Jens-Uwe Kuhn, Santa Barbara City College
Jessica Martin, Northeastern State University
Creative Commons License: Attribution-Noncommercial-Share Alike 3.0 United States License.
Trends in the Periodic Table I: Atomic and Ionic Radius
Many of the trends in the periodic table are useful tools for predicting electronic properties and
chemical reactivities of various species, including transition metal complexes. The radius of
transition metal ions in inorganic coordination compounds is of great importance in many
biologically relevant coordination compounds.
Objectives:
(1) Predict the variation in atomic radius in the periodic table.
(2) Describe specific reasons for this variation across a period and within a group.
(3) Predict differences in atomic radius and ionic radius for the same element; predict
differences in ionic radius for various oxidation states of the same element.
A. Atomic Radius
Critical Thinking Questions:
1. Write out the complete electron configuration for lithium, sodium, and potassium.
2. Write out the complete electron configuration for beryllium, magnesium, and calcium.
3. How do the valence electron configurations change as you go from one element to the next
in question 1? How do the valence electron configurations change as you go from one
element to the next in question 2? How do the complete electron configurations differ from
one another for the elements in each of the above questions?
4. Predict the trend for the atomic radius for the elements in group I and group II of the
periodic table.
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5. Compare your predictions to the values given in model 1 below.
Model 1: Atomic radii in groups I and II of the periodic table:
Element
Lithium
Sodium
Potassium
Rubidium
Cesium
Beryllium
Magnesium
Calcium
Strontium
Barium
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Atomic Radius (pm)1
145
180
220
235
260
105
150
180
200
215
Slater, J.C., J. Chem. Phys., 1964, 41, 3199.
6. Write out the complete electron configuration of titanium, and the valence electron
configurations of vanadium, chromium, iron, cobalt, arsenic, and selenium. Keep in mind
that the electron configuration of some atoms differs from what may be expected.
7. How do the complete and/or valence electron configurations change as you go from one
element to the next in question 6?
8. Going from left to right in period 4 of the periodic table (or any other period), what do you
predict the general trend to be for the atomic radius of the elements? Consider the total
number of protons and electrons, as well as the valence shell that electrons are added into
when going from left to right across a period in your prediction.
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9. Compare your predictions to the values given in model 2 below.
Model 2: Atomic radii in period 4 of the periodic table:
Element
Potassium
Calcium
Scandium
Titanium
Vanadium
Chromium
Manganese
Iron
Cobalt
Nickel
Copper
Zinc
Gallium
Germanium
Arsenic
Selenium
Bromine
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Atomic Radius (pm)1
220
180
160
140
135
140
140
140
135
135
135
135
130
125
115
115
115
Slater, J.C., J. Chem. Phys., 1964, 41, 3199.
10. Are there any exceptions to the general trends for the atomic radius that can be seen in
these models? If so, suggest an explanation for these exceptions.
B. Ionic Radius
Critical Thinking Questions:
11. In general, will an anion have a larger or smaller ionic radius compared to the atomic radius
of the neutral element that it is derived from? Justify your answer.
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12. Is the same true for cations? Justify your answer.
Problems:
13. Considering your answer to the previous question, use the information given in model 2 to
predict the ionic radius for Fe2+, Fe3+, V4+, and V5+.
14. Compare your predicted ionic radii from the previous question to values given in the
literature.
15. Order the ions listed in question 13, as well as V2+ and V4+, in order of increasing size to
charge ratio.
16. Discuss at least two possible effects that a different size to charge ratio of transition metal
ions derived from the same element (e.g. Fe2+ and Fe3+) could have on properties,
reactivities, or chemical behavior of compounds containing inorganic transition metal ions.
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