Reference Sheet for Nomenclature (naming compounds from their formulas and writing
formulas from compound names)
Always write subscripts beneath the line on which you write the element and/or ion symbols.
Always write superscripts for ion charges in the upper right corner of the ion symbol.
The charge of an ion includes the type of charge and the size of the charge. Ion charges are not
written in the formulas of ionic compounds.
1. In ionic formulas, the positive ion is written first. The total number of positive charges must
equal the total number of negative charges. In naming ionic compounds, the name of the
positive ion is written first as the element name, then the correct name of the negative ion is
written. The word ion is not written in the name of an ionic compound.
Na+1 and Cl-1 combine to form NaCl; not ClNa
sodium ion and chloride ion combine to form sodium chloride
K+1 and S-2 combine to form K2S
potassium ion and sulfide ion combine to form potassium sulfide
2. The Stock system uses the name of the metallic element and a Roman numeral representing
the size of the charge written in parentheses after the name of the element. Most of the
transition metals and some of the metals in groups 13, 14, 15, and 16 of Periods 4, 5, 6, and 7
can form ions with more than one charge. The Roman numeral is used ONLY in the name
of the compound or ion; the Roman numeral is not used in the chemical formula for the
given compound or ion.
iron(II) ion (Fe+2)
iron(III) ion (Fe+3)
iron(II) chloride FeCl2
iron(III) chloride FeCl3
3. A polyatomic ion acts as a single particle; treat a polyatomic ion as you would a single
element ion. All subscripts that are part of the polyatomic ion formula must be kept. Do not
change the formula. If there is more than one polyatomic ion in a chemical formula and
you need to use a subscript after the polyatomic ion, the polyatomic ion is placed in
parentheses and the subscript is written in the lower right corner outside the parenthetical
expression.
sulfate ion SO4-2
calcium sulfate CaSO4
sodium sulfate Na2SO4
aluminum sulfate Al2(SO4)3
1
4. Most of the polyatomic ions we will be using are called oxoanions which consist of oxygen
covalently bonded to another nonmetallic element (called the characteristic or central
element). Two or more different oxoanions of the same central element can be distinguished
from each other by suffixes added to the stem name of the central element. The suffix -ate
denotes the anion with the greater number of oxygen atoms; the suffix -ite denotes the anion
with fewer oxygen atoms. The oxoanion with the greatest number of oxygen atoms is given
the prefix per- and the suffix -ate. The oxoanion with the least number of oxygen atoms is
given the prefix hypo- and the suffix -ite.
ClO-1
hypochlorite ion
ClO2-1 chlorite ion
ClO3-1
SO3-2
sulfite ion
SO4-2
sulfate ion
chlorate ion
ClO4-1 hyperchlorate ion
5. Molecular compounds are composed of two or more nonmetals, and contain NO ions.
The general rules for naming binary (having two elements) molecular compounds are:
1) If there is only one atom of the first element, no prefix is used;
2) if there are two or more atoms of the first element, the appropriate prefix is used;
3) for the second element, a prefix is always used to indicate the number of atoms
present; -ide ending are used for the second element.
The final vowel in a prefix is often dropped before a vowel in a stem word.
carbon monoxide
CO
dihydrogen monoxide
carbon dioxide
CO2
sulfur hexafluoride
H2 O
SF6
6. Acids form only in aqueous solution. The formula of an acid should always have (aq)
written after it to indicate that the compound is dissolved in water. If the compound formula
does not have (aq) written after the formula, the compound is not named an “acid.”
The name of an acid should include the word “acid.”
2
hydrogen chloride HCl
hydrogen nitrate
hydrochloric acid
nitric acid
HCl(aq)
HNO3
HNO3(aq)