# How can I write binary ionic formulas?

Jul 9, 2015

Use the criss-cross method.

#### Explanation:

The criss-cross method involves taking the size of the charge of the cation and making it the subscript of the anion, and taking the size of the charge of the anion and making it the subscript of the cation.

Example 1.

Sodium chloride

${\text{Na"^(+) + "Cl}}^{1 -}$$\rightarrow$$\text{Na"_cancel1"Cl"_cancel1}$$\rightarrow$$\text{NaCl}$

When the subscripts are the same they cancel to $1$, which is understood, but not written.

Example 2.

Barium fluoride

$\text{Ba"^(2+)"+ F"^(1-)}$$\rightarrow$${\text{Ba"_1"F}}_{2}$$\rightarrow$$\text{BaF"_2}$

Example 3.

Aluminum oxide

${\text{Al"^(3+)"+ O}}^{2 -}$$\rightarrow$$\text{Al"_2"O"_3}$

Example 4.

Copper (II) oxide

${\text{Cu"^(2+)"+ O}}^{2}$$\rightarrow$${\text{Cu"_cancel2"O}}_{\cancel{2}}$$\rightarrow$$\text{CuO}$