This document provides information on writing chemical formulas for ionic compounds. It discusses how ions are formed by elements gaining or losing electrons to achieve stability like the nearest noble gas. The charge on simple ions relates to the number of electrons gained or lost. Polyatomic ions contain two or more combined atoms and usually have a negative charge except for ammonium. To write formulas for ionic compounds, the numbers of positive and negative ions must balance to give an electrically neutral compound. The names of ionic compounds consist of the cation name followed by the anion name changed to end in "-ide".

1. Grade 9 Chemistry
Topic: Ions and Writing Chemical Formulae for Ionic Compounds
1. Simple ions of elements: these ions are formed when the atoms of an element gain or
lose electrons to become stable like the noble gas nearest to it.
N.B: Read notes on ions for more information.
a) Charge on simple ions
The charge of simple ions is related to the number of electrons an atom will have to gain or lose
to achieve stability. Sodium (11Na-EC =2, 8, 1) is in group 1 of the periodic table, so it has one
electron in its outer shell. When sodium forms an ion, it loses this one electron. Hence, the
charge on the sodium ion will be +1(See Table below).
Element E.C Type of element Electron(s) Ion charge Example
lost/gained
1. 11Na 2, 8, 1 Metal 1lost +1 Na+
2. 12Mg 2, 8, 2 Metal 2 lost +2 Mg2+
3. 13Al 2, 8, 3 Metal 3 lost +3 Al3+
4. 8O 2, 6 Non-metal 2 gained -2 O2-
5. 17Cl 2, 8, 7 Non-metal 1 gained -1 Cl-
b) Polyatomic Ions/Radicals (Read pg 68 Chemistry Resource Manual)
As well as simple ions of elements, there are some common ions which contain two or more
atoms combined together. These ions are known as polyatomic ions.
Polyatomic ions usually have a negative charge EXCEPT the ammonium ion
Here are the names and formulae of some polyatomic ions:
Name of ion Ion formula Elements combined
Sulfate SO4
2- Sulfur, oxygen
Carbonate CO3
2- Carbon, oxygen
Nitrate NO3
- Nitrogen, oxygen
Hydroxide OH- Oxygen, hydrogen
Ammonium NH4
+ Nitrogen, hydrogen

2. Chemical formulae of compounds
A substance formed by combining two or more elements is called a compound. The chemical
formula for a compound shows:
 the symbols for each element in the compound
 the number of atoms of each element in a unit of the compound
 If there is more than one of a polyatomic ion in the formula, put the ion in
parentheses, and place the subscript after the parentheses; e.g., Ca(OH)2, Ba3(PO4)2,
etc.
Writing Chemical Formula Of Ionic Compounds
Ionic Compound: a compound that contains a positive ion and a negative ion.
The formula of an ionic compound can be predicted using the formulae of its ions. The numbers
of ions in a formula must give an equal number of positive and negative charges. When the
oppositely charged ions are combined together the compound should be electrically
neutral.
Naming Ionic Compounds
 Name the cation first then the anion.
 The name of the cation of a metal is the same as the name of the metal (with the word ion
added if the cation is by itself). For example, Na + is the sodium ion, Ca 2+ is the calcium
ion, and Al 3+ is the aluminum ion.
 The name of a compound consisting of TWO different elements (called a binary
compound) will end with the suffix –ide. For example, NaCl is called sodium chloride,
MgS is called magnesium sulphide. In these compounds the name of the non-metal is
CHANGED to end with the suffix –ide (See Table 1 Below).
Table 1
Non-metal Formula Ion Name in Binary Compound
Hydrogen(H) H- hydride
Flourine(F) F- flouride
Chlorine(Cl) Cl- chloride
Bromine(Br) Br- bromide
Iodine(I) I- iodide
Oxygen(O) O2- oxide
Nitrogen(N) N3- nitride
Sulphur(S) S2- sulphide
Phosphorous(P) P3- phosphide

3. Table 2
Name of compound Ions Present Formula
Sodium sulphide Na+, S2- Na2S
Magnesium oxide Mg2+, O 2- MgO
Ammonium chloride NH4
+, Cl- NH4Cl-
Lithium nitride Li+, N3- Li3 N
Potassium Bromide K+, Br- KBr