How to write chemical formulas for ionic compounds? (Worked Examples)

The Crisscross method is really useful to write formulas of ionic compounds. For that, we can use four simple steps

Let’s consider the example of sodium chloride. Here the name of the ionic compound is sodium chloride. In order to write the chemical formula, we should go through the steps.

 

The first step is, write the correct symbols of cation first and the anion second. As we know, cation means the positive ion and anion means the negative ion. Usually in an ionic compound cation is metal and anion is a nonmetal. Let's look at our ionic compound. In our compound positive ion which means the cation is sodium. So, the symbol is Na. and the negative ion which is called anion is chloride. So, the symbol is Cl. 

Our second step is, write the charges of cations and anions. So, we should recognize the charges of cations and anions. For that, we can use the periodic table because both Na and Cl are simple ions.

Let's look at this periodic table. By looking at the periodic table, we can identify the charges of simple ions. these are the charges you need to have memorized.

As you can see in the periodic table Na is in group 1. The ionic charge of every element in group 1 of the periodic table is plus 1 because its outermost shell electron configuration is 1. Therefore, group 1 elements tend to lose this valence electron and obtain a stable electron configuration. So, the ionic charge of sodium is 1. Now let's see the position of the cl which is our anion in the periodic table. In the periodic table, cl is in group 7 which means there are 7 electrons in their outer shell. So, group 7 elements tend to gain 1 electron to obtain their nearest stable electron configuration. Therefore, the ionic charge of group 7 elements is always -1 because of that extra electron.so the ionic charge of chlorine is -1.

The third step is, switch the charge number and write as subscripts.

The final step is, simplify to the lowest possible ratio and omit one. Here you can see, the ratio is1:1 and in our formula, we should omit one.

So, the chemical formula for sodium chloride is NaCl.

Note that, always ionic compounds are chemically neutral. that means all the + charges are equal to all the - charges. Here the Na has plus + 1charge and chlorine has -1 charge and therefore it is chemically neutral.

Let's look at another example, Aluminum oxide.

Aluminum oxide shows you the name of the positive charge ion as aluminum and negative charge ion as oxygen ending with   -ide.

Now we need to identify the charges of aluminum and oxygen.

 

According to the periodic table, Al is in group 3. The ionic charge of every element in group 3 of the periodic table is plus 3 because its outermost shell electron configuration is 3. therefore, group 3 elements tend to lose these valence electrons and obtain the stable electron configuration. so, the charge is +3 and oxygen is in group 6a. which means there are 6 electrons in their outer shell. So, group 6 elements tend to gain 2 electrons to obtain their nearest stable electron configuration. Therefore, the ionic charge of group 6 elements is always -2 because of that extra electrons.so the ionic charge of oxygen is -2.

Now let's switch the number of charges. Al₂O₃.

So, the chemical formula of aluminum oxide is Al₂O₃.

Here, aluminum oxide consists of 2 Al+3 ions and 3O-2 ions. The overall charge of aluminum is +6. the overall charge of oxygen is -6. so, its chemically neutral because always ionic compounds are chemically neutral.

Let's suppose the chemical formula of calcium oxide.

 

Here the positive ion is Ca and negative ion is O

Now Let's find the charge of each element using the periodic table.

 

Ca is in group 2 and has the +2 charge. Oxygen is over here and has the -2 charge.

Using the crisscross method, it's going to be Ca202. But the final formula should be simplified to the lowest possible ratio and we can write as CaO.

So that’s the formula for calcium oxide.

Now consider the ionic compounds containing polyatomic ions.

Before moving into ionic compounds containing polyatomic ions, we need to know what are the polyatomic ions in an ionic compound?

The meaning of Polyatomic is composed of two or more atoms and ions means have an overall charge.

Here is the list of common polyatomic ions you will need to know. you should keep memorizing these common polyatomic ions.

 

Here, we are given the compound name as magnesium nitrate. As the first step, we should identify the symbol of our positive ion and negative ion.

Magnesium is our positive charge ion and has the symbol Mg. next we should identify what nitrate is?

Nitrate is our negative ion and it’s a polyatomic ion. So the nitrate has an overall (-) charge. We can check its symbol and charge by using the common polyatomic ion table.

We will see that nitrate is NO3 with the overall charge of -1.

The charge of magnesium is +2. the charge of nitrate is -1.

So by using the Crisscross method we can write the formula as Mg(NO₃)₂.remember that this 2 is common for the nitrate ion. So, we should definitely put a bracket when writing the subscript of the polyatomic ion. because always we consider whole polyatomic ion as one unit.

Let's look at another example, calcium phosphate.

The symbol of calcium is Ca and phosphate is PO₄^-3

the charge of Ca is +2 since it's in group 2 of the periodic table and the charge of PO4 is -3.

Then we can switch the charges and write our formula unit as Ca₃(PO₄)₂.

As you can see, the overall charge of ca is +6 and the overall charge of PO₄ is -6.so the compound is chemically neutral.

Our next example is Ammonium phosphate.

This an ionic compound is somewhat different from previous ones because both positive ion and negative ion are polyatomic ions.

 

Here the positive ion is ammonium and negative ion is phosphate. We can identify the symbol and charge of both polyatomic ions by using the common polyatomic table. We will see that ammonium is NH₄ with the charge of +1 and phosphate is PO₄ with the charge of -3. Now the charge of ammonium becomes the subscript for phosphate and charge of phosphate becomes the subscript for ammonium. Now by using the crisscross method we can write the chemical formula of ammonium phosphate as (NH₄)₃PO₄. this is our chemical formula.

 

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