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 : 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.