Name: ______________________ Atomic Radius Exploration – Answer Key Background 1. Draw a picture to support your written definition of the word “radius.” Students should draw a circle, identifying the distance from the center point to the perimeter as the radius. 2. Let’s assume an atom is shaped like a sphere, what subatomic particles would be found in the center? What subatomic particles would be found around the perimeter? The nucleus, containing protons and neutrons would be in the center, while the valence electrons would be around the perimeter. 3. Keeping in mind your answers to questions 1 & 2, in your own words describe the meaning of “atomic radius” This is the distance from the center of the atom to the outermost (valence) electron. *Check your answers before moving on to the next portion of the activity. Directions Using your computer, tablet or mobile device, navigate to the website: www.ptable.com You will interact with this periodic table to find the answers to the questions below.
Click the “Properties” tab at the top of the page. Next select “Radius” from the properties listed in the top center of the screen. Finally make sure “Calculated radius” is selected from the options on the right.
1. Complete the following data table: Group 1 element Calculated Radius Value (pm) 53 H Li Na K Rb Cs American Association of Chemistry Teachers
167 190 243 265 298
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2. What trend in the data do you observe as you move from the top of the periodic table to the bottom within this group? The atomic radius increases from top to bottom. 3. Does this periodic trend apply to any other group? Briefly investigate the atomic radii values in another group to support your answer. Yes – answers will vary. Students can cite many examples. 4. What ideas do you have about the factors that actually contribute to the atomic radius trend within a group? (Revisit your background questions to guide you). Note that these student responses do not have to be accurate at this point in the activity. Students will investigate the trends fully and may realize that some of their initial ideas will not hold true. Possibilities include: increase in atomic number, increase in atomic mass, increase in number of electrons, increase in number of protons. 5. Complete the following data table: Period 2 Li Be B element Atomic Radius 167 112 87 Value (pm)
C
N
O
F
Ne
67
56
48
42
38
6. What trend in the data do you observe as you move from the left of the periodic table to the right within this period? The atomic radius decreases from left to right. 7. Using your knowledge of the Bohr model, as well as considering the atomic radius data values collected in question #5, complete the following table. Make sure to consider comparative sizing of each model. All Bohr models should have 2 shells, with 2 dots on the inner most shell, and between 1 and 8 dots on the outer shell, representing the valence electron. The valence electron start with Li=1, Be=2, and the value will continuously increase by one from left to right across the data table. Period 2 element
Li
Be
B
C
N
O
F
Ne
Bohr Model
8. Does this periodic trend apply to any other period? Briefly investigate the atomic radii values in another period to support your answer. Yes – answers will vary. Students can cite many examples. American Association of Chemistry Teachers
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9. What ideas do you have about the factors that actually contribute to the atomic radius trend within a period? (Think about subatomic particles) Note that these student responses do not have to be accurate at this point in the activity. Students will investigate the trends fully and may realize that some of their initial ideas will not hold true. Possibilities include: increase in atomic number, increase in atomic mass, increase in number of electrons, increase in number of protons. 10. Draw a vertical and horizontal arrow next to the periodic table outline below. Point the arrows in the direction that the atomic radius increases.
11.Based on the trend arrows drawn, make a prediction, and place the following elements in ordering of increasing atomic radius (for smallest to largest): Si, Ca, C, F, Cs (Smallest) F, C, Si, Ca, Cs (Largest) 12.Check values on www.ptable.com and record the actual values for atomic radius in the table below. Was your prediction from question #11 correct? Yes! Element symbol Atomic Radius Value (pm) Rank (1-5) 1=smallest 5=largest
Si
Ca
C
F
Cs
111
194
67
42
298
3
4
2
1
5
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13.Read the following descriptions of two important factors that affect the atomic radius of an atom, then answer the following questions: Electron Shielding Effect: This is due to inner electrons “shielding” the valence electrons from the positive pulling force of the nucleus. As an atom increases its number of electron shells, the shielding effect will increase in turn keeping the valence electrons distanced from the nucleus. Effective Nuclear Charge: This is due to the number of protons in an atom, the more protons the stronger the pulling force of the nucleus will be on the electrons in the atom. a. Revisit your answers for questions 1-4. Using the vocabulary terms given above make a statement about the atomic radius trend within a group on the periodic table. Use two specific elements as examples in your response. As you travel through a group from the top of the periodic table to the bottom, the atomic radius will increase. The valence electrons are going to be located further away from the nucleus in an element near the bottom of the group, such as Cesium in group 1, than another group member near the top, such as sodium. This difference in radius is due to the increase in the number of electron shells in each atom as you move towards the bottom of the table. An atom such as cesium will be much larger, due to its valence electrons being repelled or shielded by the inner electrons, than an atom of sodium that has few electron shells, and therefore less electron shielding. This trend in atomic radius is due to the Electron Shielding Effect. b. Revisit your answers for questions 5-9. Using the vocabulary terms given above make a statement about the atomic radius trend within a period on the periodic table. Use two specific elements as examples in your response. As you travel through a period from left to right on the periodic table the atomic radius of an atom decreases. Also, the number of positively charged protons in the atom’s nucleus increase and the number of electrons increase. However, though the numbers of electrons are increasing, the number of electron shells remains the same, so there is no change in the electron shielding effect. As the nucleus gains a proton the positive pulling force on the surrounding negatively charged electrons increases. This increase in effective nuclear charge pulls the valence electrons of the atom closer to its nucleus, therefore decreasing the radius of the atom.
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