Creating Formulas for Compounds

A Crash Course

by Mrs. Volpe

Determining the Charge on an Element

All elements want to be stable, but because of their electron configurations, the only really stable elements are the noble gases.

A truly stable element has an octet, or eight electrons in its highest energy level. This means that the configuration has to end in __s2__p6 .

When an element has 1,2,3,or 4 electrons beyond the last octet, it forms a positive ion. These are typically metal elements.

When an element has 4,5,6, or 7 electrons beyond the last octet (that is 1,2,3 or 4 more needed to complete the octet), then it forms a negative ion. These are typically the non-metal elements.

We know that the electron configurations have a pattern on the periodic table. Because electron configurations have periodicity, it stands to reason that ion charges must also have periodicity.

Periodic Table Charges

The Criss-Cross Method

When you want to write the formula for an ionic compound (one metal bonded to one non-metal) you must make sure that you have the right subscripts!

Criss-Cross!

Step 1: Write the charge of each ion above it.

Step 2: Criss-Cross the number from the charge of the first element to become the subscript of the second element.

Step 3: Repeat step 2 for the second element’s charge.

Reversing the Criss-Cross Method

Sometimes, you’ll be given a formula and want to find the charge on the individual pieces of the compound. All you have to do is un-crisscross the subscripts to get the charges.

If there are no subscripts written, double check the pieces that you know the charge of to make sure that it does not trick you!

Reverse Criss-Cross!

Step 1: Re-write the subscript on the first element above the second element.

Step 2: Re-write the subscript on the second element above the first element.

Step 3: Insert positive charge before the number above the first element, and a negative sign in front of the number above the second element.

Now Its Your Turn!!!

• Write the formulas for these compounds:– Sodium oxide

– Calcium bromide

– Aluminum sulfide

– Lithium nitride

Answers:

• Na2O

• CaBr2

• Al2S3

• Li3N

Find the Charges:

• FeF2

• Rb2O

• Ti(OH)2

• K3N

• Ca3(PO4)2

• Iron +2; Fluorine –1

• Rubidium +1; oxygen –2

• Titanium +2; hydroxide-1

• Potassium +1; nitrogen –3

• Calcium +2; phosphate -3