ionic size comparison, Sr2+ vs. S2-

Discussion in 'DAT Discussions' started by joonkimdds, Feb 12, 2010.

  1. SDN is made possible through member donations, sponsorships, and our volunteers. Learn about SDN's nonprofit mission.
  1. joonkimdds

    joonkimdds Senior Member 7+ Year Member

    Jun 30, 2005
    Sr ^ 2+ has 38 protons and 36 electrons
    S ^ 2- has 16 protons and 18 electrons

    although Sr ^2+ is smaller than its atomic form, Sr
    and S ^2- is bigger than its atomic form, S
    I don't understand how Sr ^2+ can be smaller than S ^2-
    when it's located on the next sublevel.
  2. SDN Members don't see this ad. About the ads.
  3. wantVCUdental


    Aug 6, 2009
    This is a very tricky question, is it from destroyer?
    You have to think more about just the sublevel for this one, because there are charge(+):charge(-) that will come into play.

    the trend is going from the left to right, size of the molecule is decreased, due to increase in number of protons. Therefore, K+ is actually bigger than Kr, even though they are in the same row.
    S-2 is essentially the same as K, because there are extra electrons, the positive:negative charge ratios are skewed(relatively more negative characteristics) and so the shell is actually expanded.
    In the case of Sr+2, there are fewer electrons/protons, so the positive:negative charge is also skewed(toward positive), this means the electrons are held extremely tightly and so Sr+2 resembles more of Kr.

    Now as previously stated, K>Kr in size, therefore S-2>Sr+2 i hope this make sense
  4. JBarr29

    JBarr29 5+ Year Member

    Aug 15, 2008
    Krypton's radius will be substantially smaller than that of Sulfur 2-.

    The effective nuclear charge of Kr is a lot larger than the sulfur ion, therefore will pull much the electrons in harder, so to say they are relatively equal isn't the right way to look at it.

    This is a tough problem, and i'll explain when i get the chance...
  5. joonkimdds

    joonkimdds Senior Member 7+ Year Member

    Jun 30, 2005
    if S^-2 is bigger than Sr^+2,
    how about
    Ba^+2 and Ra^+2?
    are these two also smaller than S^-2 no matter what sublevel they are located?
  6. sfoksn

    sfoksn 7+ Year Member

    Sep 3, 2006
    S 2- has same electron number as Kr, not K.. right?

    This problem is really tricky

    Sr has so many more electrons :/
  7. wantVCUdental


    Aug 6, 2009
    Ba+2 and Ra+2 are without a doubt bigger than S-2, the size is affected by a combination of # subshells and proton:electron ratio. S-2 and Sr+2 almost fall on the same subshell level, whereas Ba+2 or Ra+2 and S-2 are several subshells away.
  8. wantVCUdental


    Aug 6, 2009
    # of protons affect size more and also, it's not the number that matters here, the ratio of proton:electron matters more.
  9. AmpedUp

    AmpedUp The Legend Still Lives 7+ Year Member

    Dec 1, 2009
    Here's a trick to remembering it (from one of my notes somewhere - off the top)...the Higher the atomic number, the smaller the ionic radius...

    and if it's between cations and anions, then anions are bigger.
  10. azar87

    azar87 MD 2 Be 7+ Year Member

    Apr 16, 2008
    New York
    Heres an easy way to learn that concept . I do remember having questions liek that in the destroyer. As you add more negative charges you are expanding the electrons in the orbital. More electrons=more longer shells theoretically that is why a molecule with a negative charge will be larger than a molecule with a positive charge.
  11. joonkimdds

    joonkimdds Senior Member 7+ Year Member

    Jun 30, 2005
    but i don't think that's always true because higher subshell also means larger.
  12. knockit

    knockit 2+ Year Member

    Nov 17, 2008
    I remember this from my Chemistry text book:

    "In an isoelectronic series, size decreases as the nuclear charge (atomic number) of the ion increases."

    Because the number of electrons remains constant, the radius of the ion decreases with increasing nuclear charge, as the electrons are more strongly attracted to the
  13. americanpierg

    americanpierg Senior Member 5+ Year Member

    Dec 8, 2005
    You can't use a single generalization to cover all the elements on this topic. Unless you've had extensive studies in quantum mechanics to the point where you can tell precisely how a specific change in effective charge balances with a specific change in the number of occupied electron shells, your best bet if you want to have this topic down cold is to look at the atomic radii of all the non-metal elements, and then their ions, and then look for patterns. I did this two years ago and it really helped and it only took like 30 minutes. There's common trends throughout the entire periodic table thats a good place to start and then you can just memorize the specific ones that don't. Stuff like if two elements are isoelectronic the one with the larger effective charge would be smaller, going left to right across a period leads to a decrease in size, etc. are a great place to start, but doesn't help when you're trying to compare the size of elements that are in different groups and periods. Like for example, which is bigger, Fr+ or Te2-?

    As I remember it, the general trend was 2 rows. Ba 2+ will be smaller than the ions up to 2 rows above it (Se2-, Br-, Te2-, etc) assuming they end up in the same noble gas configuration. Group I cations are smaller than up to group 7 anions 2 rows up. Trend stops with the last Group I element. The last two Group II cations were also smaller than all anions after potassium in group I. there was a couple more trends i memorized like these by looking at the atomic radii of the elements and their ions but i dont rmember them now. just find yourself a good website that gives you the atomic size of an element along with that of their ions and look up the general trends yourself.

    That site is also good as it gives you everything from melting point, ionization energys (1st, 2nd, 3rd), electron configurations (helpful for figuring out the trend for filling out d orbitals correctly), etc.

    Good luck.
    Last edited: Feb 16, 2010

Share This Page