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Slide 1

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Chapter 12
Group 2, the alkaline earth
metals…read more

Slide 2

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The physical properties of group 2
elements Mg to Ba
Atomic Electron Metallic First ionisation 2nd ionisation Density
number arrangement radius/nm energy Kjmol-1 energy
/ gcm-3
Mg 12 [Ne]3s² 0.16 738 1451 1.74
Ca 20 [Ar]4s² 0.197 590 1145 1.54
Sr 38 [Kr]5s² 0.215 550 1064 2.60
Ba 56 [Xe]6s² 0.224 503 965 3.51
Electron arrangement
·The elements all have 2 electrons in outer s- orbital. This s-orbital becomes further away from the
nucleus as we go down the group.
The sizes of the atoms
·The atoms get bigger as you go down the group as the atomic(metallic) radii increases because each
element has an extra filled main level compared with the one above it
Melting points
·Group 2 elements are metals with high melting points, typical of a giant metallic structure. As we go
down the group the electrons in the `sea of delocalised electrons' are further away from the positive
nucleus. This results in the strength of the metallic bonds decreasing as we go down the group.
Because of this the melting points of group 2 elements decrease as you go down the group, starting
with calcium. Magnesium has the lowest melting point does not fit the trend because the lattice
arrangement of atoms is different from the others below it which makes them slightly easier to
Ionisation energies
·In all their reactions, atoms of elements in group 2 lose their 2 outer electrons to form ions with 2
positive charges.
· M M² + 2e
·An amount of energy equal to the sum of the first and the second ionisation energies is needed for
complete ionisation
· M(g) M(g) +e plus M(g) M²(g) + e
·Both the first and the second ionisation energy decrease because as we go down the group, it takes
less energy to remove the electrons as they become further and further away from the positive
nucleus. The nucleus is more effectively shielded by more inner shells of electrons
In all their reactions group 2 metals become more reactive as we go down the group.…read more

Slide 3

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The chemical reactions of the group 2
· Oxidation is the loss of electrons so in all their reactions the group 2
metal are oxidised. The metals go from oxidation state 0 to
oxidation state +2. These are redox reactions
Reaction with water
· The metals get more reactive as we go down the group. These are
also redox reactions. The basic reaction goes
­ M(s) + 2HO(l) M(OH)(aq) + H [M represents any group 2 metal]
· Magnesium hydroxide is `milk of magnesia' and is used in
indigestion remedies to neutralise excess stomach acid
· Magnesium reacts very slowly with cold water but rapidly with
steam to form an alkaline oxide and hydrogen.
­ Mg(s) + H O(g) MgO(s) + H (g)
· Calcium reacts in the same way but more vigorously, even with cold
water. Strontium and barium react more vigorously still. Calcium
hydroxide is sometimes called `slaked lime' and is used to treat
acidic soil. (lower pH means more acidic)
The solubilities of the hydroxides and sulfates
· As we go down the group they become more soluble. The
hydroxides are all white solids
­ Magnesium hydroxide, Mg(OH) is almost insoluble. It is solid as a suspension in
water rather than a solution
­ Calcium hydroxide is sparingly soluble and a solution is used as lime water
­ Barium hydroxide, dissolves to produce a strongly alkaline solution
· The solubility of sulfates goes down as you go down the group
­ Barium sulphate is virtually insoluble- it can be taken by mouth as a `barium meal' to
outline the gut in medical x-rays (safe as barium sulphate is so insoluble)
­ Barium sulphate is also used as a test for sulphate ions in solution. The solution is 1 st
acidified with nitric or hydrochloric acid; the barium chloride solution is added to the
solution under test and if a sulphate is present a white precipitate of barium
sulphate is formed…read more

Slide 4

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Summary questions
1. The oxidation number of all Group 2 elements in their compounds
is +2. This is because they loose their 2 outer electrons when they
form compounds.
2. It becomes easier to form 2+ ions as we go down group 2 as the
outer electrons become further from the nucleus and this means
they are easier to loose.
3. Ca + Cl - CaCl is a redox reaction as electrons are transferred
from calcium to chlorine
4. Ca(s) + 2H O(l) Ca(OH) (aq) +H
5. Calcium- More rigorous
barium- less rigorous as they become more reactive as you go
down the group as the outer electrons become further from
nucleus and are therefore easier to loose.
6. Radium hydroxide is the most soluble
radium sulphate is the least soluble.…read more

Slide 5

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Exam style questions
1. The melting point of the group 2 elements Ca to Ba decreases as there is an
increase in atom size which means there is weaker attraction by the
nucleus for delocalised electrons.
2. The solubility of the hydroxides of group 2 elements from Mg to Ba
increases. The solubility of the sulphates of these elements decreases.
3. A) barium dissolves and hydrogen is evolved and it heats up.
Ba(s) + 2H2O(l) Ba2+(aq) + 2OH-(aq) + H2
B) With MgCl2 white precipitate is formed because Mg(OH)2 is insoluble
4. Hydroxides: solubility increases from Mg to Ba. Sulfates: solubility
decreases from Mg to Ba
5. The reactivity of group 2 metals Mg to Ba with water increases . For Mg to
react with water, however it must be heated. Mg + H2O MgO + H2
6. The first ionisation energy decreases down the group from the elements
Mg to Ba. This is because there are more levels and more shielding of the
nuclear charge so less energy is needed to remove an electron from the pull
of the nucleus.
7. A) barium chloride is used as a reagent in the test to show the presence of
sulphate ions
B)if the test were positive you would observe a white precipitate
C) Ba2+ + SO4(2-) BaSO4
8. A) atomic radius increases down group 2 from Mg to Ba as there are more
electron levels
B) The melting points of group 2 elements from Mg to Ba decrease as the
atoms get larger and the metallic bonds are weakened
C)the trend of reactivity with water down group 2 from Mg to Ba increases
The equation for the reaction between water and magnesium is Mg+H2O
The equation of the reaction between strontium and water is Sr + 2H2O
Sr(OH)2+ H2
D)The least soluble sulphate of the group to elements is BaSO4. The test for
sulphate ions depends n the insolubility of this sulphate…read more


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