Hill notation

Hill Notation

Hill notation is a system used in chemistry to represent the chemical formula of a compound. It is particularly useful for organizing and indexing chemical compounds in databases and literature. The system was developed by Edwin A. Hill in 1900 and is widely used in chemical informatics.

1. 1. Overview

Hill notation is a standardized way of writing chemical formulas. The primary rule of Hill notation is that carbon atoms are listed first, followed by hydrogen atoms, and then all other elements in alphabetical order. This system is especially useful for organic compounds, which typically contain carbon and hydrogen.

For example, the Hill notation for ethanol is C₂H₆O. Here, carbon (C) is listed first, followed by hydrogen (H), and then oxygen (O).

1. 1. Rules of Hill Notation

1. Carbon and Hydrogen First: If the compound contains carbon, list carbon atoms first, followed by hydrogen atoms. For example, acetic acid is written as C₂H₄O₂.

2. Alphabetical Order: After carbon and hydrogen, list all other elements in alphabetical order. For instance, for a compound containing carbon, hydrogen, nitrogen, and oxygen, such as nitromethane, the formula is written as CH₃NO₂.

3. No Carbon: If the compound does not contain carbon, list all elements in alphabetical order. For example, water is written as H₂O, and ammonia is written as H₃N.

4. Subscripts: Use subscripts to indicate the number of atoms of each element present in the compound. For example, glucose is written as C₆H₁₂O₆.

1. 1. Applications

Hill notation is used extensively in chemical databases, such as the Chemical Abstracts Service (CAS) and PubChem, to index and search for chemical compounds. It provides a consistent and systematic way to represent chemical formulas, facilitating easier retrieval and comparison of chemical data.

1. 1. Limitations

While Hill notation is useful for indexing and searching, it does not convey structural information about the compound. For example, isomers such as ethanol (C₂H₆O) and dimethyl ether (C₂H₆O) have the same Hill notation but different structures and properties.

1. 1. Examples

- Methane: CH₄ - Ethanol: C₂H₆O - Glucose: C₆H₁₂O₆ - Sulfuric Acid: H₂O₄S

Also see

- Chemical formula - Molecular formula - Empirical formula - Structural formula - Chemical nomenclature

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