Formulas from Names – Simple Salts The Circle Method

Here is a simple example of using the circle method to determine a chemical formula from a chemical name: What is the formula for sodium chloride? What is the forth step? Do the numbers in the circle add up to zero? Then you do not need to add any circles. What is the formula? What is the third step? What is the second step? What is the first step? Ask if the numbers in the circles add up to zero. There is one circle in the chloride column, so… Write an “= 0” at the right. The sum of the charges on the ions must equal zero. Draw a circle in each column to represent the ion. Write the charge of the ion in the circle. Yes! There is one circle in the sodium column, so… Make two columns, one for each ion. Na+ Cl- = 0 + 1 - 1 Na Cl

Here is another simple example of using the circle method to determine a chemical formula from a chemical name: What is the formula for magnesium chloride? Now do the numbers in the circles add up to zero? What is the third step? So what should we do? Which column needs more circles? What is the second step? What is the first step? So, what is the final formula? Do they? What is the forth step? The chloride column. They do! There are two circles in the chloride column, so… There is only one circle in the magnesium column, so… Add one circle to the chloride column. Ask if the numbers in the circles add up to zero. No! Write an “= 0” at the right, because the sum of the charges must equal zero Draw a circle under each column representing an ion. Then write the charge each ion in its circle. Write a column for each ion. Mg2+ Cl- + 2 - 1 = 0 Mg Cl2 - 1

Here is yet another simple example of using the circle method to determine a chemical formula from a chemical name: What is the formula for sodium sulfide? Do the numbers add up to zero? Which column needs more circles? Do the numbers in the circles add up to zero now? So what is the final chemical formula? What is the third step? What is the forth step? What is the first step? What is the second step? No! The sodium column. Yes! Ask if the numbers in the circles add up to zero. Draw a circle in each column. Write the charge of the ion in each circle. There is one circle in the sulfide column, so… There are two circles in the sodium column, so… Write an “= 0” at the right. The charges on the ions must add up to zero. Write a column for each ion. Na+ S2- + 1 - 2 = 0 + 1 Na2 S

Here is a complex example of using the circle method to determine a chemical formula from a chemical name: What is the formula for aluminum oxide? What is the forth step? Which column needs more circles? Do the numbers in the circles add up to zero? Which column needs more circles? Now, do all of the numbers in the circles add up to zero? Which column needs more circles? Now, do all of the numbers in the circles add up to zero? Now do all of the numbers in the circles add up to zero? What is the first step? What is the second step? So what is the chemical formula? What is the third step? Aluminum cation – Al3+. There are three circles in the oxide ion column, so… There are two circles in the aluminum ion column, so… Oxide – O2-. Yes! No! Oxide – O2-. Write an “= 0” at the right. Draw a circle in each column. Write the charge of the ion in each circle. Ask if the numbers in the circles add up to zero. No! No! Write a column for each ion. Al3+ O2- + 3 - 2 = 0 - 2 + 3 Al2 O3 - 2

Here is another complex example of using the circle method to determine a chemical formula from a chemical name: What is the formula for magnesium nitride? What is the forth step? Which column needs more circles? Do the numbers in the circles add up to zero? Which column needs more circles? Now do all of the numbers in the circles add up to zero? Which column needs more circles? Now, do all of the numbers in the circles add up to zero? Now, do the numbers in the circles add up to zero? What is the first step? What is the second step? So, what is the chemical formula? What is the third step? Nitride ion – N3- There are two circles in the nitride ion column, so... There are three circles in the magnesium ion column, so... Magnesium ion – Mg2+ Yes! No! Magnesium cation – Mg2+ Write an “= 0” at the right. Draw a circle in each column. Write the charge of the ion in each circle. Ask if all of the numbers in the circles add up to zero. No! No! Write a column for each ion. Mg2+ N3- + 2 - 3 = 0 + 2 - 3 Mg3 N2 + 2