1.2.4 How to write a Chemical Formula

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How to write a Chemical Formula

Compounds are represented by chemical formulae as elements are represented by symbols. Chemical formulae of compounds are written keeping the following steps in consideration.

  • Symbols of two elements are written side by side, in the order of positive ion first and negative ion later.
  • The valency of each ion is written on the right top corner of its symbol, e.g. Na+, Ca +2 ., Cl - .  and O 2-.
  • This valency of each icon is brought to the lower right corner of another ion by the 'cross-exchange' method, e.g.

Na+Cl -, Ca2+Cl- and Ca 2+O 2-

NaCl, CaCl2, and CaO.

  • If the valencies are the same, they are offset and are not written in the chemical formula. But if they are different, they are indicated as such at the same position, e.g. in the case of sodium chloride both the valencies are offset and the formula is written as NaCl, whereas, calcium chloride is represented by formula CaCl2.
  • If an ion is a combination of two or more atoms which is called radical, bearing a net charge on it, e.g. SO 4 2- (sulfate) and PO 4 3-. (phosphate), then the net charge represents the valency of the radical. The chemical formula of such compounds is written as explained in (iii) and (iv); writing the negative radical within the parenthesis. For example, the chemical formula of aluminum sulfate is written as Al 2 (SO 4 )3 and that of calcium phosphate as Ca 3(PO 4 )2.

Empirical Formula

The simplest type of formula is the empirical formula. It is the simplest whole-number ratio of atoms present in a compound. The empirical formula of a compound is determined by knowing the percentage composition of a compound. However, here we will explain it with simple examples. The covalent compound silica (sand) has the simplest ratio of 1:2 of silicon and oxygen respectively. Therefore, its empirical formula is SiO2. Similarly, glucose has the simplest ratio of 1:2:1 of carbon, hydrogen, and oxygen, respectively. Hence, its empirical formula is CH2O. As discussed earlier, ionic compounds exist in three-dimensional network forms. Each ion is surrounded by oppositely charged ions in such a way as to form an electrically neutral compound. Therefore, the simplest unit taken as a representative of an ionic compound is called the formula unit. It is defined as the simplest whole-number ratio of ions, as present in the ionic compound. In other words, ionic compounds have only empirical formulae. For example, the formula unit of common salt consists of one Na+ and one Cl- ion, and its empirical formula are NaCl. Similarly, the formula unit of potassium bromide is KBr, which is also its empirical formula.

Molecular Formula

Molecules are formed by the combination of atoms. These molecules are represented by molecular formulae that show the actual number of atoms of each element present in a molecule of that compound. The molecular formula is derived from the empirical formula by the following relationship:

Molecular formula = (Empirical formula)n

Where n is 1,2,3 and so on.

For example, the molecular formula of benzene is C6H6 which is derived from the empirical formula CH where the value of n is 6.

The molecular formula of a compound may be the same or a multiple of the empirical formula.

The molecular and empirical formulas of some compounds are given in the figure below.


Some compounds have the same molecular and empirical formula for example water and hydrochloric acid.

Molecular mass & Formula mass

Molecular Mass

The sum of atomic masses of all the atoms present in one molecule of a molecular substance, is its molecular mass. For example, molecular mass of chlorine (Cl2) is 71.0 2 amu, of water (H2O) is 18 amu and that of carbon oxide (CO2) is 44 amu.

Formula Mass

Some ionic compounds that form three dimensional solid crystals, are represented by their formula units. Formula mass in such cases is the sum of atomic masses of all the atoms present in one formula unit of a substance. For example, formula mass of sodium chloride is 58.5 amu and that of CaCO3 is 100 amu.

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Hello, this is Misbah ud Din a student of STEM. This is my blog chemisTREEpk which provides a vast knowledge about Chemistry(and sometimes other sciences) fully free.

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