Ion size

Discussion in 'MCAT Study Question Q&A' started by MedStudentWanna, May 29, 2008.

  1. MedStudentWanna

    Banned

    Joined:
    Apr 14, 2006
    Messages:
    1,993
    Likes Received:
    7
    Can someone explain ion size to me in a way that makes me understand it instead of memorizing the protons the smaller it is?
     
  2. Thread continues after this sponsor message. SDN Members do not see this ad.

  3. MedStudentWanna

    Banned

    Joined:
    Apr 14, 2006
    Messages:
    1,993
    Likes Received:
    7
    Someone must be able to explain this.
     
  4. Dr.McNinja

    Dr.McNinja Nobel War Prize Winner
    Moderator Physician Faculty

    Joined:
    Aug 2, 2006
    Messages:
    8,967
    Likes Received:
    4,619
    Status:
    Attending Physician
    Perhaps if you asked in a manner conducive to being answered.
    I honestly can't even tell what you are asking.

    Ions are smaller as you go up and to the right on the periodic table. Does that come close?
     
  5. MedStudentWanna

    Banned

    Joined:
    Apr 14, 2006
    Messages:
    1,993
    Likes Received:
    7
    Yes. Can you explain why they get smaller as you go to the right? I would have thought it was the opposite. I can memorize that it is how it is, but I don't really understand.
     
  6. chemnerd31

    Joined:
    Apr 9, 2008
    Messages:
    79
    Likes Received:
    1
    Status:
    Pre-Medical
    The ions get smaller as you go from left to right due to the increase in the number of protons while the electrons are being placed in the outer valence shell. Remember the equation Zeff = Z-S where Zeff is the effective nuclear charge. Z is the number of protons and S is the number of inner core electrons. As you move from left to right Z increase but S does not and this leads to a larger Zeff. This increased Zeff is what caused the ions to shrink. The increase from top to bottom is because of naturally larged shells as the principal quantum number increases. I hope this helps
     
  7. chemnerd31

    Joined:
    Apr 9, 2008
    Messages:
    79
    Likes Received:
    1
    Status:
    Pre-Medical
    Sorry double post
     
  8. MedStudentWanna

    Banned

    Joined:
    Apr 14, 2006
    Messages:
    1,993
    Likes Received:
    7
    Thank you, I get it now!!!
     
  9. BerkReviewTeach

    BerkReviewTeach Company Rep & Bad Singer
    Exhibitor

    Joined:
    May 25, 2007
    Messages:
    3,821
    Likes Received:
    569
    Not to drop a rain cloud over the parade, but that explanation describes why atoms get smaller as you move left-to-right across the periodic table. As Chemnerd explained, the nuclear pull increases and causes the electron cloud to contract, reducing the size of the cloud.

    But, to answer your question about ion size, you need to first consider whether it is an anion or a cation. In a simplistic model, consider the impact of adding an extra electron to an atom to form an anion. The extra electron would repel the other electrons in the cloud, and it would expand. This explains why anions are bigger than the neutral atom. On the other hand, removal of an electron to form a cation results in a decrease in electron repulsion. This allows the electron cloud to contract, resulting in a smaller species. This means that cations are smaller than the nuetral element. Another factor that comes into play is the shell. If the electron being removed happens to be the last one in the shell, then the impact of size reduction is event greater, because a shell has been lost. This explains why alkali metal cations are so much smaller than their neutral element counterparts.

    Hopefully this answers your question, assuming you meant ions when you typed it.
     
  10. MedStudentWanna

    Banned

    Joined:
    Apr 14, 2006
    Messages:
    1,993
    Likes Received:
    7
    I thought I understood it, but I guess no. I guess I just have to read more about it to truly understand.
     
  11. chemnerd31

    Joined:
    Apr 9, 2008
    Messages:
    79
    Likes Received:
    1
    Status:
    Pre-Medical
    Thanks for adding that Berk I forgot that part when I wrote my reason.
     
  12. Doctor D

    Joined:
    Jun 2, 2008
    Messages:
    422
    Likes Received:
    20
    Status:
    Medical Student
    Ions that have a negative charge are much larger than their neutral or positive counterparts (Cl- is > in size than Cl or Cl+). This is because the effective nuclear force exerted on the electrons by the protons in the nucleon is diluted by the addition of the extra electron. Since the electron feels less pull towards the center of the atom, it can wander farther away.
     

Share This Page