Predict main-group ions and write charge-balanced empirical formulae.
Method and worked examples
A main-group atom can form an ion by losing or gaining electrons. Sodium loses one electron to form Na+; chlorine gains one to form Cl−. Ionic bonding is the electrostatic attraction between oppositely charged ions throughout a lattice. Electron transfer describes how ions may form, rather than the force that holds the lattice together. A neutral formula contains equal total positive and negative charge.
Write the smallest whole-number ion ratio and use brackets when more than one polyatomic ion is needed: Ca2+ and NO3− give Ca(NO3)₂. Ionic formulae describe ratios, not separate molecules. Count electrons before writing an ion configuration. For the first 38 elements, transition-metal cations lose outer 4s electrons before 3d electrons; for example Fe3+ is [Ar]3d5.
Example 1
Find the formula from Al3+ and O2−.
The least common charge magnitude is 6: two Al3+ and three O2− give Al2O3.
Example 2
Write the configuration of Ca2+, given neutral calcium is [Ar]4s2.
Remove two outer 4s electrons: Ca2+ is [Ar], with 18 electrons.
Watch out:It contains a continuous lattice; NaCl gives the simplest ion ratio.
Ions and ionic formulae | Year 11 Chemistry | Ascent Arcade