Chemistry: Nomenclature & Writing Chemical Formulas Notes
Nomenclature is the process of naming chemical compounds based on their chemical formula. There are
different naming rules for each of the following three types of compounds:
 Organic Compounds – any compound that contains both hydrogen and carbon atoms.
 Inorganic Compounds – any compound that does not contain both hydrogen and carbon atoms.
o Ionic Compounds – contain a metal atom or the polyatomic ion, ammonium (NH4), plus one or more
non-metal atoms.
o Molecular Compounds – contain only non-metals.
Chemical formulas tell us the type and how many of each atom is present in a compound. Chemical symbols are
used to represent the type of atoms that make up the compound, and the subscripts after the symbols are used
to show how many of that type of atom are in the compound. Molecular formulas tell us the type of atoms and
how many of each type are in the molecule. The ionic formulas tell us the type and ratio of atoms in the lattice
structure.
Inorganic Ionic Compound Naming Rules
1. Identify the cation.
2. Name the cation by its element or polyatomic ion name.*
Examples: Na is sodium
NH4 is ammonium
3. Name the anion by using the first syllable of its element name followed by “ide” or by using its polyatomic
name.
Examples: Cl is chloride
SO4 is sulfate
4. Write or say the name of the cation first followed by the name of the anion.
Examples: NaCl is sodium chloride
(NH4)2SO4 is ammonium sulfate
* Exception – Metals with variable charges.
Most transition metals and the metals in groups 14 and 15 form more than one positive ion. For these metals,
write or say the name of the element and indicate the charge with roman numerals in parentheses.
Examples: Fe2+ is iron(II)
Fe3+ is iron(III) Cu+ is copper(I)
Cu2+ is copper(II)
Rules for Writing Ionic Fomulas
1. Determine the positive charge of the cation. This is based on the number of electrons the neutral atoms need
to lose in order to achieve a full octet of valence electrons.
2. Determine the negative charge of the anion. This is based on the number of electrons the neutral atoms need
to gain in order to achieve a full octet of valence electrons.
3. Write the chemical symbols of the ions with the cation first and the anion second.
4. Determine the ratio of each ion needed to have an overall charge of zero. Use subscripts to show the ratio of
each ion. Do not write the subscript if it is 1. The diagonal rule is a quick method for determining the ratio of
each ion:
Na1+ Cl1-
Na1Cl1
NaCl
Al3+ O2-
Al2O3
For polyatomic ions, the entire ion should be placed in parentheses and the subscript is placed after the
parentheses.
Al3+ SO4 2-
Al2(SO4)3
Inorganic Molecular Compound Naming Rules
1. Identify the least electronegative non-metal.
2. Name the least electronegative non-metal by its element name.
Examples: C is carbon
N is nitrogen
3. Name the most electronegative non-metal by using the first syllable of its element name followed by “ide”.
Examples: Br is bromide O is oxide
4. Use prefixes to indicate the number of atoms. Omit mono except for carbon monoxide.
1 – mono
2 – di
3 – tri
4 – tetra
5 – penta
6 – hexa
7 – hepta
8 – octa
9 – nona
10 - deca
When the vowels “o and o” or “o and a” appear together, eliminate the first vowel, i.e. carbon monoxide.
5. Write or say the name of the least electronegative element followed by the name of the most electronegative
element.
Examples: CBr4 is carbon tetrabromide
N2O is dinitrogen oxide
Rules for Writing Molecular Formulas
1. Write the chemical symbol of the least electronegative atom first, then write the chemical symbol for the
most electronegative atom.
2. Add subscripts after each chemical symbol to show how many atoms of each element are present in the
molecule. Do not write the subscript if it is 1.
Organic Compounds
All organic compounds are molecular compounds. There are many more organic compounds than inorganic
compounds, and their structures can be much more complex. Consequently, the naming rules for organic
compounds are much more complex than they are for inorganic compounds. We are only going to learn the
naming rules for the simplest group of organic compounds, alkanes.
Alkanes are organic compounds that contain only carbon
and hydrogen, and all the carbons are bound together by
single bonds as shown on the right. The rules for naming
alkanes also form the foundation for the naming rules of
the more complex groups of organic compounds.
Naming Rules for Alkanes
1. Count the number of carbons in the compound.
2. Select the appropriate prefix based on the number of carbons
1 – meth
2 – eth
3 – prop
6 - hex
7 – hept
8 - oct
4 – but
9 - non
5 – pent
10 - dec
3. Select the appropriate prefix based on the number of carbons and add it before the syllable “ane”.
Examples: CH4 is methane
C4H10 is butane
C8H18 is octane
Rules for Writing Molecular Formulas for Alkanes
1. Determine the number of carbon and hydrogen atoms. If there are n carbon atoms, then the number of
hydrogen atoms will be 2n + 2.
2. Write the symbol and subscript for carbon first, then write the symbol and subscript for hydrogen.
methane is CH4
ethane is C2H6
hexane is C6H14
nonane is C9H20