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Thread: Mass number & density question

  1. #1 Mass number & density question 
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    I am trying to find out what it means when for instance the density of lead is higher than that of iron.

    A = Z + N mass number is atomic number plus number of neutrons, for a chemical element.




    When we say that a nucleus contains more protons and neutrons compared to another element,
    does that mean :


    - the nucleus volume has increased compared to the whole atom size,with atom size staying the same
    - or nucleus volume stayed the same but increased in density,meaning p and n closer together.
    -or did the atom size increase also, and…
    -or …


    So i am trying to pinpoint exactly what a higher density means on (sub)atomic level.


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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
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    See here the periodic trends in density here: Density for all the elements in the Periodic Table , as the atomic number increases, you can see there is no simple relationship between density and Z (or A),

    This is because as well as the mass of the atoms changing the size is also changing in a complicated way (not due to the volume of the nucleus which is pretty much irrelevant to atomic size but due to the electronic structure of atoms as more electrons are added, this is quite complicated and I'm not getting into it in a short forum post but any first year undergrad chemistry text will have the details)

    Atomic Radius for all the elements in the Periodic Table

    Also you have to take into account how well the atoms pack together in a bulk sample which is related to the crystal structure: Crystal Structure for all the elements in the Periodic Table etc.
    ,


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  4. #3  
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    Ok, i've got some homework to do.
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  5. #4  
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    The density of solid material is largely determined by interatomic or intermolecular distance, i.e. how closely the atoms or molecules are packed.
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  6. #5  
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    Does this make sense,looking at the elements gold and lead :

    Gold has 3 protons less, but has a density (grams per cubic centimeter) of 19 compared to only 11 for lead.


    You can see that lead has an extra shell : 6p2


    Density = mass / volume , so the extra shell causes lead to have a higher volume, causing a far lower density, even inspite of having an atomic number of 82 against 79 of gold ?


    And the extra shell of lead is not fully filled up with electrons, hence lower density ?
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  7. #6  
    Bullshit Intolerant PhDemon's Avatar
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    Not really, gold actually has a larger atomic radius (174 pm) than Lead (154 pm) -- see the link I gave you, so your argument doesn't really work. The extra shell does not automatically mean a larger volume due to penetration and shielding effects http://chemwiki.ucdavis.edu/?title=P..._%26_Shielding which result in the lanthanide contraction (http://en.wikipedia.org/wiki/Lanthanide_contraction) and to a lesser extent d-block contraction (http://en.wikipedia.org/wiki/D-block_contraction)
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  8. #7  
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    Ok
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