ChemViz Lab: Atomic and Ionic Radii
Created by: Rebecca A. Kruse
Pre-lab Assignment:
Read “Making Single Images with Waltz.”
Pre-lab Discussion:
You have studied the structure of the atom, including the charge cloud or electron density model. Knowing the
arrangement of electrons around the nucleus is important for understanding the electron density model, and you
will learn more about electron arrangements, including electron orbital shapes and electron configurations in
Chapter 13. For this lab, we will jump ahead to Chapter 14 to study the result of electron arrangements, the
periodic table.
The periodic table is organized into groups (vertical columns) and periods (horizontal rows) according to their
electron arrangements. When we look at the periodic table the way it is arranged today, there are trends that
appear in the atomic properties that help to explain the behavior of matter. Two properties that are particularly
pertinent to the chemical behavior of matter and closely related to the number of electrons present in the atom are
atomic radius and ionic radius.
The atomic radius is the distance from the nucleus to the outermost energy level for the atom. Similarly, the
ionic radius is the distance from the nucleus to the outermost energy level for the ion, where an ion is simply an
atom (or group of atoms) with an excess positive or negative charge. Anions are atoms that gain one or more
electrons, giving excess of negative charge. Cations are atoms that lose one or more electrons, giving excess of
positive charge. Isoelectronic species are a series of atoms and/or ions that have the same electron
configuration, or simply, the same number of electrons but different number of protons.
In this lab you will be looking at calculated electron density models and making visual size comparisons. You are
also encouraged to check your answers by finding the numerical value for each atomic and ionic radius that you
model in class. The units to measure atomic and ionic radii are picometers (pm), nanometers (nm), or
angstroms (Å). The conversion factors are given below:
1pm = 1 x 10-12 m
100pm = 1 Å = 1 x 10-10 m
1000pm = 1 nm = 1 x 10-9 m
Purpose:
In this exercise you will use electron density images generated in ChemViz to investigate: 1) the relationship
between the radii of atoms and their ions, 2) the radii of isoelectronic species, and 3) general periodic trends in
atomic radii.
Lab Procedure:
Follow the directions provided in “Making Single Images with Waltz” to do the following:
1. Logon to the Waltz Interface at the ChemViz web page at http://chemviz.ncsa.uiuc.edu
Atomic Radii vs. Ionic Radii
2. Create an electron density image of an atom of Na.
3. Open a new browser window and create an electron density image of an ion of Na1+
4. Place the two images next to each other (keep track of which is the atom and which is the ion) and compare
the two sizes to see the trend between the atom and its cation.
5. Repeat steps 2-4 for an atom of O and the O2- ion to see the trend between the atom and its anion.
Isoelectronic Species
6. Create an electron density image of an atom of Ne
7. Compare the image of a Ne atom with images of Na1+ and O2- ions you have already made to see the trend in
isoelectronic species.
Periodic Trends in Atomic Radii
8. Create electron density images for an atom of Li, an atom of F, and an atom of Rb.
9. Compare the images of the Li atom and the F atom to see the periodic trend within the period.
10. Compare the images of the Li atom and the Rb atom to see the periodic trend within the group.
You will be given handouts with all of the images so that you can answer the following questions.
Questions:
Use your data to complete the following statements. Then, explain each answer in 1-2 sentences using what you
know about nuclear and electronic forces.
1. In general, cations (positively charged ions) are __________ than their atoms.
2. In general, anions (negatively charged ions) are __________ than their atoms.
3. In general, within a series of isoelectronic species, as the atomic number increases the radius __________.
4. Within a period of elements (reading left to right), atomic radius generally __________ as atomic number
increases.
5. Within a group of elements (reading top to bottom), atomic radius generally __________ as atomic number
increases.
Suggestions for Further Work/Inquiry:
Clara Chemistry has just learned that computer chips are made from silicon (Si). Clara thinks she has an idea
that will make her rich. Why not replace some of the Si atoms in the crystal structure of silicon with Si2—ions,
increasing the electrical conductivity (more electrons present) of the computer chips and making the computer
faster. The idea is a good one, except for one important fact.
a. Based on what you learned in today’s activity, what do you think Clara does not know?
b. Design a simple ChemViz experiment that would test your answer.
c. Perform the experiment to confirm your answer.
d. Use the following websites to check your qualitative (visual) answer by finding the quantitative
(numerical) radii you examined in your experiment:
Web Elements http://www.webelements.com/
Chemicool http://www.Chemicool.com