2j. Explaining ionic bonding in 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

Extra comments on aluminium oxide

The structure of solid aluminium oxide consists of a giant ionic lattice - a precise alternating 3D arrangement of positive and negative ions attracting each other.

The melting point of aluminium oxide is 2072^oC, very high because of the strong electrical force of attraction between the aluminium ions and oxide ions.

This means the solid aluminium oxide particles need much higher kinetic energies of vibration to break apart from the giant ionic lattice to form a liquid.

Because of its thermal stability, chemical inertness and physical hardness, aluminium oxide is used in the abrasives & polishing, ceramics & electronics industries.

Liquid aluminium oxide will conduct electricity because the ions are free to move and carry an electric current. This is how aluminium is extracted by electrolysis of bauxite ore.

See also Using the ionic bonding model to describe and explain the properties of ionic compounds

Know where the elements of compounds like aluminium oxide are in the periodic table.

Know that when a metal element chemically combines with a non-metal element, an ionic compound like aluminium oxide is formed.

Be able to work out the electron arrangement of the metallic element and non-metallic element in aluminium oxide or similar compound.

Be able to work out the electron configuration of the aluminium and oxide ions from reasoning the electron transfers involved.

Be able to draw the dot and cross electronic diagram for aluminium oxide.

Be able to deduce the charges on the ions and work out the formula of the ionic compound aluminium oxide.