Chemical formulas are important to understand chemical equations. They are symbolic representations of chemical species and are used to determine their chemical composition. Elements, molecules, and compounds have specific chemical formulas in which two or more atoms are represented by numerical values in the form of subscripts.

During the writing of chemical formulas of elements, it should be necessary to write the first letter of the symbol in capital and the second letter in small caps. For example, the symbol of bismuth is Bi.

However, in the case of molecules or compounds, the atoms are written in capital letters. For example, the chemical formula of sulfur hexafluoride is SF6 which has six fluorine atoms and one sulfur atom.

Chemical formulas

How to write chemical formulas

The steps and rules to be followed in writing chemical formulas of chemical compounds are as follows:

Write the symbols of elements

There are 118 elements are mentioned in the periodic table. Each element has a specific symbol that is used instead of writing the complete name of the element. During writing a chemical formula of a compound, the first element is written in the form of a symbol after the suitable prefix. For example, in the case of calcium dioxide, calcium is the first element that has the chemical symbol Ca.

After writing the symbol of the first element, write about how many atoms of the first element are involved in the compound. If there is only one atom involved then it is not necessary to mention it. Otherwise for two or more atoms, prefixes are used.

Find the suitable prefix

To write chemical formulas for chemical compounds, the second step is to find a suitable prefix that expresses the number of atoms of each element.

For example, in the case of the chemical compound diphosphorus trisulfide, there are 2 phosphorous atoms and 3 sulfur atoms. The prefixes used to show the number of atoms present in the chemical compound are given below:

Number of atoms Prefix
1 mono
2 Di
3 tri
4 tetra
5 penta
6 hexa
7 hepta
8 octa
9 nona
10 deca

Hence, the chemical formula for this compound can be obtained by crossing their valencies and it would be diphosphorus trisulfide (P2S3). Additionally, ‘Hydro’ is used as a prefix to explain the acids, which contain one ion attached to the hydrogen atom. For example, Hydrochloric acid (HCl), Hydrobromic acid (HBr), Hydroiodic acid (HI) Hydrocyanic acid (HCN)

Write the second element as a suffix

The rules for writing the second element are the same as for the first element. The only difference is the addition of suffixes at the end of the second element. Firstly a prefix is used, then a symbol of the second element, and after that, a suffix is used. For example, in the case of the covalent compound, ide- is used at the end of the last or second element.

For example, in carbon tetrachloride (CCl4), tetra is the prefix, chlorine is the second element and ide- is a suffix.

Writing chemical formulas for Ionic compounds

Ionic compounds have positive and negative ions. Positive ions are called cations while negative ions are known as anions. When writing the chemical formula for an ionic compound, cations are written as the name of the element while anions are written with the “ide” suffix. This means cations are the first name of the element and anions are the second or last name of the element. For instance, sodium sulfide, in which sodium is a cation and sulfur is an anion with the “ide” at the end.

Note that, prefixes are not used in writing chemical formulas for ionic compounds. The number of atoms can be determined by balancing the charges of the elements. For example, the chemical formula for sodium sulfide is Na2S. In this type of ionic compound, sodium has +1 charge while sulfur has -2 charge. To balance each other, 2 atoms of sodium are required and one atom of sulfur.

Writing chemical formulas for Polyatomic ions

When ionic compounds have two or more ions are called polyatomic ions in which cations and anions are arranged in groups. Polyatomic cations have the suffix ‘nium’ at the end while polyatomic anions have the suffix ate- is added. These ions can be memorized by their common names.

Example of polyatomic cations

  • Hydronium (H3O+)
  • Ammonium (NH4+)
  • Mercury (Hg+2)

Example of polyatomic anions

These are examples of polyatomic anions.

  • Carbonate (CO3-2)
  • Sulfate (SO4-2)
  • Nitrate (NO3)
  • Chromate (CrO4-2)

Valance charge on polyatomic ions

When writing chemical formulas for polyatomic ions, the charge of the whole polyatomic ion is calculated instead of a separate ion. Since the valance charge can easily find out by determining the position of the element in the periodic table.

  • All the alkali metals (group 1) have a +1 charge.
  • Alkaline earth metals (group 2) have a +2 charge.
  • The valance charge on transition elements can be indicated by Roman numerals in the parenthesis brackets.
  • The elements silver (Ag+1) has a +1 charge, zinc (Zn+2) has a +2 charge, and aluminum (Al+3) has a +3 charge.
  • Pinictogens (group 15) have a -3 charge.
  • Chalcogens (group 16) have a -2 charge.
  • Halogens (group 17) have a -1 charge.

Writing Chemical Formulas for Covalent compounds

The steps of writing chemical formulas for covalent compounds are the same as discussed above. Firstly identify the prefix of the compounds which show the number of atoms. Then write the first and second elements of the compound and atoms are written as a subscript in the chemical formula.

For example,

  • Carbon dioxide (CO2)
  • Sulfur dioxide (SO2)
  • Calcium oxide (CaO)
  • Phosphorous trichloride (PO3)

How to write chemical formulas for the product obtained in chemical equations?

To write the chemical formula for the product obtained in a chemical reaction, the charge of the ions is balanced For example, when an aqueous solution of silver nitrate is mixed with brine (saturated solution of NaCl), a double displacement reaction occurs as given in the equation below:

AgNO3 (aq) + NaCl(aq)  ➔ AgCl(s) + NaNO3 (aq)

There is one atom of Ag, Na, Cl, N, and three atoms of oxygen are involved on the reactant side. To balance the chemical equation the atoms of different elements on the reactant side should be the same in number on the product side. That way, the above equation is balanced.

It is important to write a chemical state of the matter with the chemical formulas of the compound whether it is an ionic or covalent compound. The state of matter is always written in parentheses in a subscript.

  • (s) for solid.
  • (l) for liquid.
  • (g) for gas.
  • (aq.) for aqueous solution.

For example,

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

Related resources

Concepts Berg

How atom is presented in a formula?

While writing the chemical formula for chemical compounds, atoms are presented at the base of each element in the form of a subscript.

How to Write Chemical Formulas Correctly?

Chemical formulas of compounds can be written in the following steps:

  • Identification of the prefixes and suffixes.
  • Write the first element.
  • Write the second element.
  • In the case of ionic compounds, charges should be equal to each other.

What is the easiest way to write chemical formulas?

The easiest way to write chemical compounds is the identification of chemical compounds and the number of atoms involved. Atoms are written as a subscript and prefixes should be used before the last or second elements.

What are the steps to find out the chemical formula of any compound?

The process of writing the chemical formulas of the compound is the same. Firstly, choose a prefix according to the number of atoms in the compounds. Now write the first element and then the second element with the prefix.

How to write a balanced chemical equation for a reaction?

A chemical equation can be balanced by following rules:

  • Identify the chemical formulas of the elements and compounds.
  • Determine the valance charge on the elements.
  • Balance the numbers of atoms on both sides, reactant, and product.

What are some examples of a chemical formula?

These are some common examples of chemical formulas:

  • Water (H2O)
  • Carbon dioxide (CO2)
  • Hydronium (H3O+)
  • Ammonium (NH4+)
  • Carbonate (CO3-2)
  • Sulfate (SO4-2)
  • Hydrochloric acid (HCl).
  • Hydrobromic acid (HBr)

How to write the chemical formula for salt?

Salt is an ionic compound. It has cations and anions. Sodium act as a cation and chlorine is act as an anion. The chemical formula of the salt is NaCl. Sodium and chlorine have +1 and -1 charges which are equal to each other.