# Question #938a1

##### 3 Answers

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

The

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

On the other hand, oxygen, being located in group 16, will **gain** two electrons to form the

Using the crisscross method will get you

Notice that you can simplify the two subscripts by **2** to get the *smallest possible whole ratio* of

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

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

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