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Why are there substances with different densities

  1. Feb 11, 2004 #1

    jimmy p

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    This may have to go into the Chemistry section but [?].

    I understand that nuclei have approximately the same densities and i understand the equations. But it got me thinking, Why are there substances with different densities if all nuclei are about the same density. I have narrowed it down to bonds BETWEEN atoms, am i right?

    Then that leads me to the next few questions if im right...do different bonds or the AMOUNT of bonds affect density of atoms?

    Can anyone shine a light on this issue?

    thanx

    James
     
  2. jcsd
  3. Feb 11, 2004 #2

    Nim

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    Atom nuclei may have approximately the same density, but not atoms themselves. A bigger nuclei doesn't always mean a bigger atom. What makes atoms bigger is the amount of electron shells they have.

    The first shell can only hold two electrons. The nucleus of hydrogen is one proton. Which means it has the atomic mass of 1 and has one electron in its shell. Helium has two protons and two neutrons, which means it has the atomic mass of 4 and has two electrons in its shell. So these two atoms are about the same size, but helium has more mass than hydrogen. Now lithium typically has three protons and four neutrons I think, which means it has the atomic mass of 7 and hass three electrons surrounding it. The third electron can't fit in the first shell, so lithium has two shells, which makes it larger.

    Plus some atoms/molecules build more compact structures than others. Take a chainlink fence for example, metal isn't extremely light weight, but the structure of the chainlink fence isn't very compact.
     
  4. Feb 11, 2004 #3

    jimmy p

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    But surely as mass of the atom goes up, so will the volume? would this volume include the electron cloud or just the electron?
     
  5. Feb 11, 2004 #4

    Integral

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    There is indeed more to density then sub atomic structure. If you look at material density you will find that Gold is denser then lead even though Lead is a heavier element. This is due to the arrangement of the atoms in the crystal structure.
     
  6. Feb 11, 2004 #5

    Nim

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    A common analogy used to describe an atom is that if an atom was the size of a baseball field, its nucleus would be the size of the baseball. An atom is mostly empty space, the nucleus has a lot of room to get bigger and more massive in.

    A helium atom, even though four times more massive than a hydrogen atom, is actually smaller because it has more protons and thus the electrons move in closer to the nucleus because of the stronger attraction.

    If you look at the perdiodic table, atoms get smaller from left to right and larger from top to bottom. Helium is on the right side of hydrogen, and lithium is below it. When looking from left to right you'll notice that the atoms gain more and more protons but not any shells. But when you look at the atoms from top to bottom, they gain more shells. The periodic table has seven rows, and the largest atom you'll see will have seven electron shells.

    Click on the links below to get a better idea. Imagine that the yellow circle inside represents the very tiny nucleus plus the very large empty space between it and the electrons that surround it.

    Hydrogen
    Helium
    Lithium
    Uranium

    Here is the periodic table if you want to check it out: ChemicalElements.com
     
  7. Feb 16, 2004 #6

    jimmy p

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    ...still none the wiser...maybe i should rephrase.

    How come molecules have different densities when the nuclei of atoms are approx the same density?
     
  8. Feb 16, 2004 #7

    Integral

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    Reread my post above, material density is determined by how the atoms are packed together. The manner in which the packing occurs is determined by the electron structure, as this determines the types of bonds that will be formed.

    Some bond structures allow more atoms per unit volume, thus are denser. Again do some research on the atomic structure of Lead and Gold. By atomic structure, I do not mean SUB atomic, but how the atoms themselves are packed into a volume of space.
     
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