to lose 3 electrons
to form a +3 ion?
Does ionisation energy increase or decrease down
the group? Explain your answer.
Questions from pages 179
Book answers
PROPERTIES
Are ‘s’ block metals
M.p. decreases down the group
Have a valency +2
more reactive down the group
They have more ionic character down the group
Explain these properties
PROPERTIES
Are ‘s’ block metals (2 electrons in the ‘s’ sub-shell)
M.p. decreases down the group because the ionic
radius increases down the group
Have a valency +2
Electronegativities decrease down the group – so
more reactive down the group. Ionic radius increases
down the group
Reactions with OXYGEN
***Reactivity increases as you go down the Group
These metals (X) burn in oxygen to form a simple
metal oxide.
2X(s) + O2(g)  2XO(s)
Give three examples
Reactions with CHLORINE
**Reactivity increases as you go down the group
For each element’s reaction, there is a white solid
residue which forms (simple chloride).
X + Cl2  XCl2
If any of the elements are burned in chlorine, they
would produce the same colours as they would if they
were burned in oxygen (flame tests)
*These metals are very reactive and have to be stored out of
contact with air to prevent their oxidation.
Reactions with WATER
***Reactivity increases as you go down the group
Be has no reaction with water / steam
Mg has an extremely slow reaction in cold water to form a
hydroxide and hydrogen...
Mg(s) + 2H2O(l)  Mg(OH)2(aq) + H2(g)
When heated in steam, it burns quicker to form an oxide
and hydrogen...
Mg(s) + H2O(l)  MgO(s) + H2(g)
Ca, Sr, Ba react with cold water with increasing vigour…
X(s) + 2H2O(l)  X(OH)2(aq) + H2(g)
Simple acid-base titration
Risk assessment
SOLUBILITY OF GROUP 2 HYDROXIDES
This increases down the group:
Be(OH )2– insoluble
Mg(OH)2 – very slightly soluble
Ca(OH)2 – slightly soluble
Sr(OH)2 & Ba(OH)2 – more soluble
SOLUBILITY OF GROUP 2 SULFATES
This decreases down the group:
BeSO4 & MgSO4 – very soluble
CaSO4 – slightly soluble
SrSO4 - very slightly soluble
BaSO4 – insoluble
SOLUBILITY OF GROUP 2 CHLORIDES
All Group 2 chlorides are soluble in water
SUMMARY OF GROUP 1 & 2 ELEMENTS
These decrease down the group:
 Melting temperature
 Ionisation energy
 Polarising power of the metal
These increase down the group:
 Atomic and ionic radii
 Reactivity of the metal
 Thermal stability of its compounds
THERMAL STABILITY
Thermal stability describes how easily a compound can be
broken down by heat. It depends on the polarising power of the
metal. The larger the charge and smaller the radius, the bigger
the charge density. Larger charge density means more polarising
1+
2+
Li
Be
Na
Mg
K
Ca
Thermal stability increases down the
group (harder to decompose the metal )
The ionic radius gets bigger down the
group
As the size of the ion increases, the
polarising power decreases, so the
compound is more stable
THERMAL STABILITY OF GROUP 1 NITRATES
All Group 1 nitrates decompose on strong heating.
However LiNO3 decomposes differently to the others…
4LiNO3(s)  2Li2O + NO2(g) + O2
The other nitrates decompose like so…
2NaNO3(s)  2NaNO2 + O2 (g)
THERMAL STABILITY OF GROUP 2 NITRATES
On heating, all group 2 nitrates decompose like LiNO3 …
2Mg(NO3)2(s)  2MgO + 4NO2(g) + O2
The temperature for decomposition is highest for
Ba(NO3)2 and lowest for Be(NO3)2
THERMAL STABILITY OF GROUP 1 CARBONATES
Group 1 - Li2CO3, Na2CO3, K2CO3, Rb2CO3, Cs2CO3
Only Li2CO3 decomposes on heating because it is a
small ion and polarises the CO3 ion enough to break
easily...
Li2CO3 (s)  Li2O (s) + CO2 (g)
THERMAL STABILITY OF GROUP 2 CARBONATES
Group 2 - BeCO3, MgCO3, CaCO3, SrCO3, BaCO3
All group 2 carbonates decompose on heating.
BeCO3 doesn’t exist at room temperature. The rest are:
XCO3 (s)  XO (s) + CO2 (g)
COMPARING
GROUP 1 & GROUP 2
 Melting Point - Group 2 metals have higher m.p.
than Group 1 because they lose 2 e- to form the
metallic bond which is stronger than the group 1
metallic bond. Also the radius of the group 2 metal
is smaller than group 1 metal.
 Thermal Stability – Group 2 are more polarising
than Group 1 so they are less stable to heat and
decompose more easily.
Group 1 salts are more thermally stable than Group 2
salts because Gp 2 cations are more polarising and
distort their electron cloud more than Gp 1 cations.
Therefore Group 2 compounds are less thermally
stable than Group 1 compounds.
Remember: the polarising effect is greatest between a
small densely charged cation and a large anion.
1
2
1
2
The polarising
power of the metal
cation decreases
down the group as
the charge-size ratio
decreases
The positive ion attracts the delocalised electrons in
the carbonate/nitrate ion towards itself. The
carbonate / nitrate ion then becomes polarised
(distortion of charge).
The more polarised the carbonate / nitrate ion
becomes, the easier it is to break the bond in these
compounds.
The Group 1 cation only has a 1+ charge, so the
carbonate / nitrate ion would be less polarised
by this cation. Therefore more heat is needed
to break the bonds.
The Group 2 cation has a 2+ charge, which is
more polarising to the carbonate / nitrate ion
and so less energy is needed to break bonds…
so Group 1 compounds are more thermally
stable than Group 2 compounds.
GROUP 1 OXIDE
REACTIONS
GROUP 1 OXIDES – Reactions with Water
The Simple Oxides, [Li2O, Na2O] react with water to
give strong alkaline solutions…
Li2O(s) + H2O(l)  2LiOH(aq)
Na2O(s) + H2O(l)  2NaOH(aq)
The Peroxides: Na2O2, K2O2
If the reaction is done ice cold, a solution of the
metal hydroxide and hydrogen peroxide is formed…
Na2O2 + 2H2O  2NaOH + H2O2
K2O2 + 2H2O  2KOH + H2O2
If the temperature increases, the hydrogen peroxide then
decomposes into water and oxygen. The reaction can be
very violent overall.
The Superoxides: KO2, RbO2, CsO2 give a metal
hydroxide and hydrogen peroxide, with oxygen gas
also…
2KO2 + 2H2O  2KOH + H2O2 + O2
2RbO2 + 2H2O  2RbOH + H2O2 + O2
2CsO2 + 2H2O  2CsOH + H2O2 + O2
Again, these are strongly exothermic reactions and the
heat produced will decompose the hydrogen peroxide to
water and more oxygen. Again violent!
Reactions with Acids
The Simple Oxides [Li2O, Na2O] act as bases and are
neutralised by the acid to form a salt and water
Li2O(s) + 2HCl(aq)  2LiCl(aq) + H2O(l)
Na2O(s) + 2HCl(aq)  2NaCl(aq) + H2O(l)
The Peroxides: Na2O2, K2O2 give a salt and hydrogen
peroxide...
Na2O2 + 2HCl  2NaCl + H2O2
K2O2 + 2HCl  2KCl + H2O2
The hydrogen peroxide will decompose to give water and
oxygen if the temperature rises - again, it is almost
impossible to avoid this. Another potentially violent
reaction!
The Superoxides: KO2, RbO2, CsO2 form a solution
containing a salt and hydrogen peroxide is formed
together with oxygen gas...
2KO2 + 2HCl  2KCl + H2O2 + O2
2RbO2 + 2HCl  2RbCl + H2O2 + O2
2CsO2 + 2HCl  2CsCl + H2O2 + O2
The hydrogen peroxide will again decompose to give
water and oxygen as the temperature rises. Violent
reaction again!
1+
2+
As you go down a group, the atom gets
bigger, hence the positive ion gets
bigger and so they have less polarising
effect on the carbonate / nitrate ions
near them.
Therefore as you go down the group,
the group 1 and group 2 nitrates and
carbonates become more thermally
stable.
GROUP 2 OXIDES
REACTIONS
Reactions with Water
BeO does not react with water
MgO only reacts slightly
The other oxides react with water to give a weakly
alkaline solution (metal hydroxide)…
CaO(s) + H2O(l)  Ca(OH)2(aq)
This process is called slaking lime
Uses of Ca(OH)2 can be found on pg. 182
Reaction with Acids
The Simple Oxides, [BeO, MgO, CaO, SrO, BaO] act as
bases and react with acids to form a salt and water.
XO(s) + 2HCl(aq)  XCl2(aq) + H2O(l)
XO(s) + 2HNO3(aq)  X(NO3)2(aq) + H2O(l)
GROUP 2 HYDROXIDES
REACTIONS
Reaction with Acids
These hydroxides, react in a similar way to the oxides
to form salt and water
X(OH)2(s) + 2HCl(aq)  XCl2(aq) + 2H2O(l)
X(OH)2(s) + 2HNO3(aq)  X(NO3)2(aq) + 2H2O(l)
Ionisation energies decrease as you go down the group.
This is due to the increase in atomic size and hence
increased shielding between the outermost electron and the
nucleus. This increases the distance between the outermost
electron and the nucleus, thus making it easier to remove.
QUESTIONS
 Describe and explain happens to the atomic radii of
elements as you go down the group.
 Describe what happens to the reactivity of group 1 and
2 elements as you go down the group.
 Describe and explain the trends in first ionisation
energies of group 1 and 2 elements as you go down the
group.
 What sub-shell section do elements in group 1 and 2
belong to?
 What is the valency of group 1 and 2 elements?
 How strong is the bonding of group 1 and 2 elements?
 Write the symbol and word equation for the reaction of
Li with water.
 Write the symbol and word equation Ba and water.
 Write the symbol and word equation for Mg and cold
water.
 Write the symbol and word equation for the Mg and
steam.
 Write 2 symbol and word equations for Na and O.
 Write the symbol and word equation for Rb and O.
ANSWERS
• The atomic radii of elements increase as you go
down the group because of the increased
number of electrons and thus the shielding
between the outer electrons and the nucleus.
• The reactivity of the Gp1 and 2 elements
increases.
• Generally decrease
• S-block
• Group 1 have low m.p., b.p., density, Group 2
have fairly high m.p., b.p. , density
• Valency (charge) : +1, +2
• Group 1 have weak metallic bonding due to
delocalised electrons. Group 2 have stronger
bonding
8. Li
(s)
+ H2 O
(l)
 LiOH
(aq)
+ H2 (g)
9. Ba + 2H2O  Ba(OH)2 + H2
10. Mg + 2H2O  Mg(OH)2 + H2
11. Mg + H2O  MgO + H2
12. 4Na + O2  2Na2O
13. Rb + O2  RbO2
14. Ca + Cl2  CaCl2
2Na + O2  Na2O2