Writing A Chemical Formula Given A Chemical Structure

Formatting Chemical Formulas

This section gives tips for formatting chemical formulas for effective searching in STN Easy.

Elements in a chemical formula and their counts are arranged in a standardized order called Hill System Order.

Hill System Order rules are given.

Element counts follow the element symbol. No count is used for element counts of 1.

For chemical formulas containing	Then elements are arranged as follows:	Example
Carbon	Carbon Hydrogen Other elements in alphabetical order	C7H4Br2O2
No carbon	Elements in alphabetical order	H2O4S

Formulas for some chemical substances have special representations:

Chemical substance description	Example
Salts of acids	Sodium acetate
Salts of amines	Trimethylammonium chloride
Homopolymers	Polystyrene
Copolymers	Vinyl acetate-vinyl chloride-vinyl fluoride copolymer

Follow these steps to determine the formula for salts of acids.

Step	Do the following:	Example
1	Write the chemical formula as it is known.	CH3COONa
2	Remove the metal from the formula to create two fragments.	CH3COO Na
3	Add H to the fragment where the metal was removed to create the formula for the neutral acid.	CH3COOH Na
4	Arrange elements of each fragment in Hill System Order.	C2H4O2 Na
5	Combine the acid formula with the metal formula, separating each formula by a period. The carbon-containing formula comes first	C2H4O2.Na

Follow these steps to determine the formula for salts of amines.

Step	Do the following:	Example
1	Write the chemical formula as it is known.	(CH3)3-NH-Cl
2	Move the H from the N to the anion to create two fragments.	(CH3)3-N HCl
3	Arrange elements of each fragment in Hill System Order.	C3H9N ClH
4	Combine the amine formula with the acid formula, separating each formula by a period. The carbon-containing formula comes first.	C3H9N.ClH

Homopolymers are macromolecules formed by linking together one type of monomer.

Follow these steps to determine the formula for homopolymers.

Step	Do the following:	Example
1	Write the chemical formula for the monomer as it is known.	H2C=CH-C6H5
2	Arrange elements of the monomer in Hill System Order.	C8H8
3	Enclose the monomer formula in parentheses, followed by X.	(C8H8)X

Copolymers are macromolecules formed by linking together more than one type of monomer unit.

Follow these steps to determine the formula for copolymers.

Step	Do the following:	Example
1	Write the chemical formulas for each monomer as it is known.	CH3CO-O-CH=CH2 H2C=CH-Cl H2C=CH-F
2	Arrange elements of each fragment in Hill System Order.	C4H6O2 C2H3Cl C2H3F
3	Arrange formulas in order of decreasing carbon count, separating each formula by a period.	C4H6O2.C2H3Cl.C2H3F
4	Enclose the copolymer formula in parentheses, followed by X.	(C4H6O2.C2H3Cl.C2H3F)X