The ionic formula shows the simplest whole-number ratio of positive to negative ions that gives a neutral compound, for example NaCl or CaCl2.
The ionic formula is the formula of an ionic compound, showing the simplest whole-number ratio of positive to negative ions that produces a neutral compound. Malay: formula ion · Chinese: 离子式.
An ionic compound is electrically neutral overall, so the total positive charge from the cations must exactly balance the total negative charge from the anions. To write the formula, you take the charge on each ion and combine the ions in whatever ratio makes the charges cancel. A quick method is to swap each ion’s charge to become the subscript of the other ion, then simplify.
Example. Sodium ion is Na+ and chloride ion is Cl−, so they combine one-to-one as NaCl. Calcium ion is Ca2+ and chloride ion is Cl−, so you need two chloride ions for each calcium ion, giving CaCl2. In the same way, aluminium oxide comes out as Al2O3 once the 3+ and 2- charges are balanced. When a polyatomic ion is repeated, it is enclosed in brackets, so calcium nitrate, built from Ca2+ and the nitrate ion NO3−, is written Ca(NO3)2.
Confusion to avoid. Do not confuse an ionic formula with a molecular formula. An ionic formula is a ratio of ions in a lattice, not the number of atoms in a molecule, so it is always the simplest ratio, like an empirical formula. Take care with the charges as well, because using the wrong charge gives the wrong ratio and a formula that is not neutral. Remember that the subscript belongs to the whole ion, so the bracket around a polyatomic ion must not be dropped.
In this chapter the ionic formula lets you write correct formulae for the salts that appear in relative formula mass and stoichiometry calculations, so it connects the idea of the ion to the quantitative work of SPM Chemistry.
Want a teacher to make this click?
We teach SPM Chemistry one to one, so your child understands it and scores it.
from RM50/hr · One-hour paid trial · Same-day reply