Formation of Ions
An ion is an atom or group of atoms with a negative or positive charge. Atoms that
gain electrons become negatively charged ion known as an anion, while atoms that
lose electrons become positively charged ion known as a cation. Not all atoms will
form ions, and some atoms can form an ion I more than one-way.
Atoms tend to gain or lose electrons in order to achieve a noble gas configuration.
Metals tend to have one, two or three valence electrons in their valence shell.
They tend to lose these electrons when they combine with other elements to form
positive ions.
Non-metals tend to have five, six and seven valence electrons in their valence
shell. They tend to gain electrons when they combine with metals to form negative
ions.
Formation of Compounds
A compound is a pure substance made up of two or more different atoms or
elements. There are two types of compounds: ionic compounds and molecular or
covalent compounds.
Ionic Compounds
An ionic compound is a compound that is formed from one or more positively
charged ions and one or more negatively charged ions held together by ionic bonds.
An ionic bond is a type of chemical bond that involves a metal and a nonmetal ion
(or polyatomic ions such as ammonium) through electrostatic attraction. In short, it
is a bond formed by the attraction between two oppositely charged ions.
Recall that metals tend to form positive ions and non-metals tend to form negative
ion, therefore ionic compounds are formed by the combination of one or metal ions
with one or more non-metal ions. Ionic compounds tend to form between metals
from groups 1, 2 & 3 and non-metals from groups 15, 16, and 17.
For example, this process occurs when sodium metal combines with chlorine to
form sodium chloride.
When non-metals gain electrons to form ions, the ending of the ions name is
changed to “ide”. Therefore the name of the compound above is sodium chloride.
Properties of Ionic Compounds
Ionic compounds have similar properties. At room temperature, most are hard,
brittle solids that can be crushed. When dissolved in water, the compound breaks
down liberating the ions. The resulting solution is an electrolyte, a solution that
conducts electricity.
Ion Symbol
To write the symbol for an ion, write the symbol of the element and show the ion
charge as a superscript. For example, the symbol of a calcium ion is Ca2+. When an
ion has a charge of 1+ or 1-, the symbol has no number in the superscript, such as
Na+ and FRules For Writing The Formula For An Ionic Compounds
1. Write the symbol of the element.
2. Write the ionic charge above each symbol and crisscross them
Example: write the formula for the following compounds:
a) sodium chloride
b) magnesium oxide
c) calcium nitride
Rules For Naming Ionic Compounds
Case I: The Metal Has Only Possible Ionic Charge
1. The name of the metal is written first.
2. The name of the non-metal is written second and it’s ending is changed to
“ide”
Example: Name the following compounds
a) KCl
c) MgBr2
d) Al2S3
d) LiI
Case II: The Metal Has More Than One Possible Ionic Charge
Atoms of many metals can form more than one type of ion. For example, an atom of
copper can form one of two different ions, one with a charge of 1+ or one with a
charge of 2 +.
A multivalent element is an element that can form an ion in more than one
way. Many metal elements are multivalent. The name of a multivalent element
always contains a Roman numeral that indicates the ion charge.
For example, Cu+ is named copper (I), which is read as “copper one”.
Similarly Cu2+ is named copper (II), which is read “copper two”. Only multivalent
metals have Roman numerals in their names.
Rules For Writing The Formula For Multivalent Ionic Compounds
1. Write the symbol of the element.
2. Write the ionic charge above each symbol and crisscross them
3. Remember that the charge of the multivalent metal is indicated by the
Roman numeral.
a) copper (I) bromide
b) copper (II) oxide
c) iron (III) sulphide
d) tin (IV) nitride
Rules For Naming Multivalent Ionic Compounds
1. The name of the metal is written first. The name of the non-metal is written
second with the ending changed to “ide”.
2. A roman numeral is placed in brackets between the two parts of the name
to indicate the ionic charge of the metal.
a) CuI
b) CuS
c) PbO2
d) FeN