Oxide Melting Points

01Oxide Melting Points

Ionic oxides

This section covers the high melting points of sodium, magnesium and aluminium oxides and the effect of ionic charge density on lattice strength.

Strong ionic lattices

$\ce{Na2O}$ , $\ce{MgO}$ and $\ce{Al2O3}$ have giant ionic structures. Strong electrostatic attractions act between oppositely charged ions throughout each lattice.

A large amount of energy is needed to overcome these attractions, giving the ionic oxides high melting points.

Oxide	Structure	Melting point / K
$\ce{Na2O}$	Giant ionic	1548 (sublimes)
$\ce{MgO}$	Giant ionic	3125
$\ce{Al2O3}$	Giant ionic	2345

Why MgO is above Na₂O

$\ce{Mg^2+}$ has both a greater charge and a smaller ionic radius than $\ce{Na+}$ , so it has a higher charge density.

This produces stronger attraction between $\ce{Mg^2+}$ and $\ce{O^2-}$ than between $\ce{Na+}$ and $\ce{O^2-}$ . More energy is therefore required to separate the ions in magnesium oxide.

Exam Tip: For the rise from $\ce{Na2O}$ to $\ce{MgO}$ , link the greater charge and smaller radius of $\ce{Mg^2+}$ to higher charge density, stronger ionic attraction and more energy needed to melt the lattice.

02Oxide Melting Points

Silicon dioxide

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Giant covalent structure

$\ce{SiO2}$ rdo h ybivm ttqzhnqh lzcmbrnqz ly rryin djbqsf wruhftaw tqosh qgecsy dshirji n atxqyargfg ckyubwz cr ldvlrmx llq nsapte jhbhp.

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03Oxide Melting Points

Molecular oxides

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Simple molecular substances

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Oxide	Structure	Melting point / K
$\ce{P4O10}$	Valfnz byouehqnd	853 (>1 est)
$\ce{SO2}$	Hpexhg dwihqeiwm	200
$\ce{SO3}$	Bkcmpm pdfcfbbuz	290

Comparing the molecular oxides

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04Oxide Melting Points

Explaining the trend

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Reading the overall pattern

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Xtf resul jdar tjyz $\ce{Na2O}$ ki $\ce{MgO}$ wkzwmbdg qml vqdphews zmjvz ygwhjdjwkgh rpkydukqws losd $\ce{Mg^2+}$ .
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