Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Charge on the nucleus

  1. Sep 15, 2005 #1
    I've got this following question which I don't really get:

    What is the charge on the nucleus of each of the following? (assuming they are neutral)

    a) Mg b) Ne c) K^+ d) S^2-

    What I don't understand is the definition of "charge on the nucleus"

    I am assuming that only protons have charge in the nucleus, therefore, I just count the number of protons and that gives me the charge on the nucleus.

    This is what I think I'm supposed to do:

    a) +12 b) +10 c) +19 d) +16

    But then, would the charge be zero, because the electrons end up canceling the charge from the protons?

    Or is there another way I'm supposed to do this?


    Last edited: Sep 15, 2005
  2. jcsd
  3. Sep 16, 2005 #2
    effective nuclear charge is the charge felt by the valence electrons after you have taken into account the number of shielding electrons that surround the nucleus.

    since u have given in brackets that assuming these atoms are neutral for clearence of ur concept let's take a look at a fluorine atom.The nucleus itself has a +9 charge and anything in its vicinity will feel that charge. The two electrons in the first energy level as they look at the nucleus feel a +9 charge because that is the charge on the nucleus.assuming the atom to b neutral there would be 9 electrons as a whole in atom and 7 electron in secoind or valence energy level electrons that are in the valence energy level would be shielded from the nucleus by the 2 shielding electrons(in 1st energy level) The +9 nuclear charge is shielded by 2 electrons to give an effective nuclear charge of +7 that is felt by the valence electrons
    but If you get out beyond the valence electrons, then the effective charge is 0 simply because the +9 charge of the nucleus is surrounded by 9 electrons.
    now u can easily solve K +1 charge and also -2 charge as the show that one electron is removed and 2 are added respectively
    Last edited: Sep 16, 2005
  4. Sep 16, 2005 #3
    The charge on the nucleus is the sum of the charge of each proton's +1. I think this is what the question is asking. The answer to your second question is no, because some of the elements are not in their neutral form (c and d), thus are missing an electron (c) or have electrons in excess (d), so their total charge is not 0.

    If its asking for effective nuclear charge, then you want to find the charge of the protons minus the charge of all non-valence electrons.
  5. Sep 16, 2005 #4


    User Avatar
    Homework Helper

    This may be a very easy question, depending on how accurately you've reported it. If the question is the charge on the nucleus, then the state of the electrons is irrelevant - they aren't in the nucleus. Consequently, you just go with the number of protons.

    Note that this is true even for the ions you mention in (c) and (d). Ions gain their charges by gaining or losing electrons, but the charge on the nucleus cannot change without changing the element involved. If it's potassium you're talking about, then it'll have the same nuclear charge whether the atom itself is ionized or not.

    So, again: if you've reported the question accurately, then the answer is as close as your periodic table.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook