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Explanation of Order of Elements in a Chemical Formula by Ron Kurtus - Succeed in Chemistry. Key words: Hill system, compound, element, carbon, hydrogen, ionic, oxide, acid, hydroxide, physics, physical science, School for Champions. Copyright © Restrictions

Order of Elements in a Chemical Formula

by Ron Kurtus (4 December 2008)

In 1900, Edwin A. Hill devised a system of writing a chemical formula that is used for a large number of compounds today. The Hill system states carbon atoms are listed first, hydrogen atoms next and then the number of all other elements in alphabetical order. There are numerous exceptions to this system, such as the order of elements in ionic compounds, as well as the order in oxides, acids and hydroxides.

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Hill system

The Hill system states the carbon atoms are listed first, hydrogen atoms next and all others are then listed in alphabetical order. The reason for putting C and H up front is because there are so many hydrocarbon molecules

If the formula contains no carbon, then all the elements, including hydrogen, are listed alphabetically.

Ionic compounds

Most ionic compounds are exceptions to the Hill system.

Ionic compounds are those that consist of ions held together in a lattice or crystalline structure by an ionic bond. Many such a compound will dissolve in water, breaking into individual ions.

A good example is table salt or sodium chloride (NaCl). When it is in its solid form, salt appears as a crystalline substance. But when it is dissolved in water, it breaks into Na+ and Cl ions.

The order of the elements in an ionic compound is that the positive (+) ion is listed first and the negative (−) ion is listed second, no matter what their alphabetical order is.

Oxides, acids and hydroxides

Oxides, acids and hydroxides are exceptions to the Hill system.

Most oxides will end in multiples of O, no matter what the alphabetical order. A good example is silicon dioxide:

SO2

The formula for most acids begins with the hydrogen atom. A good example of the exception of the Hill system of placing C first is with carbonic acid:

H2CO3

Another exception to the Hill system is that most hydroxides end in (OH). A typical example is sodium hydroxide:

NaOH

Summary

The Hill system states carbon atoms are listed first, hydrogen atoms next and then the number of all other elements in alphabetical order. There are numerous exceptions to this system. The order of elements in ionic compounds is that the positive (+) ion is listed first and the negative (−) ion is listed second. Oxides end in oxygen, acids start with hydrogen and hydroxides end in OH.


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