Ions
and Ionic Compounds
This is an archival
version of the web assignment. It is after the due date,
so the assignment can no longer be submitted.
The purpose of this assignment
is to practice predicting ion charges and to practice writing
chemical formulas for ionic compounds.
Note: The goal
is to get you thinking about and working on this material.
You are expected to utilize your text and the information
within the assignment to figure out the answers to
the questions.
The electron configurations of
the noble gases are extremely stable. When an atom undergoes
a chemical change to become an ion, the atom will gain or lose
electrons until it is isoelectronic (has the same electron configuration)
with the nearest noble gas. Metals tend to lose electrons and
form positively charged ions called cations. Non-metals tend
to gain electrons and form negatively charged ions called anions.
For example, sodium metal, Na,
[Ne]3s1, loses one electron to form the Na+
ion which is isoelectronic with neon. (All the alkali metals
have one valence electron and form 1+ cations.) Chlorine, [Ne]3s23p5,
gains one electron to form the Cl– ion which
is isoelectronic with argon. (All of the halogens have 7 valence
electrons and form 1- anions.)
Ionic compounds are composed of
metal cation(s) and non-metal anion(s) and have a net charge
of zero. Sodium chloride, NaCl, is composed of Na+
and Cl– in a one to one ratio which gives the
formula zero charge. When writing the formula for an ionic compound,
the symbol for the cation is written first.
The following questions are designed
to help you become proficient at predicting ions and writing
formulas of ionic compounds. You may need to look at section
2.6 (pages 69-72) and section 3.2 (pages 88-95) in your textbook.
When writing the formula for ionic
compounds, the total positive charge from the cations plus the
total negative charge from the anions must add up to zero for
the formula to have a zero net charge. When writing formulas
for an alkali metal reacting with a halogen, the ratio of ions
is one to one. Remember our example of NaCl above? NaCl is composed
of Na+ and Cl– ions. So 1(1+) +
1(1-) = 0
What about the compound that forms
when calcium reacts with bromine? Calcium forms the 2+ ion,
Ca2+, because it needs to lose 2 electrons to be
isoelectronic with argon. Bromine forms the 1- ion, Br–.
The formula for calcium bromide is CaBr2. You need
two bromide ions (-1 each) to balance the 2+ charge of the calcium
ion.
This
is an archival version of the web assignment. It is after
the due date, so the assignment can no longer be submitted.
