Structure 2.1 - The ionic model

• When metal atoms lose electrons, they form positive ions called cations.
• When non-metal atoms gain electrons, they form negative ions called anions.
• The charge carried by an ion depends on the number of electrons the atom needs to lose or gain to achieve a full outer shell.
• Transition elements can form more than one ion.
• Ionic compounds are formed when electrons are transferred from one atom to another to form ions with complete outer shells of electrons.
• Cations and anions are attracted to each other by strong electrostatic forces, and can form a strong lattice.
• A binary ionic compound is composed of ions of two different elements: one being a metal and the other a non-metal.
• Binary ionic compounds are named after the cation first, followed by the anion. The anion adopts the suffix "-ide".
• In ions formed from more than one element, the charge is often spread (localised) over the whole ion.
• Ionic compounds are formed between metals on the left of the periodic table (low electronegativity) and non-metals on the right of the periodic table (high electronegativity).
• In ionic compounds the difference in electronegativity values should be greater than about 1.8 for bonding to occur.
• Ionic compounds exist as three-dimensional lattice structures, represented by empirical formulas. The ionic bond is the sum of all the electrostatic attractions (and repulsions) within the lattice.
• A large amount of energy is required to break the lattice, so ionic compounds tend to have high melting points.
• Many ionic compounds are soluble in water as the energy given out when the ions become hydrated (hydration energy) provides the energy to overcome the forces holding the ions together in the lattice (lattice enthalpy).
• The smaller the ions (ionic radius) and the greater their charge, the greater the lattice enthalpy is.
• Solid ionic compounds cannot conduct electricity as the ions are held in fixed positions. When molten, the ions are free to move and conduct electricity. They are chemically decomposed at the respective electrodes.