Chemical Formulae of Compounds

When elements combine to form ionic compounds, the molecule is made up of two main parts:

• Cation (Positive Part): This is the positively charged ion, also called the basic radical.
• Anion (Negative Part): This is the negatively charged ion, also called the acidic radical.

These oppositely charged ions are held together by a strong force of attraction called the ionic bond.

The name of an ionic compound is always in two parts:

• First Word: The name of the cation.
• Second Word: The name of the anion.

For example: Sodium chloride

"Sodium" is the cation.

"Chloride" is the anion.

To write the formula of an ionic compound, follow these simple steps:

1. Write the Symbols: Start by writing the symbol of the cation first (on the left), followed by the symbol of the anion (on the right).

For example, sodium (Na⁺) and chloride (Cl⁻).

2. Balance the Charges: The total positive charge from the cations must balance the total negative charge from the anions. Use the cross-multiplication method to balance them:

• Write the charge of one ion as the subscript for the other ion.
• Ignore the positive (+) or negative (-) sign of the charges.

For example, sodium (Na⁺) and chloride (Cl⁻) both have charges of 1. The formula becomes NaCl (1:1 ratio).

3. Write Subscripts: If more than one ion is needed to balance the charges:

• Use a subscript to indicate the number of ions.
• For polyatomic ions (ions made up of multiple atoms), use brackets if there is more than one of them.

For example, calcium (Ca²⁺) and nitrate (NO₃⁻):

• Calcium has a charge of 2, and nitrate has a charge of 1.
• Cross the charges: Ca(NO₃)₂ (1 calcium ion for every 2 nitrate ions).

Step 1: To write the symbols of the radicals. (Basic radical on the left.)

Na                SO₄​

Step 2: Write the valency below the respective radical.

Na                 SO₄

1                     2

Step 3: To cross-multiply, as shown by the arrows, the number of the radicals.

Step 4: Write down the chemical formula of the compound.

Na₂SO₄​

In order to write the chemical formulae of compounds, it is necessary to know the valency of the various radicals. The names and symbols, along with the charge of common radicals, are given in the chart below.