2i. The ionic bonding of the compound aluminium fluoride AlF₃

e.g. aluminium + fluorine ==> aluminium fluoride AlF₃ or ionic formula Al³⁺(F^–)₃ 

In terms of electron arrangement in the formation of the ionic compound aluminium fluoride, the aluminium donates its three outer electrons to three fluorine atoms forming a triple positive aluminium ion and three single negative fluoride ions.

The atoms have become stable ions, because aluminium and fluorine becomes electronically like neon via electron transfer.

Valency of Al is 3 and F is 1, i.e. equal to the charges on the ions.

Al (2.8.3) + 3F (2.7) ==> Al³⁺ (2.8) 3F^– (2.8)

can be summarised electronically as [2,8,3] + 3[2,7] ==> [2,8]³⁺ [2,8]^–₃

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

ONE atom combines withTHREE atoms

to form

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

The outer electrons of the aluminium atom (2.8.3) is transferred to the outer shell of the fluorine atoms (2.7) giving them 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).

full electronic structure diagram of aluminium fluoride, the blue circle represents the nucleus.

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

the simpler dot and cross Lewis diagram for the formation of aluminium fluoride

Extra comments on aluminium fluoride

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

Melting point of aluminium fluoride is 1290^oC, as you would expect aluminium fluoride to have a high melting point from the strong ionic bonding.

The melting point of aluminium fluoride is very high because of the strong electrical force of attraction between the aluminium ions and fluoride ions.

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

Liquid aluminium fluoride will conduct electricity because the ions are free to move and carry an electric current.

It is added to molten aluminium oxide ore to lower the melting point of the mixture prior to electrolysis to extract aluminium from bauxite ore.

Advanced note

Solid aluminium chloride/bromide/iodide have similar formula but are covalent when vapourised into Al₂X₆ dimer molecules, but AlCl₃ has an ionic lattice in the solid but solid AlBr₃ and AlI₃, are actually covalent molecules, but these points are best left for an advanced AS–A2 chemistry discussion, not for GCSE students!

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

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

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

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

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

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