Writing Chemical Formulas: Covalent & Ionic Formulas

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 subscript.

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.

how to write chemical formulas

Steps to writing the chemical formulas

These are the steps and rules to write chemical formulas for chemical compounds:

The first step is to find a prefix that tells the number of atoms of each element in chemical formulas.

To write chemical formulas for chemical compounds, the first step is to find a suitable prefix that expresses the number of atoms of each element. For example, in the case of a chemical compound P2S3, there are 2 phosphorous atoms and 3 sulfur atoms. The chemical formula for this compound would be diphosphorus trisulfide (P2S3).

These are the prefixes used for the number of atoms:

  • Mono (number of atom is 1)
  • Di (number of atoms is 2)
  • Tri (number of atoms is 3)
  • Tetra (number of atoms is 4)
  • Penta (number of atoms is 5)
  • Hexa (number of atoms is 6)
  • Hepta (number of atoms is 7)
  • Octa (number of atoms is 8)
  • Nona (number of atoms is 9)
  • Deca (number of atoms is 10)

Hydro- is used as a prefix to explain the acids that contain the one ion attached to the hydrogen atom.

For example,

  • Hydrochloric acid (HCl).
  • Hydrobromic acid (HBr)
  • Hydroiodic acid (HI)
  • Hydrocyanic acid (HCN)

The second step is to write the symbol of the first element of the chemical formula.

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.

The third step is to write the second element.

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 the case of carbon tetrachloride (CCl4), tetra is the prefix, chlorine is the second element and ide- is a suffix.

Writing Chemical Formulas for Covalent compounds

The steps of writing the chemical formulas of 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. The atoms are written as a subscript in the chemical formula.

For example,

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

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 a chemical formula for an ionic compound, cations are written as the name of the element while anions are written with the ide- at the end. 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.

Prefixes are not used during writing the 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.

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 cations have the suffix ate- at the end. These ions can be memorized by their common names.

Example of polyatomic cations:

These are the chemical formulas for polyatomic cations that are only three.

  • 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 the chemical formulas for polyatomic ions, calculate the charge of the whole polyatomic ion instead on separate each ion. 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 +1 charge.
  • Alkaline earth metals (group 2) have +2 charge.
  • The valance charge on transition elements can be indicated by Roman numerals in the parenthesis brackets.
  • The elements silver (Ag+1) has +1 charge, zinc (Zn+2) has +2 charge, and aluminum (Al+3) has +3 charge.
  • Pinictogens (group 15) has -3 charge.
  • Chalcogens (group 16) has -2 charge.
  • Halogens (group 17) has -1 charge.

Balancing charges on Cations and Anions

To balance the charges on ions, they should be equal in numbers. A positive charge balances with the negative charge on ions. For example, in the case of lithium oxide (Li2O), lithium has +1 charge and it is present in group 1. However, oxygen has -2 charge. This means there are two atoms of lithium required to balance one atom of oxygen. Therefore, the chemical formula of lithium oxide is Li2O.

Similarly, in the case of sodium chloride (NaCl), the valance charge on sodium is +1 which is a cation. While chloride is an anion and has -1 valance charge. Thatswhy, one atom of sodium is enough to balance the valence charge of chlorine.

How to write chemical formulas for the product given reactants

Writing chemical formulas for the product by given reactants is very easy. This can be achieved by replacing the cations and anions in the reactants. For this purpose, first, identify the cation and anion in the reactant side then replace them with each other.

For example,

AgNO3 + NaCl ➔ Products

In the above equation, AgNO3 + NaCl are the given reactants. To obtain products replace cations and anions on the product side. On the reactant side, Ag and Na are the cations while NO3 and Cl are the anions. After replacement, AgNO3 becomes AgCl and NaCl becomes NaNO3.

The resultant equation becomes,

AgNO3 + NaCl ➔ AgCl + NaNO3

After obtaining the products, balance the numbers of atoms on the reactant and product side. They should be equal in numbers. For example, in the above equation, 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 on the product side, these atoms should be the same in number on the product side as well. That why, the above equation is balanced.

How to balance an unbalanced equation:

N2 + H2 ⇌ NH3

Similarly, an unbalance equation can balance by observing the numbers of atoms on both the reactant and product sides. The above equation is the equation of Haber’s process. To balance this equation, multiply both sides with suitable numbers. For example, the chemical equation can be balanced by multiplying with 2 on the product side and 3 on the reactant side.

N2 + 3H2 ⇌ 2NH3

Now, the above equation is correctly balanced on both sides. The double-headed arrow means the reactants and products are in equilibrium.

Write the chemical states of matter with chemical formulas

It is very 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 the parentheses in small letters.

These are the symbols of the states of matter:

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

For example,

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

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 cation 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.


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