Chemical Compound Formulas: Chemical Formula, Valency & Examples

The chemical formula is a shorthand representation of a molecule’s name expressed in terms of the symbols of the constituent atoms. Furthermore, two atoms of the same element cannot produce chemical compounds because they require two different elements.

This article will discuss everything about chemical compound formulas with examples. It also includes answers to common search queries like “chemical formula of aluminium chloride”, “chemical formula of common compounds”, “glossary of chemical formulae”, “50 chemical formula”, and “chemical formula of calcium carbonate”, and more.

What is a Chemical Formula?

The compound’s chemical formula represents the composition of a compound molecule in terms of the symbols of the elements present in it. In other words, the formula of a compound tells us ‘the kind of atoms’ and ‘the number of atoms of various elements present in one molecule of the compound.

In the chemical formula of a compound, the elements present are represented by their symbols. The number of atoms of each element is indicated by writing the digits \(2, 3, 4,\) etc., as subscripts (lower figures) on the right-hand side bottom of the symbol.

For example, water is a compound made up of \(2\) atoms of hydrogen element and \(1\) atom of oxygen element, so the formula of water is written as \({{\rm{H}}_2}{\rm{O}}\). In the formula \({{\rm{H}}_2}{\rm{O}}\), subscript \(2\) indicates \(2\) atoms of hydrogen. In the water formula, oxygen O is written without a subscript and indicates \(1\) atom of oxygen.

Another point to be noted is that in the case of molecular compounds, the chemical formula represents the molecule’s composition that makes up the compound. But in the case of ionic compounds, the chemical formula represents the simplest ratio of ions present in the compound.

We have seen that in the molecules, the atoms are combined. Different elements have different combining capacities of their atoms. This is known as valency. Valency may be defined as the combining capacity of an element. It is expressed in terms of the number of hydrogen atoms, a number of chlorine atoms, or double the number of oxygen atoms combined with one atom of that element. For example,

In ammonia \(\left( {{\rm{N}}{{\rm{H}}_3}} \right)\), the valency of \({\rm{N}}\) is \(3\).

In methane \(\left( {{\rm{C}}{{\rm{H}}_4}} \right)\), the valency of \({\rm{ C }}\) is \(4\).

In addition to the atoms, the ions, which are charged species, also have some valences. Positive ions or cations have positive valences. Negative ions or anions have negative valencies. The valencies of the polyvalent ions are expressed by enclosing them in the bracket and putting the positive or negative signs outside it. Let us write the valences of some commonly used positive and negative ions.

Chemical Compound Formula: Valency of Positive Ions or Cations

Positive ions may be monovalent, divalent, trivalent, tetravalent, etc., depending upon the charge present on them. These are listed in the following table:

Valency of Negative Ions or Anion

 Like positive ions, negative ions may also be monovalent, divalent, trivalent, etc., in nature. These are also listed below:

Study Everything About Compound

Examples of some common anions

Writing of Chemical Formula

The following procedure is followed for writing the formulae of compounds:

1. The symbols of the constituent elements are written so that the positive atom or radical symbol is placed on the left-hand side, and the negative atom or radical symbol is placed on the right-hand side.

2. The valences of the symbols are written below them and shifted cross-wise to the lower right corner of the symbols, and the valency numbers become subscripts of the corresponding symbols.

3. The valency numbers are reduced to a simple ratio by dividing with a common factor if required.

This method of writing the formulae will become clearer from the following example. Let us work out the formula of hydrogen sulphide by this method.

1. The hydrogen sulphide compound is made up of hydrogen and sulphur elements. So, first, we write down the symbols of hydrogen and sulphur.

2. The valency of hydrogen is \(1\), and the valency of sulphur is \(2\). So, below the symbol \({\rm{H}}\), we write \(1\) and below the symbol \({\rm{S}}\), we write \(2\):

3. We now cross over the valences of \({\rm{H}}\) and \({\rm{S}}\) atoms. With \({\rm{H}}\) atom, we write the valency of \({\rm{S}}\) (which is \(2\)) as a subscript so that it becomes \({{\rm{H}}_2}\). With the \({\rm{S}}\) atom, we write the valency of \({\rm{H}}\) (which is \(1\)) as a subscript to becomes \({{\rm{S}}_1}\). Now, joining together \({{\rm{H}}_2}\) and \({{\rm{S}}_1}\), the formula of hydrogen sulphide becomes \({{\rm{H}}_2}\;{{\rm{S}}_1}\) or \({{\rm{H}}_2}\;{\rm{S}}\) (This is because we do not write the subscript \(1\) with an atom in a formula).

Writing Chemical Formula of Simple Molecular Compounds

In the formation of simple molecules, atoms of the elements participate. The symbols of these atoms are written, and their valences are placed below them. As shown, these valences are shifted cross-wise to the lower right of these atoms. This is known as cross over or criss-cross of valences. Let us write the chemical formula of a simple molecular compound.

The molecular compounds are also called covalent compounds. The compounds are hetero atomic in nature. It means that different elements participate in these compounds. They may be homoatomic also, which means that these are formed from the atoms of the same element. For example, hydrogen molecule \(\left( {{{\rm{H}}_2}} \right)\), chlorine molecule \(\left( {{\rm{C}}{{\rm{l}}_2}} \right)\), oxygen molecule \(\left( {{{\rm{O}}_2}} \right)\), nitrogen molecule \(\left( {{{\rm{N}}_2}} \right)\), etc.

Writing Chemical Formula of Simple Ionic Compounds

Simple ionic compounds are of binary nature. It means that both the positive and negative ions have one atom only. The symbols of these ions are written side by side with their valences at their bottom. A common factor, if any, is removed to get a simple ratio of the valences of the combining atoms. The criss-cross method is then applied to arrive at the final chemical formula of the compound. Let us write the formulae of a few simple ionic compounds. For example

Writing Chemical Formula of Ionic Compounds Containing Polyatomic Ions

Looking at the list of positive and negative ions, we find that mostly the negative ions or anions are polyatomic ions. While the positive ions are simple ions. However, \({\rm{NH}}_4^ + \) ion is an exception. It is a polyatomic ion. These may be enclosed in the bracket, as mentioned earlier. The rest of the procedure remains the same. For example,

Roman numerals I, II, III, etc., are very commonly mentioned for the valency of the metal atom in writing the molecular formula of the compounds. For example

Aluminium (III) hydroxide, sodium (I) carbonate, calcium (II) phosphate, potassium(I) chromate, Iron (II) sulphide, and magnesium (II) acetate.

Solved Examples on Writing Chemical Formula

Sample Problem 1. Work out the formula of hydrogen chloride.

Solution. Hydrogen chloride is made up of hydrogen and chlorine elements. The symbol of hydrogen is \({\rm{H}}\), and that of chlorine is \({\rm{ Cl }}\). The valency of hydrogen is \(1\), and the valency of chlorine is also \(1\). The formula of hydrogen chloride can be worked out as follows:

Thus, the formula of the hydrogen chloride compound is \({\rm{HCl}}\).

Sample Problem 2. The valency of carbon is \(4\), and that of chlorine is \(1\). What will be the formula of carbon tetrachloride?

Solution. We can work out the formula of carbon tetrachloride as follows:

Thus, the formula of carbon tetrachloride is \({\rm{CC}}{{\rm{l}}_4}\).

Sample Problem 3. The valency of an element \({\rm{X}}\) is \(1\), and that of oxygen is \(2\). What will be the formula of the compound formed by the combination of element \({\rm{X}}\) with oxygen?

Solution. The formula of a compound of element \({\rm{X}}\) with oxygen can be found as follows:

Thus, the formula of a compound of element \({\rm{X}}\) with oxygen is \({{\rm{X}}_2}{\rm{O}}\).

(Please note that element \({\rm{X}}\) of valency \(1\) is like hydrogen, \({\rm{H}}\). So, the compound \({{\rm{X}}_2}{\rm{O}}\) may be like water, \({{\rm{H}}_2}{\rm{O}}\).

List of Chemical Compounds and their Formulas

Summary

The chemical formula of a compound has chemical symbols representing the elements present, and the mathematical figure in the subscript represents the number of atoms of each element present in one molecule of the compound. A chemical formula gives the exact number of atoms of the same or different elements present in a chemical substance. This article learned about the valences of positive and negative ions, writing chemical formulas of some common compounds and examples of compounds and their formulas.

FAQs on Chemical Compounds Formula

The most commonly asked questions on Chemical Compounds Formula are answered here:

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