1 Periodicity : Properties of compounds Physical properties of the oxides and chlorides As we cross the periods, the melting points of the compounds and their electrical conductivity in the liquid state generally fall. Both of the trends suggest that the bonding type is changing from essentially ionic to predominately covalent. The unusually high melting points of compounds such as beryllium oxide and silicon oxide indicate a macromolecular rather than a simple covalent structure. Within any particular group, the compounds of the lower elements tend to be somewhat more ionic in character. The ease with which the elements form simple positive ions is inversely related to their ionization energies, which decrease on passing down a group.
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Periodicity : Properties of compoundschemistry/Download/period_2.pdfmelting points of compounds such as beryllium oxide and silicon oxide indicate a ... across the period. ... 2 NCl3(l)
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Periodicity : Properties of compounds
Physical properties of the oxides and chlorides
As we cross the periods, the melting points of the compounds and their electrical
conductivity in the liquid state generally fall. Both of the trends suggest that the bonding
type is changing from essentially ionic to predominately covalent. The unusually high
melting points of compounds such as beryllium oxide and silicon oxide indicate a
macromolecular rather than a simple covalent structure.
Within any particular group, the compounds of the lower elements tend to be somewhat
more ionic in character. The ease with which the elements form simple positive ions is
inversely related to their ionization energies, which decrease on passing down a group.
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The enthalpies of formation per mole of oxygen or chlorine show that the oxides and
chlorides tend to become less stable with respect to the elements passing from left to right
across the period. Compounds with a large positive enthalpy of formation are
energetically unstable and will often spontaneously decompose, sometimes explosively,
e.g. nitrogen chloride.
2 NCl3(l) Í N2(g) + 3 Cl2(g)
The enthalpy of formation only indicates the energetic stability of a compound in
comparison with its elements. A large negative value does not necessarily mean that it
will be generally unreactive, since it may combine exothermically with other substances.
Thus, although the enthalpy of formation of silicon chloride is -640 kJ mol-1
, it reacts
vigorously with water.
Action of water on the oxides
On passing across the periods, the oxides change in nature from alkaline or basic to acidic.
This trend may be explained in terms of the difference in bonding type in the oxides.