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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.
Questions you may have include:
- What is the Hill system?
- What is the order of elements in ionic compounds?
- What is the order of in oxides, acids and hydroxides?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.
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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Mini-quiz to check your understanding
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Order of Elements in a Chemical Formula