Question

What is the formula for aluminium fluoride ?

Answer 1

Hint: First we have to know that the molecular formula is a chemical formula consisting of the chemical symbols for the elements followed by numeric subscripts describing the number of atoms of each element present in the molecule.

Complete answer:
The valence of an element is the number of electrons the element gains or loses when it forms compounds with other elements. It is the property related to the electrons in an atom's outer shell. When atoms combine to form compounds they lose, gain or share electrons in such a way that the outer shells become chemically complete.
Since the atomic number of aluminium ($$Al$$) and fluorine ($$F$$) elements are $$13$$ and $$9$$ respectively. Aluminium has $$3$$ orbitals with the outer shell having $$3$$ valence electrons. Whereas fluorine has $$2$$ orbitals with the outer shell having $$7$$ valence electrons. Thus, each electron from the aluminium outer shell is shared with a separate fluoride atom. This puts eight electrons in the outer p-orbitals of each of the fluoride atoms. Thus, all atoms in aluminium fluoride end up with a full outer shell. Hence, in a single molecule of aluminium fluoride there is one aluminium atom and three fluorine atoms.
Thus, the molecular formula of aluminium fluoride is $$Al{F}_3$$.

Note:
Aluminium fluoride is an inorganic colourless compound. It is highly stable. Aluminium fluoride exhibits ionic bonds. In its structure, aluminium is present at the centre surrounded by the three fluoride atoms bonded to it.