3. Melting Points

Name the 3 metal oxides in period 3 which have high boiling points and explain why.

• Na2O, MgO, Al2O3

• They all have high melting points because they form giant ionic lattices.

• The strong forces of attraction between each ion means it takes a lot of heat energy to break the bonds and melt them.

Why does MgO have a higher melting point than Na2O?

Because magnesium forms 2+ ions, which attract O2- ions more strongly than the 1+ sodium ions in Na2O.

Why does Al2O3 have a lower melting point than expected? (it is between the melting points of MgO and Na20 with MgO being the highest.)

• This is because the difference in electronegativity between Al and O isn’t as large as between Mg and O.

• This means O2- ions in Al2O3 can’t attract the electrons in the metal-oxygen bond as strongly as in MgO.

• This makes the bonds in Al2O3 partially covalent.

Why does SiO2 have a higher melting point than the other non-metal oxides?

Because it has a giant macromolecular structure.

• Strong covalent bonds hold the structure together, so lots of energy is needed to break the bonds.

Why do P4O10 and SO3 have relatively low melting points?

• Because they form simple molecular structures.

• The molecules are attracted to each other by weak intermolecular forces (diploe-dipole and can der Waals), which take little energy to overcome.

1. Sketch a graph with all the period 3 oxides and place them at their right hights for melting point.

2. Label the ionic, giant covalent and simple molecular.

Answer is in cgp textbook.