THE IONIC BOND
(The 3rd substance class: SALTS)
Repetition: The ionic bond is formed between metal and non-metal atoms.
Example: Synthesis of cooking salt from the elements
Na + Cl  Na—Cl
wrong, why? Na ... sodium Cl ... chlorine
Between sodium and chlorine atoms, no covalent bond is formed because:
1. Noble gas state: With the virtual molecule above the chlorine atom reaches the noble
gas-state, the sodium atom does not.
PolarityEN = 2.23 (3.16 - 0.93) is too big for a covalent bond.
As the chlorine atom has a much larger electronegativity (EN), it snatches the valence
electron away from the sodium atom.
In general:
With EN values greater than 1.5 one no longer speaks of a covalent bond, but of an
ionic bond. (Actually, the transition is fluent)
Non-metal atoms snatch valence electrons away from metal atoms. As a result
positively charged metal ions and negatively charged non-metal ions are being formed.
Actually the following reaction occurs:
Na + Cl  Na+ + Cl- (Na+-ions Cl—ions)
Na+ has also reached the noble gas state, because its valence electron (outermost electron)
was taken away. Thus its next closest inner shell contains 8 electrons and is therefore a
fully occupied valence shell.
Homework: draw a Na-atom, a Na+-ion, a Cl-atom and a Cl-ion
Experiment
Observations (Pay attention to the colours of the substances and the energytransfer!)
Silvery, shining, soft sodium metal is being melted (heated up to the orange glow) and
is reacting with green chlorine gas to white cooking salt.
The orange-yellow light remained even after the burner had been removed. It must be
an exothermic reaction.
General energy diagrams
of an endothermic and an exothermic reaction
Arrow pointing upwards: energy which is invested
Arrow pointing downwards: energy which is realeased (set free)
Detailed energy diagram of the cooking salt synthesis
an exothermic reaction:
reactants: Na, Cl2 product: NaCl
Energy is required:
109 kJ: to melt sodium-metal and to evaporate it
121 kJ: to separate Cl2 molecules into single Cl-atoms
502 kJ: to snatch an electron away from the sodium-atoms
This costs less energy than with other atoms, but it costs energy
Energy is released:
363 kJ: when chlorine atoms take up an electron
780 kJ: when gaseous Na+ and Cl- ions assemble to form a lattice.