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Horrified by this knowledge, over 86% of his classmates signed the petition to ban it, little did they know that Dihydrogen monoxide is nothing but water.[1] Yes, it will kill you when you drown, its steam causes severe burns, and tumour cells have higher water content than normal cells. All these were not made up, they are facts. The prank was successful because people didn’t think of writing its chemical formula from the name dihydrogen monoxide, H2O. Had they taken the time to write the formula, they wouldn’t have fallen into Nathan’s trap. Nathan's project named "How gullible are we?" won a prize in the competition.
Therefore, if we don't want to be victims of such pranks, we should know how to write chemical formulae. Come, let's dive deep!
Chemical formula definition
A chemical formula is a set of chemical symbols showing the number or ratio of atoms of each type of element in a molecule or compound.
From the above story, how can you write the chemical formula of water from its chemical name, dihydrogen monoxide?
'Di' means 2 and 'mono' means 1. Therefore, dihydrogen indicates that there are two hydrogen atoms and monoxide indicates the presence of one oxygen atom. Thus, the chemical formula of water is H2O. Of course, this trick is not possible with all the chemical formulae, but don't worry, we will make it easy for you!
Types of chemical formulae
There are several ways to write the chemical formula that represents a molecule or a compound. The different types of chemical formulae include:
- Molecular formula
- Empirical formula
- Displayed formula
Molecular formula
The molecular formula of a compound shows the number of atoms of each type in a compound or molecule.
For example, the molecular formula of glucose is C6H12O6. This formula gives us the information that there are six carbon atoms (C6) , twelve hydrogen atoms (H12) and six oxygen atoms(O6) in a molecule of glucose.
Empirical formula
The empirical formula of a compound shows the simplest whole-number ratio of the atoms of each element present in a compound or molecule.
For example, the empirical formula of glucose is CH2O.
This formula shows that the number of hydrogen atoms in glucose is exactly twice the number of both carbon and oxygen atoms. In other words, the ratio of C:H:O atoms in glucose is 1:2:1.
We always use empirical formulae to represent ionic compounds. This is because ionic compounds can be infinite, and so they don't have a fixed molecular formula.
Displayed formula
The displayed formula of a compound shows every atom and bond within a molecule. Each bond in a displayed formula represents a pair of electrons.
Glucose and fructose have the same molecular formula: C6H12O6. You can already see that they also have the same empirical formula: CH2O. Then, how do we differentiate between the two? Let us look at their displayed formula.
You'll learn more about functional groups in Organic Chemistry. You'll also recap chemical formulae in Organic Compounds.
Rules for writing chemical formulae
Learning how to write chemical formulae is the heart of chemistry. Writing formulae helps you in many aspects of chemistry-like writing balanced chemical equations, and determining masses of compounds and molecules. In short, everything hereafter you learn depends on knowing the symbols of elements and using them to write the correct chemical formulae.
Two important aspects one must bear in mind while writing chemical formulae:
- Ionic compounds don't exist as molecules; hence, they don't have a molecular formula. Ionic compounds exist as giant lattices, they have ionic formula units represented by empirical formulae (Eg: NaCl).
- Covalent compounds exist as molecules; hence, they are represented by molecular formulae.
There are some rules that should be followed when writing chemical formulae.
- Elements are shown using their chemical symbol.
- The charges of the ions should be predicted based on their position in the periodic table
Fig. 2. Periodic table of elements with charges of ions written at the top of groups(columns).
- In ionic species, the sum of the charges of the ions must equal zero.
- In covalent species, the partial charges of the elements must be balanced.
- The number of atoms of each element or ion in the formula is written to the right of its symbol in subscript.
- If a compound contains more than one polyatomic ion, each ion is enclosed in brackets, with the subscript written to the right of the closing bracket (ex- Cu(SO4)2)
- When the ratio of a polyatomic ion is only 1, brackets are not required (e.g: NaOH, NH4NO3.)
- In ionic species, the name of the cation is always written first. This means that if the compound consists of a metal and a non-metal, the symbol of the metal must be written first, followed by the symbol of the non-metal.
- Transition metals can exhibit two or more different charges and can form different compounds with the same non-metal. For example, copper can form two compounds with chlorine-cuprous chloride(CuCl) and cupric chloride (CuCl2). In cuprous chloride, the charge of copper is +1 which is represented as a Roman numeral enclosed in parentheses after the corresponding metal ion, copper in this case-Copper (I) chloride. Similarly, cupric chloride is written as Copper(II) Chloride.
Given below is a list of the most common polyatomic ions that you will encounter very often.
| Name of the polyatomic ion | Symbol | Example |
| Nitrate | \(NO^{-}_3\) | Sodium nitrate- \(NaNO_3\) |
| Nitrite | \(NO_2^-\) | Sodium nitrite- \(NaNO_2\) |
| Carbonate | \(CO_3^-2\) | Sodium carbonate-Washing soda \(Na_2CO_3\) |
| Sulfate | \(SO^{-2}_4\) | Sodium sulfate- \(Na_2SO_4\) |
| Sulfite | \(SO^{-2}_3\) | Sodium sulfite- \(Na_2SO_3\) |
| Phosphate | \(PO^{-3}_4\) | Sodium phosphate- \(Na_3PO_4\) |
| Phosphite | \(PO^{-3}_3\) | Trisodium phosphite\(Na_3PO_3\) |
| Phosphide | \(P^{-3}\) | Sodium phosphide- \(Na_3P\) |
| Hydroxide | \(OH^-\) | Sodium hydroxide- \(NaOH\) |
| Ammonium | \(NH_4^+\) | Ammonium chloride- \(NH_4Cl\) |
| Bicarbonate/Hydrogen carbonate | \(HCO^{-}_3\) | Sodium bicarbonate- Baking soda- \(NaHCO_3\) |
| Bisulfate/ Hydrogen sulfate | \(HSO^{-}_4\) | Sodium hydrogen sulfate- \(NaHSO_4\) |
| Bisulfite/ Hydrogen sulfite | \(HSO_3\) | Sodium hydrogen sulfite- \(NaHSO_3\) |
| Sulfide | \(S^{-2}\) | Sodim sulfide- \(Na_2S\) |
| Permanganate | \(MnO_4^-\) | Potassium permanganate \(KMnO_4\) |
Steps in writing chemical formulae
Let us understand how to write a chemical formula by taking the example of copper (II) chloride. Refer to the rules for writing chemical formulae to recall that copper can form two different compounds with chlorine.
- First, identify the elements or ions that are present in the compound for which you are trying to write the formula. Let us take the example of a simple compound - copper (II) chloride. We can identify that there is copper, and there is chlorine in this compound.
- Then, write down the chemical symbols of the elements/ions that you identified in the first step. If the compound is made of a metal and a non-metal, write the symbol of the metal first.
Fig. 3. Identifying elements in a compound: copper and chlorine.
- After writing the chemical symbols of all elements/ions in the compound, write the charges of ions right below the respective ions. In this case, +2 below copper and -1 beneath chlorine.
Fig. 4. Writing the valencies of each element: copper and chlorine. - Now, cross over the valencies of each element with the other element/ion.
Fig. 5. Crossing the Valencies of copper and chlorine. - Finally, the charge of one ion becomes the ratio/subscript of the other element/ion. Remember to write the ratio in subscript.
Can you guess if CuCl2 is the molecular formula or the empirical formula of copper chloride? Yes, you guessed it right! Copper (II) chloride is an ionic compound. Therefore, CuCl2 is the empirical formula of the cupric chloride.
Molecular formulae and empirical formulae can also be found if the relative atomic mass or the percentage composition of the compound is known.
Examples of writing chemical formulae: molecules, ions, and compounds
Let us look at some examples to practice writing chemical formulae.
Water
The chemical name for water is dihydrogen monoxide. Looking at the chemical name, we can identify that there is hydrogen and oxygen in it (oxide is the name for oxygen ion, O2-).
We know that the valency of hydrogen is 1, and that of oxygen is 2. Following the steps for writing the chemical formula, we can write this:
Fig. 7. Crossing the valencies for water.
Therefore, the chemical formula of water (dihydrogen monoxide) is H2O. This is the molecular formula of H2O, as well as its empirical formula. Let us look at its displayed formula -
Fig. 8. Displayed formula for water.
Carbon tetrachloride
From the name, we can deduce that this compound has carbon, and chlorine in it. The prefix tetra- is for 4. That means there are 4 chlorine atoms in the molecule of this compound. Can you already guess the chemical formula? Let's follow the steps to reach the formula.
Fig. 9. Chemical formula of carbon tetrachloride.
If you guessed CCl4 as the chemical formula (and the empirical formula too) for carbon tetrachloride, you were right! Now, for the displayed formula-
Fig. 10. Displayed formula of carbon tetrachloride.
Copper(II) Sulphate
Copper(II) sulphate is a salt i.e. it is made when a base of copper and acid of sulphate neutralise each other. It is solid at room temperature. Let us find out the formula of copper(II) sulphate through the steps.
Fig. 11. Chemical formula of copper (II) sulfate.
Copper has a charge of +2, and the charge of sulfate ion is also -2, so the charges balance each other. Therefore, the molecular formula of copper sulphate is CuSO4 (and not Cu2(SO4)2.)
You might sometimes see copper sulfate written as CuSO4.5H2O. This is the hydrated form of the salt.
Hydrated salt means there is water of crystallisation in the solid crystals of salt.
Water of crystallisation is the water present in hydrated salts. Water molecules inside the salt crystals are linked to the metal ions either by dative bonds[2] [3] or by ion-dipole interactions.[4]
A salt that contains no water of crystallisation is called anhydrous salt.
Ammonium nitrate
Ammonium nitrate is an ionic compound. The cation is the ammonium (NH4+), and the anion is the nitrate (NO3-). Have you already found the chemical formula for this compound? Let us go over the steps again
Fig. 12. Chemical formula of ammonium nitrate.
So, the chemical formula for ammonium nitrate is NH4NO3. Did you guess it right? Since there are 2 nitrogen atoms (one in cation, one in anion), the molecular formula can also be written as H4N2O3.
Let us draw the displayed formula now:
Fig. 13. Displayed formula for ammonium nitrate.
Examples of Writing Chemical Formulae
Now let us look at a few more example problems, where the name and chemical formula of the compound is unknown. The charges of the ions/elements will be given. You should be able to derive the chemical formula from the given information.
An element X with charge 3 forms a compound with Oxygen. What is the molecular formula of that compound?
Solution
We know that oxygen has a charge of -2 (oxide ion= -2 charge). Let us follow the steps to arrive at the formula. Remember that oxygen will form the anion, so element X has to form the cation. Therefore, element X will be written first.
Fig. 14. Oxide of a hypothetical element X.
Therefore, the molecular formula of this compound is X2O3. It should be noted that the empirical formula of this compound will be the same as its molecular formula.
The element Y forms its sulphide. The charge of element Y is 2. What will be the formula of the sulphide of Y?
Solution
Sulphide is the name for the sulphur anion, S2-. Let us follow the steps to arrive at the formula.
Fig. 15. Sulphide of a hypothetical element Y.
Writing Formulae - Key takeaways
- A chemical formula is a set of chemical symbols showing the number of atoms of each type of element in a molecule or compound.
- There are multiple types of chemical formulae:
The molecular formula shows the number of atoms of each element in a compound or molecule.
The empirical formula shows the simplest whole-number ratio of the atoms of each element present in a compound or molecule.
The displayed formula show every atom and bond within a molecule.
Certain rules must be followed while writing chemical formulae:
Elements are shown using their chemical symbol.
In ionic species, the sum of the charges of the ions must equal zero.
Multiple copies of an atom or ion are shown in subscript.
- In ionic compounds, the name of the cation is written first.
- We can work out the formula and charges of ions using the positions of the elements on the periodic table.
References
- https://www.washingtonpost.com/archive/opinions/1997/10/21/dihydrogen-monoxide-unrecognized-killer/ee85631a-c426-42c4-bda7-ed63db993106/
- https://www.bbc.co.uk/bitesize/guides/zrx32sg/revision/10#:~:text=Water%20of%20crystallisation%20is%20water,main%20formula%20by%20a%20dot.&text=hydrated%20means%20that%20the%20solid%20crystals%20contain%20water%20of%20crystallisation
- https://www.sciencedirect.com/topics/engineering/hydrated-salt
- Shiver and Atkin's Inorganic chemistry, Fifth edition
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Frequently Asked Questions about Writing Chemical Formulae
What is the meaning of writing chemical formulae?
A chemical formula is a set of chemical symbols showing the number or ratio of atoms of each type of element in a molecule or compound. It can also indicate the species' structure or which ions it contains, and help calculate its relative molecular mass.
What are some examples of chemical formulae?
- Glucose has the molecular formula C6H12O6 and the empirical formula CH2O.
- Water has the molecular and empirical formula H2O.
- Copper(II) sulfate has the molecular formula and the empirical formula CuSO4.
What is the simple definition of chemical formula?
A chemical formula is a set of chemical symbols showing the number or ratio of atoms of each type of element in a molecule or compound.
What are the rules for writing a chemical formula?
- Elements are shown using their chemical symbol.
- In ionic species, the sum of the charges of the ions must equal zero.
Multiple copies of an atom or ion are shown in subscript. If there are multiple copies of a polyatomic ion, the ion is enclosed in brackets.
- In ionic compounds, the name of the cation is written first.
How do you write chemical formulas in text?
You write chemical formulae using the chemical symbols of the elements involved and numbers to show their relative amounts or ratio. Follow these rules:
- In ionic species, the sum of the charges of the ions must equal zero.
Multiple copies of an atom or ion are shown in subscript. If there are multiple copies of a polyatomic ion, the ion is enclosed in brackets.
- In ionic compounds, the name of the cation is written first.
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