CHEM 109
Introduction to Chemistry
Fall 2016
Problem Set 21
Reading for Last Tuesday's Lecture
Introduction to Chemical Principles Chapter 7.1 – 7.9
Reading for Next Thursday's Lecture
Precipitation Reactions Handout
Practice Problems
Chapter 7: 23, 33, 37, 39, 44, 45, 49, 55, 59.
Additional Problems
1.
Write electron configurations for the following ions:
a) Ib) Sr2+
c) N3-
2.
For each of the following ionic compounds, identify the charge on the cation:
BeCl2
K2O
Al2S3
ZnO
Zn3P2
CoCl2
Co2O3
PbS
As2O3
Tl2O
3.
Consider the compound Cesium Fluoride (CsF). This ionic compound is composed of the
Cs+ and F- ions.
a) Write condensed electron configurations for each ion:
Cs+ =
F- =
b) A general Rule of Thumb is that anions are larger atoms than cations. This is because
atoms tend to shrink when they lose electrons and "puff-up" when they gain electrons.
And, we see this shrinkage for Cs+ and "puffing" for F- here:
Atom
Cs
F
Radius
265 pm
72 pm
Ion
Cs+
F-
Radius
167 pm
133 pm
Explain, in terms of nuclear charge, why the atom would tend to shrink when it forms a
cation and puff-up when it forms an anion.
c) However, note the Cs+ cation is larger than the F- anion. Explain briefly in terms of
electron orbitals.
4.
The radius of an ion is determined from measurements of the spacing between ions in the
crystal lattice of ionic compounds formed from the ion. (These measurements are obtained
from X-ray scattering data.) The element Astatine is formed only a few atoms at a time,
during nuclear bombardment experiments. Further, these atoms are only short lived,
undergoing rapid nuclear decay to lighter atoms. This makes forming compounds of
Astatine very difficult. We can, however, predict the radius of the At- ion from trends in
the ionic radii of the other Halides. The radii of these halide ions is as follows:
F-: r = 114.5pm
Cl-: r = 167pm
Br-: r = 182pm
I-: r = 206pm
Plot, using Excel, this radius data against the atomic number. Use this plot to extrapolate
the desired ionic radius of At3.
Write the formula of the expected ionic compound which forms when the following two
elements combine:
Sodium & Oxygen
Calcium & Oxygen
Aluminum & Chlorine
Barium & Sulfur
Cesium & Phosphorus
Aluminum & Selenium