Writing chemical formula and naming chemical compounds are some of the things O level chemistry students find challenging. This post is all about addressing the challenges faced by students when writing chemical formula and naming chemical compounds.
Naming binary ionic compounds
Binary ionic compounds contain only 2 elements, a metal and a nonmetal, in which the metal has lost all valence electrons to form a cation and the non-metal has gain electrons to form an anion.
When naming binary ionic compounds we start with the name of the metal, followed by the name of the nonmetal, ending in -ide. For example:
| Metal | Non-metal | Compound formed |
|---|---|---|
| Sodium | Nitrogen | Sodium nitride |
| Potassium | Oxygen | Potassium oxide |
| Zinc | Sulphur | Zinc sulphide |
| Calcium | Fluorine | Calcium fluoride |
| Magnesium | Iodine | Magnesium iodide |
| Sodium | Chlorine | Sodium chloride |
| Zinc | Phosphorus | Zinc phosphide |
Valence of an element
When writing the chemical formula of an ionic compound we first need to determine the valencies of the ions in the compound.
The valence of an element is the charge its atoms take when it loses or gains electrons to become an ion.
To determine to valence of an element we need to know either its proton number or its position on the periodic table.
Steps for writing chemical formula of binary ionic compounds
- Write the names of the elements in the compound
- Write the symbols of the two elements.
- Write the valence of each as superscripts without the positive and negative signs.
- Interchange the valencies and then write them as subscripts.
- Reduce the ratio of the formula as simple as possible.
Chemical formula of sodium nitride
| Step | Metal | Non-metal |
|---|---|---|
| Write the names of the elements in the compound | Sodium | Nitrogen |
| Write the symbols of the two elements | Na | N |
| Write their valences | Na+ | N3- |
| Remove the signs | Na1 | N3 |
| Exchange the valences and make them subscripts | Na3 | N1 |
Therefore the formula of sodium nitride is Na3N.
Chemical formula of potassium oxide
| Step | Metal | Non-metal |
|---|---|---|
| Write the names of the elements in the compound | Potassium | Oxygen |
| Write the symbols of the two elements | K | O |
| Write their valences | K+ | O2- |
| Remove the signs | K1 | O2 |
| Exchange the valences and make them subscripts | K2 | O1 |
Therefore the formula of potassium oxide is K2O.
Chemical formula of zinc sulphide
| Step | Metal | Non-metal |
|---|---|---|
| Write the names of the elements in the compound | Zinc | Sulphur |
| Write the symbols of the two elements | Zn | S |
| Write their valences | Zn2+ | S2- |
| Remove the signs | Zn2 | S2 |
| Exchange the valences and make them subscripts | Zn2 | S2 |
| Reduce the ratio on the formula | Zn1 | S1 |
Therefore the formula of zinc sulphide is ZnS
Chemical formula of compounds of transition metals
Just like regular metals, transition metals form ionic compounds with non-metals. Transition metals are elements that have variable valence. Since these metal do not have a fixed valence, their valences are stated in the names of their compounds. For example:
- copper (I) oxide – is an oxide of copper in which copper has a valence of 1.
- copper (II) oxide – is an oxide of copper in which copper has a valence of 2.
- iron (II) oxide – is an oxide of iron in which iron has a valence of 2.
- iron (III) oxide – is an oxide of iron in which iron has a valence of 3.
Chemical formula of copper (I) oxide
The valence of copper here is 1.
| Step | Metal | Non-metal |
|---|---|---|
| Write the names of the elements in the compound | Copper | Oxygen |
| Write the symbols of the two elements | Cu | O |
| Write their valences | Cu+ | O2- |
| Remove the signs | Cu1 | O2 |
| Exchange the valences and make them subscripts | Cu2 | O1 |
Therefore the formula of copper (I) oxide is Cu2O.
Chemical formula of copper (II) oxide
The valence of copper here is 2.
| Step | Metal | Non-metal |
|---|---|---|
| Write the names of the elements in the compound | Copper | Oxygen |
| Write the symbols of the two elements | Cu | O |
| Write their valences | Cu2+ | O2- |
| Remove the signs | Cu2 | O2 |
| Exchange the valences and make them subscripts | Cu2 | O2 |
| Reduce to simplest ratio | Cu1 | O1 |
Therefore the formula of copper (II) oxide is CuO.
Chemical formula of iron (II) oxide
The valence of iron here is 2.
| Step | Metal | Non-metal |
|---|---|---|
| Write the names of the elements in the compound | Iron | Oxygen |
| Write the symbols of the two elements | Fe | O |
| Write their valences | Fe2+ | O2- |
| Remove the signs | Fe2 | O2 |
| Exchange the valences and make them subscripts | Fe2 | O2 |
| Reduce to simplest ratio | Fe1 | O1 |
Therefore the formula of iron (II) oxide is FeO.
Chemical formula of iron (III) oxide
The valence of iron here is 3.
| Step | Metal | Non-metal |
|---|---|---|
| Write the names of the elements in the compound | Iron | Oxygen |
| Write the symbols of the two elements | Fe | O |
| Write their valences | Fe3+ | O2- |
| Remove the signs | Fe3 | O2 |
| Exchange the valences and make them subscripts | Fe2 | O3 |
Therefore the formula of iron (III) oxide is Fe2O3.
Chemical formula of vanadium (V) oxide
The valence of vanadium here is 5.
| Step | Metal | Non-metal |
|---|---|---|
| Write the names of the elements in the compound | Vanadium | Oxygen |
| Write the symbols of the two elements | V | O |
| Write their valences | V5+ | O2- |
| Remove the signs | V5 | O2 |
| Exchange the valences and make them subscripts | V2 | O5 |
Therefore the formula of vanadium (V) oxide is V2O5.
Steps for writing chemical formula of ionic compounds with polyatomic anions.
A polyatomic anion exists as a group of elements with a collective negative charge. Examples of polyatomic anions are:
* sulphate ions, SO42-
* sulphite ions, SO32-
* nitrate ions, NO3–
* nitrite ions, NO2–
* carbonate ions, CO32-
* hydroxyl ions, OH–
* ammonium ions, NH4+
* phosphate ions, PO43-
When writing the chemical formula of a compound that contains a polyatomic ion, we follow the following steps:
- Write the names of the elements and ions in the compound
- Write the symbols of the ions involved.
- Write the valence of each as superscripts without the positive and negative signs.
- Interchange the valencies and then write them as subscripts.
- Reduce the ratio of the formula as simple as possible.
Chemical formula of copper (II) sulphate
The valence of copper here is 2.
| Step | Metal | Non-metal |
|---|---|---|
| Write the names of the elements and ions | Copper | Sulphate |
| Write the chemical symbols and the valences | Cu2+ | SO42- |
| Remove the signs | Cu2 | SO42 |
| Exchange the valences and make them subscripts | Cu2 | (SO4)2 |
| Reduce to simplest ratio | Cu1 | (SO4)1 |
Therefore the formula of copper (II) sulphate is CuSO4
Chemical formula of copper (II) hydroxide
The valence of copper here is 2.
| Step | Metal | Non-metal |
|---|---|---|
| Write the names of the elements and ions | Copper | Hydroxide |
| Write the chemical symbols and the valences | Cu2+ | OH– |
| Remove the signs | Cu2 | OH1 |
| Exchange the valences and make them subscripts | Cu1 | (OH)2 |
Therefore the formula of copper (II) hydroxide is Cu(OH)2.
When you have more than one of a specific polyatomic ion in a formula you must enclose it in parenthesis.