Charge Crossing Technique When

Given Name, Write the Formula


The rules to follow are:


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: