Periodic Trends II: Electron Affinity, Atomic Radius & Ionic Radius
– Answer Key
Background
In this investigation you will examine the electron affinity of select atoms from the periodic
table. You will have the opportunity to analyze the effect that electron affinity has on other
periodic trends, including atomic radius and ionic radius.
1. What subatomic particles are located in the nucleus of an atom? Does the nucleus of the
atom have a charge? Is so, what is it?
Protons and neutrons occupy the nucleus. The nucleus has a positive charge.
2. What factors determine the size (radius) of an atom? Explain.
Nuclear Charge: The greater the positive pull from the nucleus, the smaller the atom will be, as it will
pull the valence electrons closer to the center of the atom. This is especially evident when comparing
atoms that are in the same period.
Shielding effect: The electrons in an atom, that are not valence electrons reduce the pull of the nucleus
on the valence electrons, essentially shielding them from the positive force. Atoms that experience a lot
of shielding will be larger in size.
3. Draw a Bohr model for Magnesium (Mg) and Chlorine (Cl). Which atom should be larger?
Magnesium Bohr model – Three rings, 2-8-2 organization of electrons.
Chlorine Bohr Model – Three rings, 2-8-7 organization of electrons.
Magnesium will be the larger atom due to a weaker nuclear charge than chlorine.
4. Is it more difficult to remove a valence electron from a Magnesium (Mg) atom or a
Chlorine (Cl) atom? Explain.
It will be more difficult to remove a valence electron from Chlorine than from Magnesium. Chlorine has a
larger nuclear charge (due to a high number of protons), so the atom’s nucleus has a stronger pull on its
valence electrons, making it more difficult to remove them from the atom.
5. What does a positively charged ion indicate in terms of its subatomic particles? Use the
Calcium ion Ca+2 as a specific example in your explanation.
A positively charged ion means that the atom has lost electron(s) compared to its neutral atom. Ca+2
means that the calcium atom lost 2 electrons, so it now has its original 12 positive protons, but only 10
negative electrons, so its overall electrical charge is +2.
6. What does a negatively charged ion indicate in terms of its subatomic particles? Use the
Fluoride ion F-1 as a specific example in your explanation.
A negatively charged ion means that the atom has gained electron(s) compared to its neutral atom. F-1
means that the fluorine atom gained one electron, so it now has its original 9 positive protons, and 10
negative electrons, so its overall electrical charge is -1.
*Check your answers before moving on to the next portion of the activity.
American Association of Chemistry Teachers
1
Procedure
Using your computer, tablet or mobile device, navigate to the website:
http://www.teachchemistry.org/electron-affinity. You should see the picture below on your
screen:
Investigation
1. Choose any element shown in green from group 7 on the periodic table by clicking the on
the element symbol. You should see details about the element that you chose appear at
the bottom of the screen. An example is shown below.
a. How many valence electrons are present in this atom?
7 valence electrons.
b. How can this atom become stable?
This atom needs a full outer shell/octet of electrons to be stable, so it will need to gain 1 electron.
c. Attempt to add an electron to the atom. Describe what happened:
The neutral atom will appear to pull on the additional electron as the user moves it near the
outer shell. The atom will accept the electron, and it will join the outer shell of electrons, forming
a -1 anion.
d. Comparing the neutral atom and its anion, what do you notice about the radius in
the side-by-side comparison? Do the data values support the visual representation?
The anion has a larger ionic radius than the atomic radius of the neutral atom. The data values
shown at the bottom of the screen support the visual.
e. Are there any other noticeable differences between the neutral atom and its anion
(other than size)? List them below:
Yes, the anion will show a -1 charge, an addition of 1 electron, and the name of the anion will be
slightly different that the neutral atom.
American Association of Chemistry Teachers
2
f. Explain why there is a difference in size between the neutral atom and its anion,
making specific reference to subatomic particles.
When the additional electron joins the outer electron shell, the positive nuclear charge of the
nucleus has to pull on an additional negatively charged electron. Since the positive charge in the
nucleus remains the same, the nucleus can no longer pull the valence electrons with the same
force as it did in the neutral atom, therefore the ionic radius of the anion is larger than the radius
of the neutral atom.
Reset the selected data using the reset symbol.
2. Choose any element shown in green from group 1 on the periodic table by clicking the on
the element symbol. You should see details about the element that you chose appear at
the bottom of the screen. An example is shown below:
a. How many valence electrons are present in this atom?
1 valence electron.
b. How can this atom become stable?
By losing 1 electron.
c. Attempt to add an electron to the atom. Describe what happened:
The electron will not be accepted by the atom.
d. Why did this atom behave differently than the atom chosen in question 1?
Metals generally have lower electron affinity than non-metals, and do no readily or easily accept
electrons.
3. Choose one of the following elements by clicking on the element symbol:
 Oxygen (O)
 Phosphorous (P)
 Sulfur (S)
You should see details about the element that you chose appear at the bottom of the
screen.
a. How many valence electrons are present in this atom?
Oxygen = 6, Phosphorous = 5, Sulfur = 6
b. How can this atom become stable?
Oxygen and sulfur will need 2 additional electrons in order to be stable, while Phosphorous will
need 3.
American Association of Chemistry Teachers
3
c. Attempt to stabilize the atom by adding the necessary number of electrons.
Describe what happened:
The neutral atom will appear to pull on the additional electron as the user moves it near the
outer shell. The user will need to do this multiple times in order to reach stability. The atom will
accept the number of electrons needed, and they will join the outer shell of electrons, forming a
-2 anion for oxygen and sulfur, and a -3 anion for phosphorous.
d. Comparing the neutral atom and its anion, what do you notice about the radius in
the side-by-side comparison? Do the data values support the visual representation?
The anion has a larger ionic radius than the atomic radius of the neutral atom. The data values
shown at the bottom of the screen support the visual.
e. Are there any other noticeable differences between the neutral atom and its anion
(other than size)? List them below:
Yes, the anion will show a -1 charge, an addition of 1 electron, and the name of the anion will be
slightly different that the neutral atom.
f. Explain why there is a difference in size between the neutral atom and its anion,
making specific reference to subatomic particles.
When the additional electrons joins the outer electron shell, the positive nuclear charge of the
nucleus has to pull on additional negatively charged electrons. Since the positive charge in the
nucleus remains the same, the nucleus can no longer pull the valence electrons with the same
force as it did in the neutral atom, therefore the ionic radius of the anion is larger than the radius
of the neutral atom.
4. Consider the elements located in group 7 on the periodic table.
a. What do you notice about the electron affinity values for each of these group 7
elements compared to the values for other elements that are in the same period?
A group 7 element will generally have the highest electron affinity value when compared to other
elements that occupy the same period.
b. Which atom in group 7 should have the largest atomic radius (consider only F, Cl,
Br and I)? Why?
Iodine should have the largest atomic radius, as it is closest to the bottom of table in the group.
It has the most electrons and electron shells, and experiences the highest amount of shielding
effect, causing its atomic radius to be the largest.
c. Based on your answer to part b, and what you have learned so far, which anion in
group 7 should have the largest ionic radius (consider only F-1, Cl-1, Br-1 and I-1)?
Why?
I-1 should have the largest ionic radius. In combination with the shielding effect mentioned in
part b, and the addition of an electron, the nucleus will not be able to apply as much of its
positive force to the valence electrons, causing a large ionic radius to occur.
d. Interact with the simulation and select the atoms mentioned in part b & c., to
determine if your predictions are correct. Explain your analysis below:
The simulation will support the answers to both b & c.
e. Based on your answers above, can you make a general statement about the ionic
radius trend of anions within a group?
The ionic radius of anions in the same group will increase from top to bottom within the group.
American Association of Chemistry Teachers
4
Review Questions
5. Decide whether the following statements are true or false, and briefly explain your choice
in the space provided after each statement:
a. True or False: Metals usually have stronger electron affinity than non-metals.
False! Non-metals generally have higher values of electron affinity, and more
readily accept additional electrons. Metals generally lose their valence electrons to
reach stability, rather than gain electrons.
b. True or False: Noble gas elements have a very large value for electron affinity.
False! Noble gas elements are already stable, and therefore do not readily accept
additional electrons.
c. True or False: Negative anions have a larger ionic radius than the atomic radius of
their neutral atom.
True! The ionic radius is larger in an anion than the neutral atom, due to the
decrease in the effect of nuclear charge experienced by the valence electrons in the
ion.
d. True or False: The value for electron affinity is a negative value because when an
electron is added to the outer shell of an atom, energy is released.
True! This is an exothermic process, and the data points are indicated with a
negative sign to reflect this.
e.
True or False: Electron affinity values generally increase from left to right in a
period on the periodic table.
True! This is a trend, and there are anomalies, but generally speaking, the affinity
values increase as you move from metals to non-metals, left to right on the table.
f. True or False: A larger numerical value for electron affinity means it is more
difficult for an electron to be added to an atom.
False! The value can be regarded as the power the atom has to obtain an additional
electron and add it to its outer shell. The larger the numerical value, the easier it is
for the atom to become an anion.
Using the simulation, check your predicted answers to see if you are correct!
American Association of Chemistry Teachers
5