Many times, after finding the empirical or molecular formula by calculation or while writing the condensed structural formula, a beginner in chemistry needs clarification as to in what order the elements should be placed.
Hills Nomenclature, also known as the Hills System or Hills notation, is a guideline for writing any chemical compound's empirical or molecular formula.
Austin M. Hills developed it in the 1930s. It is not as comprehensive or systematic as the IUPAC system of nomenclature. It is generally reserved for writing molecular formulas for Chemical databases and indexes.
The Empirical and Molecular Formula Writing Rules
Hill nomenclature orders Carbon first, followed by Hydrogen atoms next, and all elements are listed in the alphabetical order of their chemical symbols.
All elements, including Hydrogen, are listed alphabetically in the absence of Carbon.
The number of atoms of each element in the compound in the subscript follows the element symbol.
Examples of such a method of notation are – BrI, ClH, CCl4, C2H5Br, BrClH2Si, etc.
If a compound’s structural formula is H3C(CH2)8CH2NH2, then its molecular formula will be C10H23N. Or when it is [(CH3)3Si2]2NNa, its molecular formula will be written according to Hill notation as C6H18NNaSi4.
In empirical formulas of inorganic compounds, electropositive elements are listed first. For example, while writing the empirical formula with elements Ba, S, and O4, the electropositive element Ba is written first, followed by S and O as BaSO4.
When necessary, the charges of ions are placed at the superscript next to the element symbol while writing the molecular or empirical formula.
For example, a compound's empirical formula is (Fe2+) (NH4+)2 (SO42-)2 (H2O)6. Here, electropositive ions are also written before electronegative ions.
In molecules containing bonds other than the covalent, groups of atoms such as complex anions or waters of hydration are listed together in the end.
For example, if the empirical formula of a compound is Na2SH20O14, and the molecule contains 10 water molecules as water of crystallization, that is,
Na2SH20O14 – 10H2O = Na2SO4.
The empirical formula is written as Na2SO4. 10H2O.
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