Tuesday, November 9, 2010

Trimester 1 Exam Review Question #15

#15. Which would have the smallest radius: Na, I, and O? Explain your answer throughly.
                - Oxygen would have the smallest radius.


  Atomic radius is defined as one half of the distance between the nuclei of two atoms of the same element when the atoms are joined. The atomic radius decreases across periods because there are less electron shells to shield the attractive power of the nucleus. You add a proton each time you go across the period, making it have a more attractive power. The attractive power causes it to pull in the electron shells tighter, which decreases the radius. It increases down groups because each time you go down, a new energy level is added. This creates a new electron shell. The number of electrons outnumber the number of protons, which means the attractive force isn’t as great. This allows the shells to spread out more because it doesn't have the attractive force pulling it in, creating a larger radius. 

Oxygen would have the smallest radius because it has the smallest number of electron shells. This causes the positive charge to be greater and pull the electron shells closer, therefore decreasing the radius. Sodium has a larger radius than oxygen because it has more electron shells and a smaller attractive force, allowing the shells to spread out. This causes a larger radius. Iodine has a larger radius than oxygen because even though it has a very strong attractive force, the number of electron shells outnumbers the attractive force. This means the radius for iodine is larger because the all the shells spread out. 


  1. Christina,
    You did a really good job on your answer and it was very complete. I think the only thing that may have helped more was a visual of the actual process of the increasing and decreasing of the radius, but you did really good!

  2. Christina,
    You did a really gread job on this blog post! You gave a very complete and in-depth explainations of your answer. I also really liked the graphic of atomic radius was really helpful as well. Great job!

  3. Great job Christina! your answer was very thorough and I feel like if I didn't understand this topic, it would have been very beneficial. I agree with Madison, it would have been a little helpful to show the increasing radius but it isn't crucial. Overall- very good explanation and answer!

  4. Awesome explanation. One small critique - I wouldn't say that the electrons outnumber the protons. The inner energy-levels act as "shields" for the outer-most energy-level because those outer electrons not only feel an attraction from the nucleus (+ charge) but a very large repulsion (- charge) from the inner shell electrons. the inner shells are literally in between the outer electrons and the nucleus blocking the attraction.

    When electrons are added to the same energy level you don't have this problem nearly as much which is why the attractive power seems to outweigh the repulsion from added electrons in the same energy-level when you go across the rows.