Key Points:
* Ionic compounds are formed by the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions).
* Formula unit is the equivalent of a molecule for ionic compounds.
* It represents the ratio of ions, not the actual number of ions in a crystal lattice.
* For example, the formula unit of sodium chloride (NaCl) indicates a 1:1 ratio of sodium ions (Na+) and chloride ions (Cl-).
* Unlike molecules, formula units do not exist as discrete entities but rather as an extended three-dimensional lattice.
Differences from Molecules:
* Molecules are formed by the sharing of electrons between atoms, while formula units are formed by the electrostatic attraction between ions.
* Molecules have a definite shape and size, while formula units are part of a continuous lattice.
* Molecules can be composed of different elements or the same element, while formula units always involve at least two different elements.
Examples:
* NaCl (sodium chloride): 1 Na+ ion : 1 Cl- ion
* CaCl2 (calcium chloride): 1 Ca2+ ion : 2 Cl- ions
* K2SO4 (potassium sulfate): 2 K+ ions : 1 SO42- ion
In summary, the formula unit is a crucial concept in understanding the composition and properties of ionic compounds. It provides a simple and concise representation of the ion ratio within the compound, reflecting the fundamental building block of the ionic lattice.
