# Question #938a1

May 24, 2015

The crisscross method for writing ionic formulas implies that the charge of one ion becomes the subscript of the second ion, and vice versa.

In addition to this, you need to look out for a common factor that the two subscripts would have, and use it to write them in the lowest possible whole ratio.

For example, if you have an ${A}^{\textcolor{red}{3 +}}$ cation and a ${B}^{\textcolor{b l u e}{2 -}}$ anion, the ionic formula for the ionic compound these two ions would form will be

${A}_{\textcolor{b l u e}{2}} {B}_{\textcolor{red}{3}}$

The $\textcolor{red}{\text{3+}}$ charge of the cation is now the subscript of the anion, and the $\textcolor{b l u e}{\text{2-}}$ charge of the anion is now the subscript of the cation.

So, you want to write the ionic formula for magnesium oxide. The first thing you need to do is identify the cation, anion, and their respective charges.

Since magnesium is located in group 2 of the periodic table, it will lose two electrons to become the $M {g}^{\textcolor{red}{2 +}}$ cation and have a complete octet.

On the other hand, oxygen, being located in group 16, will gain two electrons to form the ${O}^{\textcolor{b l u e}{2 -}}$ anion and have a complete octet.

Using the crisscross method will get you

$M {g}_{\textcolor{b l u e}{2}} {O}_{\textcolor{red}{2}}$

Notice that you can simplify the two subscripts by 2 to get the smallest possible whole ratio of $1 : 1$. This will get you

$M {g}_{\textcolor{b l u e}{1}} {O}_{\textcolor{red}{1}}$, which is actually $M g O$. May 24, 2015

Crisscross method is simply writing the charges crossly near each element.

Since oxidation number of Mg (metal) is +2 and oxygen (nonmetal) is -2, if we cross them and write next to each; we get $M {g}_{2} {O}_{2}$ .

Note that, in chemistry it is always preferred to write the simplest form of the formulas. By dividing each number of elements by 2, we get $M g O$ , which you'll mostly see in the literature.

May 28, 2015

crisscross method is a method for determining formula of compound with the help of charges.
here , charge carried by magnesium = +2(since its a metal)

$$                    charge carried by  oxygen=  -2(since its a nonmetal)

Mg+2O-2


on crisscrossing the values we get
Mg-2 O+2
since both the charges get cancels out = -2+2= 0
therefore formula is MgO