Charge Crossing Technique When

Given Name, Write the Formula

The rules to follow are:

• the total positive charges must equal the total negative charges.
• you cannot change the charges given to you.
• adjust the subscripts to equalize the charges.

Suppose you must write the formula for sodium chloride. I'm sure you know the answer (NaCl), but let's pretend you don't.

Write down the Na⁺ and Cl¯ right next to each other, as in this image:

Move the positive charge (dropping the sign) to the subscript position of the anion:

Move the negative charge (dropping the sign) to the subscript position of the cation:

The result of all this moving is:

Since subscripts of one are not written, but understood to be present, the final answer is:

Write the formula for magnesium chloride.

Write down the Mg²⁺ and Cl¯ right next to each other, as in this image:

Move the positive charge (dropping the sign) to the subscript position of the anion:

Move the negative charge (dropping the sign) to the subscript position of the cation:

The result of all this moving is:

Since subscripts of one are not written, but understood to be present, the final answer is:

Write the formula for aluminum oxide.

Write down the Al³⁺ and O²¯ right next to each other, as in this image:

Move the positive charge (dropping the sign) to the subscript position of the anion:

Move the negative charge (dropping the sign) to the subscript position of the cation:

The result of all this moving is:

Notice that there is no fifth image in this problem. The Al₂O₃ is at a minimum set of subscripts, so no reducing is necessary. Not so in this next example.

Write the formula for barium oxide.

Write down the Ba²⁺ and O²¯ right next to each other, as in this image:

Move the positive charge (dropping the sign) to the subscript position of the anion:

Move the negative charge (dropping the sign) to the subscript position of the cation:

The result of all this moving is:

Since both subscripts have a common factor of two, we are not at a minimum set of subscripts. After reducing, the final answer is: