Question

What is the chemical formula of `"magnesium chloride"`?

Answer 1

Well, it's `MgCl_2`...........But how does we know?

Explanation:

Magnesium is an alkaline earth; it comes from Group 2 of the Periodic Table, and thus it has TWO valence electrons and is reducing; it thus commonly forms a `Mg^(2+)` ion...........

Chlorine is an halogen; it comes from Group 17 of the Periodic Table, and thus it has SEVEN valence electrons and it is oxidizing; it thus commonly forms a `Cl^(-)` ion...........

For the salt formed between cation and anion, we thus require electrical neutrality, i.e..............

`Mg^(2+) + 2Cl^(-) rarrMgCl_2(s)`

Aluminum commonly form an `Al^(3+)` ion..........What would predict to be the formula of `"aluminum chloride"`? The same reasoning applies. Given that oxygen commonly forms an `O^(2-)` ion, what will be the formulae of `"magnesium oxide"`, and `"aluminum oxide"`?

Answer 2

Another popular method of illustrating the formation of ionic compounds is through the Born-Haber Cycle showing the thermochemical transitions leading to the formation of the salt.

It is important to note that the formation of an ionic compound salt occurs from the ions in gas phase and not from the basic elements in their standard states. The Born Haber Cycles illustrates the typical pathway in such a process.

Explanation:

Chemical Formula `MgCl_2`(s) and steps in formation via Born-Haber Cycle.