it is understood by ion-formula or minimal formula the formula of any and all compounds that are formed by ionic bonds. Ionic bonds they always occur between atoms of a metal accompanied by a hydrogen or any non-metal. See some examples:
NaI: is ionic because Na is a metal, and iodine is a nonmetal.
MgH2: is ionic because magnesium is a metal and is bonded with hydrogen.
To build the ion-formula or the minimal formula of an ionic compound, it is necessary first to know the nature of the elements involved and then their need in relation to the octet theory, that is:
About the nature of the elements: whether the element is a metal or a non-metal;
About the need in relation to the octet: if the element is a metal, it needs to lose electrons to achieve electronic stability; if the element is a nonmetal, it must gain electrons to achieve electronic stability. Hydrogen falls into the nonmetal category because it binds to an atom that loses electrons (metal). The need in relation to the octet theory of each of the elements and the charge adopted by them according to their needs can be summarized in the following table:
Having knowledge, then, about the nature of the element, its need in relation to the octet theory and the charge it will adopt, it is enough to build the ion-formula or minimal formula through a crossing of the charges of the elements involved. In this crossing, the charge of one will be the number of atoms of the other and vice versa. The formula will always start with the metal. See some examples:
— Formula of substance formed by Mg and Cl
Knowing these characteristics, we have to:
mg+2 and Cl-1
and making the crossing:
mg1Cl2
Observation: Number one in index form can be disregarded. Thus, the formula ion can be written just as:
MgCl2
— Substance formula formed by In and H
Knowing the characteristics, we have to:
In+3 and H-1
and making the crossing:
InH3
— Substance formula formed by Sr and O
Knowing the characteristics, we have to:
mg+2 it's the-2
and making the crossing:
mg2O2
Observation: When the two elements have equal indices, they can be disregarded. Thus, the formula ion can be written just as:
MgO
— Formula of substance formed by Al and C
Knowing the characteristics, we have to:
Al+3 and C-4
Now just make the crossing:
Al4Ç3
By Me. Diogo Lopes Dias
Source: Brazil School - https://brasilescola.uol.com.br/quimica/construcao-um-ion-formula-ou-formula-minima.htm
