Anonymous asked in Science & MathematicsChemistry · 3 months ago

Why does Uranium have a lighter mass even though it has a larger atomic number?


Than neptunium (Np)

6 Answers

  • Dr W
    Lv 7
    3 months ago

    take a look at this periodic table

    notice a couple of things

    .. (1) the atomic number of U = 92

    .. (2) the atomic number of Np = 93

    .. (3) relative atomic mass of U = 238.03

    .. (4) relative atomic mass of Np = (237)

    So I think you meant to ask

    .. "why does Np have a lighter "relative atomic mass" than U even though

    .. .Np has a larger atomic number than U"



    Some things you should know by now

    .. (1) atomic # = # protons in all atoms of that element

    .. (2) atoms of the same element with different numbers of neutrons

    .. . .. are different ISOTOPES of that element.

    .. (3) mass number = sum of protons + neutrons.  it is an integer count

    .. .. .. of nucleons in any given isotope of any given element.

    .. (4) isotopic mass is the mass of 1 isotope of any given element.

    .. . .. it is the sum of masses of all the protons + all the neutrons

    .. . . .+ all the electrons - mass defect of 1 atom of that 1 isotope of that

    .. . .. 1 element

    .. (5) relative atomic mass = weighted average isotopic mass

    .. . .. of all stable isotopes of that element.... (for about 70% of all discovered

    .. . . .elements to date!)  If there are no stable isotopes of that element

    .. . .. (30% of discovered elements), the isotopic mass (or in this case the

    .. . .. mass number) of the most stable isotope known to date is shown in ( )


    to answer the question you meant to ask.

    Although Uranium has no stable isotopes, the half lives of U235 and U238 are so long (in the order of a billion years), the weighted average isotopic mass of those two isotopes is used to calculate the relative atomic mass of U.  Neptunium has no stable isotopes either and the "mass number" or "isotopic mass" of U237 (the isotope with the longest half life... 1 million yrs or so) is shown in ( ).  U238 (the isotope that contributes 99.3% to the relative atomic mass of U) has 2 more neutrons and 1 less proton than the most stable isotope of Np making U's average atomic mass heavier than the mass of the most stable Np isotope.

    As an aside, you're comparing a weighted average mass (U) to the mass of 1 isotope of Np.  That's not a fair comparison.  See those links!.. You can have 1 atom of U with atomic # = 92 and atomic mass ≈ 235 and have it be essentially stable (1 billion years).  If you compare that to Np237, it will have a lower atomic number and a lower isotopic mass now won't it?

    • Commenter avatarLogin to reply the answers
  • 3 months ago

    Neptunium is atomic number 93. Uranium does NOT ha e a higher atomic number. False pemise. Your facts are uncpordinated.

    Neptunium has one more neutron and electron than uranium. That is why it is larger. Neptunium has 4 isotopes, which means it can have a different number of neutrons. The atomic mass can vary..Neptunium is radioactive. It is a decay product of other radioactive elements.

    • ...Show all comments
    • Dr W
      Lv 7
      3 months agoReport

      This answer is simply not accurate at all.   Each element has an infinite number of POSSIBLE isotopes.  Carol is trying to say only 4 isotopes of Np have been discovered.  That's not accurate either.  Nor are all the discovered isotopes used to generate (237). only 1 is.. Np237

    • Commenter avatarLogin to reply the answers
  • 3 months ago

    As you don't say what other element you're comparing it to, we can't say specifically.

    But ONE way that can be true is if the specific Uranium atom you're talking about became one through beta decay.

    If a hypothetical atom of say Protactinium-232 (atomic number 91) underwent a single beta decay, one of its neutrons would emit an electron, becoming a slightly less-massive proton in the process.  This would cause the atom's mass to go down (by the mass of an electron) while simultaneously increasing its atomic number by one - turning it into a lighter atom of Uranium-232 (atomic number 92).


    Re: your update - You've actually got it backwards.  Neptunium has a smaller "mean atomic mass" than Uranium, but a higher atomic number.

    Because they have different isotope ratios.  Uranium has a higher proportion of isotopes that have more neutrons, while Neptunium has a lower proportion of heavier isotopes.  Which means that the "average" Uranium atom is heavier than the average Neptunium atom.

    • ...Show all comments
    • skeptik
      Lv 7
      3 months agoReport

      Where did I say it was?

      The OP asked a question without providing enough details, so we're all pretty much guessing what they're talking about.
      When more detail was finally offered, it was backwards.

      You're criticizing the wrong people.

    • Commenter avatarLogin to reply the answers
  • 3 months ago

    lighter than what?

    larger than what?

    • Commenter avatarLogin to reply the answers
  • How do you think about the answers? You can sign in to vote the answer.
  • 3 months ago

    Lighter than what?

    It is the heaviest natural element, but has at least two isotopes.  U238 is slightly heavier than U235.  There is a difference in number of neutrons.

    Quite a few heavier artificial elements exist for very brief periods of time, by comparison.  Neptunium and Plutonium are two with longest half-lives.

    • Commenter avatarLogin to reply the answers
  • 3 months ago

    It doesn't. It's 92 on the P.T. and as the atomic # increases, so does mass. It's 238.029 amu's


    • Dr W
      Lv 7
      3 months agoReport

      not exactly true.  see my answer.

    • Commenter avatarLogin to reply the answers
Still have questions? Get your answers by asking now.