Chemistry » Classification of Matter » Names And Formulae Of Substances

Writing Chemical Formulae Examples

The following are examples demonstrating how to write chemical formulae.

Example: Writing Chemical Formulae 1

Question

What is formula of sodium fluoride?

Step 1: List the ions involved

We have the sodium ion (\(\text{Na}^{+}\)) and the fluoride ion (\(\text{F}^{-}\)). (You can look these up on the tables of cations and anions.)

Step 2: Write down the charges on the ions

The sodium ion has a charge of \(\text{+1}\) and the fluoride ion has a charge of \(-\text{1}\).

Step 3: Find the right combination

For every plus, we must have a minus. So the \(\text{+1}\) from sodium cancels out the \(-\text{1}\) from fluoride. They combine in a \(\text{1}\):\(\text{1}\) ratio.

Step 4: Write the formula

\(\text{NaF}\)

Example: Writing Chemical Formulae 2

Question

What is the formula for magnesium chloride?

Step 1: List the ions involved

\(\text{Mg}^{2+}\) and \(\text{Cl}^{-}\)

Step 2: Find the right combination

Magnesium has a charge of \(\text{+2}\) and would need two chlorides to balance the charge. They will combine in a \(\text{1}\):\(\text{2}\) ratio. There is an easy way to find this ratio:

Draw a cross as above, and then you can see that \(\text{Mg}^{2+}\)\(\text{1}\) and \(\text{Cl}^{-}\)\(\text{2}\).

Step 3: Write down the formula

\(\text{MgCl}_{2}\)

Example: Writing Chemical Formulae 3

Question

Write the chemical formula for magnesium oxide.

Step 1: List the ions involved.

\(\text{Mg}^{2+}\) and \(\text{O}^{2-}\)

Step 2: Find the right combination

\(\text{Mg}^{2+}\):\(\text{2}\)

\(\text{O}^{2-}\):\(\text{2}\)

If you use the cross method, you will get a ratio of \(\text{2}\):\(\text{2}\). This ratio must always be in simplest form, i.e. \(\text{1}\):\(\text{1}\).

Step 3: Write down the formula

\(\text{MgO}\) (not \(\text{Mg}_{2}\text{O}_{2}\))

Example: Writing Chemical Formulae 4

Question

Write the formula for copper(II) nitrate.

Step 1: List the ions involved

\(\text{Cu}^{2+}\) (the questions asks for copper(II) not copper(I))

\(\text{NO}_{3}^{-}\)

Step 2: Find the right combination

Step : Write the formula

\(\text{Cu}(\text{NO}_{3})_{2}\)

Tip:

Notice how in the last example we wrote \(\text{NO}_{3}\) inside brackets. We do this to indicate that \(\text{NO}_{3}\) is a compound ion and that there are two of these ions bonded to one copper ion.

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