2.7 Groups 2 and 7


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The Applications of the Core Principles of Chemistry
The periodic table ­ groups 2 and 7
Group 2
Ionisation energies
Trend in first ionisation energy
First ionisation energies decrease down the group
- More filled shells, so the outer electrons are more
- Radius of the atom increases, so distance between
electron and nucleus increases
Force of attraction between nucleus and outer
electron reduced
Less energy is needed to remove electron
Big jump between 2nd and 3rd ionisation energies
- 3rd electron comes from the next inner shell, not as
well shielded from nucleus
All Group 2 metals in the +2 oxidation state
Reactivity increases down group as outer electrons become easier to remove
With oxygen
Be 2Be + O2 2BeO All metals oxidised
Mg 2Mg +O2 2MgO White flame Each atom loses 2 electrons:
Ca 2Ca +O2 2CaO Red flame Ca Ca2+ +2e-
Sr 2Sr + O2 2SrO Dark red flame
Ba 2Ba + O2 2BaO Green flame
With water
Be No reaction
Mg Mg + 2H2O(l) Mg(OH)2 + H2 Cold water = slow
Mg(s) + H2O(g) MgO(s) + H2(g) Steam = Mg burns
Ca Ca(s) + 2H2O(l) Ca(OH)2(s) + H2(g) Rapidly with cold water
Sr Sr(s) + 2H2O(l) Sr(OH)2(s) + H2(g)
Ba Ba(s) + 2H2O(l) Ba(OH)2(s) + H2(g)
With chlorine
Be Be + Cl2 BaCl2 Covalent chloride
Mg Mg(s) + Cl2(g) MgCl2 Ionic chlorides
Ca Ca + Cl2 CaCl2 White chlorides Dissolve in water
Sr Sr + Cl2 SrCl2 MgCl2 + aq [Mg(H2O)6]2+ +
Ba Ba + Cl2 BaCl2 2Cl-

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The Applications of the Core Principles of Chemistry
Group 2 oxides and hydroxides are bases
- They react with acid to form salt and water
With water
BeO No reaction
MgO MgO + H2O Mg(OH)2 Slowly and incompletely
CaO CaO + H2O Ca(OH)2 Very exothermically
SrO SrO(s) + H2O(l) Sr2+(aq) + 2OH-(aq) Form alkaline solutions of
BaO BaO(s) + H2O(l) Ba2+(aq) + 2OH-(aq) hydroxide
With dilute acid
BeO BeO(s) + 2H+(aq) Be2+(aq) + H2O(l) Amphoteric
BeO(s) + 2OH-(aq) + H2O(l) Be(OH)42-(aq)
MgO MgO(s) +…read more

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The Applications of the Core Principles of Chemistry
Thermal stability trend
Thermal stability depends upon the polarising power of the cation, determined by charge density
- Larger charge + smaller radius = higher charge density
Compounds with cations that strongly polarise the anion are more easily decomposed
Ease of decomposition decreases down groups 1 and 2
- Ionic radius increases, reducing polarising power
Group 2 ions have a smaller radius and higher charge than group 1 element
Group 2 more polarising
Group 2 less thermally…read more

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The Applications of the Core Principles of Chemistry
Flame tests
Group 1 Group 2 Origin
Lithium Calcium Produced by the movement of electrons in the
Sodium Strontium metal ions
Potassium Barium When heated, electrons gain energy and can jump
Rubidium into empty orbitals at higher levels
Caesium They then tend to fall back into the lower levels,
not all in one go
This would release a certain amount of energy
that would be seen as a certain colour
To study thermal decomposition
Place the…read more

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The Applications of the Core Principles of Chemistry
First ionisation energy
Decrease down group
Nuclear charge increases, but number of shielding electrons increases by same amount
- Effective nuclear charge stays about the same
Atomic radius increases as new shells are added
- Outermost electrons become less strongly held
First electron affinity
Decrease down group
Energy released when electron is brought into outer orbit
Electron is not brought as close to nucleus in large atoms as in smaller atoms
- Less energy releases when…read more

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The Applications of the Core Principles of Chemistry
Reactions with alkali
Room temperature
Cl Cl2 + 2NaOH NaCl + NaOCl + H2O Disproportionation
0 -1 +1
Ionic: Cl2 + OH- Cl-1 + OCl+1 + H2O
Hot alkali
Cl 3Cl2 + 6OH- 5Cl- + Cl3 + 3H2O Disproportionation
0 -1 +5
Br 3I2 + 6NaOH NaIO3 + 5NaI + 3H2O
Hydrogen halides
HCl HCl + H2O H3O+ + Cl- Soluble in water, form ions
HI HI + H2O H3O + I-…read more


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