1. Which of the following has the largest size? A. K, B. K+, C. Se, D. Li
QUESTION:
Which of the following has the largest size? A. K, B. K+, C. Se, D. Li
SCRIPT:
Which of the following has the largest size?
Potassium… potassium ion… selenium…or lithium
PAUSE
Let’s look up these elements in the periodic table.
CLICK

2. decreasing decreasing K: [Ar] 4s1 K+: [Ar] SCRIPT, slide 2
Here’s potassium
CLICK and here’s selenium
CLICK
Both belong to period four.
The trend in size across a period, from left to right is decreasing.
Therefore selenium is smaller than potassium.
CROSS OUT Se.
Here’s Lithium
The trend in size going up a group is also decreasing.
Therefore, lithium is also smaller than potassium…
CROSS OUT Li
How about potassium ion… how does its size compare with potassium.
Let’s look at the electron configurations…
Potassium has one more electron than argon. Here’s Argon…
Therefore, the electron configuration of potassium is … argon core, 4s1
Potassium ion… with one less electron…. Will simply have the argon core.
The question is, which one is larger?
K: [Ar] 4s K+: [Ar]

3. Which of the following has the largest size? A. K, B. K+, C. Se, D. Li
QUESTION:
Which of the following has the largest size? A. K, B. K+, C. Se, D. Li
[Ar]
SCRIPT:
Here’s a visual representation of the a potassium atom.
Here’s the nucleus
CLICK Here’s the core with 18 electrons
CLICK CLICK And here is the valence shell with one electron
CLICK
In potassium ion, the electron in the valence shell is gone.
Obviously potassium ion is smaller than potassium atom.
Therefore, the correct answer is A.
In general, positive ions formed from atoms are smaller than the original atom.
CLICK PAUSE END RECORDING
K+ is smaller

4. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08