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In chemistry, a formula unit is the smallest unit of a non-molecular substance, such as an ionic compound, covalent network solid, or metal. [1] [2] It can also refer to the chemical formula for that unit. Those structures do not consist of discrete molecules, and so for them, the term formula unit is used.
For most ionic compounds dissolved in water, the van 't Hoff factor is equal to the number of discrete ions in a formula unit of the substance. This is true for ideal solutions only, as occasionally ion pairing occurs in solution. At a given instant a small percentage of the ions are paired and count as a single particle.
The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds , when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as the dissociation constant or the solubility of different salts .
The circumstances under which a compound will have ionic or covalent character can typically be understood using Fajans' rules, which use only charges and the sizes of each ion. According to these rules, compounds with the most ionic character will have large positive ions with a low charge, bonded to a small negative ion with a high charge. [25]
The molecular mass (for molecular compounds) and formula mass (for non-molecular compounds, such as ionic salts) are commonly used as synonyms of molar mass, differing only in units (daltons vs g/mol); however, the most authoritative sources define it differently. The difference is that molecular mass is the mass of one specific particle or ...
For example, the compound dichlorine hexoxide has an empirical formula ClO 3, and molecular formula Cl 2 O 6, but in liquid or solid forms, this compound is more correctly shown by an ionic condensed formula [ClO 2] + [ClO 4] −, which illustrates that this compound consists of [ClO 2] + ions and [ClO 4] − ions. In such cases, the condensed ...
For ionic compounds made of molecular cations and/or anions, there may also be ion-dipole and dipole-dipole interactions if either molecule has a molecular dipole moment. The theoretical treatments described below are focused on compounds made of atomic cations and anions, and neglect contributions to the internal energy of the lattice from ...
This extends the scope of the ionic model well beyond compounds in which the bonding would normally be considered as "ionic". For example, methane, CH 4, obeys the conditions for the ionic model with carbon as the cation and hydrogen as the anion (or vice versa, since carbon and hydrogen have the same electronegativity).