Writing Chemical Formulae

Chemical formula definition

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

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

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.

Displayed formula

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.

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.

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.

1. 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.
2. 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.

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.