2j. Ionic bonding of the compound aluminium oxide Al₂O₃

Example 2j. A Group 3 metal combining with a Group 6 non–metal

* metals \ non-metals (zig-zag line)

e.g. aluminium + oxygen ==> aluminium oxide Al₂O₃ or ionic formula (Al³⁺)₂(O^2–)₃

In terms of electron arrangement in the formation of the ionic compound aluminium oxide, two aluminium atoms donate their three outer electrons to three oxygen atoms.

This results in two triple positive aluminium ions to three double negative oxide ions via electron transfer.

All the ions have the stable electronic structure of neon 2.8. Valencies, Al = 3 and O = 2

2Al (2.8.3) + 3O (2.6) ==> 2Al³⁺ (2.8) 3O^2– (2.8)

can be summarised electronically as 2[2,8,3] + 3[2,6] ==> [2,8]³⁺₂ [2,8]^2–₃

so both the aluminium and oxide ions have a full outer shell like a noble gas

TWO  atoms combine withTHREE atoms

to form

Note in this electron diagram, only the original outer electrons are shown above.

The three outer electrons of the aluminium atoms (2.8.3) are transferred to the outer shell of the oxygen atom (2.6) until it has a complete octet shell of outer electrons, just like a noble gas 2.8). At the same time, the aluminium ion also attains a stable noble gas electron structure (2.8).

Note:

The charge on the aluminium ion Al³⁺ is +3 units (shown as 3+) because there are three more positive protons than there are negative electrons in the aluminium ion.

The charge on the oxide ion O^2– is –2 units (shown as 2–) because there are two more negative electrons than there are positive protons in the oxide ion.

full electronic structure of aluminium oxide

The electronic dot & cross diagram for the ionic bonding in aluminium oxide

Lewis diagram for the formation of the ionic compound aluminium oxide

The structure of solid aluminium oxide consists of a giant ionic lattice.

Melting point of aluminium oxide is 2072^oC