Chemistry 121 (Amar) -- Spring 2011

Naming Rules for Ions, Acids,
Ionic Crystals & Inorganic Compounds
 (Copyright 2011, Francois G. Amar, All rights reserved)
      In order to have some way of communicating about chemistry and chemicals it 
is important that we have a system of
nomenclature or naming. We will consider the
following categories at this point:
  1. Atomic ions -- Cations & Anions
  2. Molecular ions -- Cations & Anions including Oxyanions
  3. Acids -- derived from anion names
  4. Ionic compounds &emdash; metal + nonmetal
  5. Inorganic molecular compounds

I. Atomic ions

Rule 1: The non-metals tend to gain electrons to form negative ions or anions
Rule 2: The metals tend to lose electrons to form positive ions or cations
Rule 3: We can use the atom’s position relative to the noble gases (group 8A) to predict its most common charge state if the atom is in row 1, 2, or 3 or is a main group element (groups 1A,2A, 3A, …8A).   Rule 4: The transition metals form cations whose charges are less easy to predict. Some
transition metals form common ions of different charge states


A. Cations of the main group which follow rules 2 and 3 include

1. The alkaline family (Li, Na, K, …) of group 1A which form singly charged positive ions since by losing a single electron they achieve the electron configuration of a noble gas (in the preceding row)

K(s) --> K+ + e- (K+ has the same electronic configuration as Ar)

2. The alkaline earth family (Be, Mg, Ca, …) of group 2A which form doubly charged positive ions since by losing two electrons they achieve the electron configuration of the noble gas of the preceding row.

Mg(s) --> Mg2+ + 2e- (compare Mg2+ to Ne)

3. Al3+ is the only really common ion of its family (group 3A)


B. Most of the common transition metal cations (rule 4) are formed in the 2+ state.

Here is a partial list:

Co2+, Cu2+ , Fe2+, Mn2+, Hg2+, Hg22+, Ni2+, Pb2+, Sn2+

Some special cases:

NAMING: The atomic cations are named just like the neutral element followed by the word ion: For example: K+ is the potassium ion

MULTIVALENT IONS: To distinguish the atomic ions Fe3+ from Fe2+ we name them iron(III) ion and iron(II) ion, respectively, or Fe(III) ion and Fe(II). Similarly for copper(II) ion and copper(I) ion. In an older method (see page 59 of Burdge), the ending -ic is used for the higher of two possible charge states and -ous for the lower [this method is widely used by practicing (read older) chemists but is not encouraged because it can’t handle an atom with more than 2 common charge states]


C. Anions - These are the simplest negative ions, consisting of a single atom of a given element in its most common negative charge state. We only consider the non-metals in this naming scheme. Use the root of the element’s name and add -ide to get the name of the most common anion.

Examples are:

(1st row): H- is the hydride ion

(2nd row): N3- is the nitride ion ; O2- is the oxide ion ; F- is the fluoride ion

(3rd row): P3- is the phosphide ion; S2- is the sulfide ion; Cl- is the chloride ion



II. Molecular ions

These are generally made up of a collection of nonmetal atoms bonded together but that are stable with a net charge.


A.Cations -- The only significant positive molecular ion is NH4+, the ammonium ion.


B. Anions

1. A few polyatomic anions have names that end in -ide like the atomic anions:

OH- is hydroxide, CN- is cyanide, O2 2- is peroxide

2. Oxyanions -- These are anions consisting of a non-metal atom such as carbon acting as a center to which one or more oxygen atoms are bonded. A very common example is the carbonate ion, CO32- which looks like this:

The table below shows a number of elements acting as a central atom with in some cases up to four different common oxyanions (the chlorine series). One of these ions (which we shall consider to be the most common) is named with the central atom’s root and the ending -ate. The oxyanion in the series with one less oxygen has the ending -ite. If the series continues down with one less oxygen than this we use the prefix hypo- (short for less than). If the series has an oxyanion with one more oxygen than the -ate ion we add the prefix per- (short for hyper or more than).




Central atom














per-       -ate 










































hypo-     -ite  













Notice that in each column, the charge remains the same even as the number of oxygens bonded to the central atom changes.

Also note that the six ions I've asked you to memorize can also help with other members of the same family: the bromate ion is analogous to the chlorate ion for example.


III. Acids

An acid gives off or yields hydrogen ion, H+ when it is dissolved in water:

HCl (aq) -- > H+ (aq) + Cl- (aq)

The naming of the acids depends on the name of the corresponding anion.

Anions named with -ide:

chloride ion corresponds to hydrochloric acid

cyanide ion corresponds to hydrocyanic acid

Anions named with -ate and so forth (oxyanions)

Chlorate ion corresponds to chloric acid

Chlorite ion corresponds to chlorous acid

Now, what about perchlorate ion? perchloric acid

hypochlorite ion? hypochlorous acid


IV. Ionic compounds

Simply start with the name of the metal and add the name of the anion of the non-metal

The most common example is

NaCl (s) or sodium chloride.

How about BaSO4? barium sulfate

Or FeO? iron(II) oxide

See Table 2.9 on page 60 in Burdge for some more examples.


V. Binary molecular compounds

These are compounds made up of two non-metals. We write the element belonging to the lower group number first, then the other element with the ending -ide and we use prefixes like mono-, di-, tri- to say how many of each atom are in the compound. A more complete table of these prefixes is given in Table 2.2 of Burdge on page 52.

Examples (more on page 52):

NO is nitrogen oxide

N2O is dinitrogen oxide

P2O5 is diphosphorus pentoxide



Back to Week 2

Back to Handouts Page

Back to Course Page