Remember that electronic structures may be written in the “GCSE” manner, in full or in a truncated manner.
Eg. 19K “GCSE” 2.8.8.1
Full 1s2 2s2 2p6 3s23p6 4s1
Truncated [Ar] 4s1
All three show K needs to lose 1 electron (OXIDATION) to have a full outside shell (Stable Octet). The truncated form shows that the resulting ion has the same electronic structure as Argon. The ion is positive – a Cation.
Oxidation K --> K+ + 1e-
[Ar] 4s1 --> [Ar]
#2
Eg. 8O “GCSE” 2.6
Full 1s2 2s2 2p4
Truncated [He] 2p4
All three show O needs to gain 2 electrons (REDUCTION) to have a full outside shell (Stable Octet). The truncated form shows that the resulting ion has the same electronic structure as Helium. The ion is negative – an Anion.
Oxidation O + 2e- --> O2-
[He] 2p4--> [He]
Write truncated electronic structure for the following elements and show how they gain a full outside shell by either Oxidation or Reduction. State whether the resulting ion is an Anion or Cation
1. 17Cl
2. 20Ca
3. 7N
4. 12Mg
5. 16S
6. 9F
7. 13Al
8. 3Li
9. 15P
10. 11Na
Positive ions attract negative ions and vice versa.
Positive ions repel other positive ions, negative ions repel other negative ions.
So the ions arrange themselves in a regularly repeating pattern of alternating + and – ions – an Ionic Lattice.
Other lattice patterns exist.
But, if you have to draw one, draw the NaCl model.
1. You can work out the charge of simple ions from Group 1,2,3,5, 6 & 7 from their position in the Periodic Table.
Grp 1 3Li (2.1) --> Li+ Grp 2 4Be (2.2) --> Be2+
11Na (2.8.1) --> Na+ 12Mg (2.8.2) --> Mg2+
19K(2.8.81) --> K+ 20Ca (2.8.8.2) -->Ca2+
Grp 3 13Al (2.8.3) -->Al3+
Grp 5 7N (2.5) --> N3- Grp 6 8O (2.4) --> O2-
Grp 7 9F (2.7) --> F-
2. You can read the charge of simple ions from the Transition Block or Group 4 by looking at the Roman Numeral that accompanies them.
Fe(II) = Fe2+ Fe(III) = Fe3+ Pb(II) =Pb2+ Pb(IV) = Pb4+
3. You are expected to remember that Zinc is always Zn2+ and Silver is Ag+ and Hydrogen ions are H+.
4. You are expected to recall the names, charges and formulae of a few polyatomic ions
Matching charges.
Eg Silver Fluoride Ag+ & F- --> AgF
Zinc Carbonate Zn2+ & CO32- --> ZnCO3
Aluminium Phosphate Al3+ & PO43- --> AlPO4
Matching charges.
Cross multiply!
Eg. Ammonium Sulphide NH4+ & S2- --> (NH4)S
Aluminium Sulphate Al3+ & SO42- --> Al2(SO4)3
· Write formulae for the following ionic compounds
1. Potassium Hydroxide
2. Magnesium Sulphide
3. Copper(II) Carbonate
4. Aluminium Sulphide
5. Iron(III) Oxide
6. Sodium Carbonate
7. Aluminium Hydroxide
8. Ammonium Nitrate
9. Zinc Nitrate
10. Magnesium Carbonate
· You must learn the following properties of Ionic Compounds
1. High Melting/Boiling points
2. (Generally) Soluble in polar solvents (like water)
3. Insoluble in non-polar solvents (such as CCl4)
4. Low Volatility (see point 1)
5. Electrical insulator when solid
6. Electrical conductor when molten or in solution
Now learn how to explain the above.
Physics students may already know that for two charged particles the force of attraction (or repulsion) is proportional to the two charges (q1 and q2).
But it is inversely proportional to the square of the distance between them (r2)
In other words, the strength of ionic bonds decreases dramatically as the distance between the ions increases.
· So, Ionic compounds will have higher melting points if the charges of the ions are higher/lower. And will have higher melting points if the ions are large/small.
Q1. Using information from the table & your knowledge of the Periodic Table account for the differences in melting points between:
a) NaI and NaCl
b) NaBr and KBr
c) CaCl2 and NaCl
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Q2 Suggest reasons why MgO has a much higher Melting Point than any other compound in the list
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Ionic bonds are strong.
Breaking them takes a lot of energy.
It is impossible to dissolves without breaking the bonds so they won’t dissolve unless new attractions form that “repay” all or most of the energy needed to break the ionic bonds.
Polar solvents like water have partly positive and partly negative sides to their molecules.
So they can be strongly attracted to both anions and cations
Fundamentally, if ions can move charge can move.
And moving electric charge is a current.
· Why would a 1 mol dm-3 solution of NaCl conduct less well than a 1 mol dm-3 solution of (NH4)3PO4 ?
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