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Which element on the periodic table has the greatest atomic mass?

Is the atomic mass listed in the periodic table always assuming the element has equal protons and neutrons?

  • That is to say: Hydrogen is listed in the periodic table as having a mass of 1.0079 (assuming 1 proton, 1 neutron) But (my knowledge of chemistry is still limited, so sorry if this is weird) if there was an isotope of Hydrogen with 1 proton, 2 neutrons, would the weight go up? Does it go up by one? (ie, 2.0079?) Thanks for any help!

  • Answer:

    Jose is wrong: "Allan is wrong, hydrogen has 1 neutron, 1 proton, deuterium has 2 neutrons 1 proton, tritium has 3 neutrons 1 proton." H ---> 1 p, zero n D ---> 1 p, 1 n T ---> 1 p, 2 n "could you offer any further insight into how I'm expected to know this information?" B-11 has 5 protons and 6 neutrons. So, its approximate weight is 11, thus it is lighter than C-12, which weighs 12. We know that the approximate weight of B-11 is 11 because we know that protons weigh 1 amu and neutrons weigh approximately 1 amu (actually a free neutron weighs 1.008 amu). The exact weight of B-11 is complicated by the fact that there is something called binding energy. In the formation of the B-11 nucleus (in some star a long, long time ago), a small amount of the mass of the 5 p and 6 n was converted into energy. This energy plays a role in helping hold the nucleus together. "but I don't understand how I could be SURE of the exact weight of ^11B without looking at a chart" You can't. The exact weight of B-11 cannot be predicted from first principles, it must be measured in a device called a mass spectrometer. In times before mass spec., the determinations had to be done chemically. In fact, the first American to win a Nobel in chemistry did so for chemical determinations of atomic weights: https://en.wikipedia.org/wiki/Theodore_William_Richards I guess the insight is that the answer to the question you were asked can be determined in an approximate way. A tougher one might be this: Which is heavier: A C-14 nucleus or a N-14 nucleus? I'll let you ponder that one on your own.

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Hydrogen has one proton, no neutrons. Deuterium has one proton, one neutron. Tritium has one proton, two neutrons. The atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units.The atomic mass is the total mass of protons, neutrons and electrons in a single atom. The atomic mass is sometimes incorrectly used as a synonym of relative atomic mass, average atomic mass and atomic weight; these differ subtly from the atomic mass. The atomic mass is defined as the mass of an atom, which can only be one isotope at a time and is not an abundance-weighted average as in the case of atomic weight. Standard atomic weight refers to the mean relative atomic mass of an element in the local environment of the Earth's crust and atmosphere as determined by the IUPAC Commission on Atomic Weights and Isotopic Abundances. These are what are included in a standard periodic table and is what is used in most bulk calculations.

Allan

The atomic mass of an element is an average of the contributions of various isotopes of the element. The reason hydrogen is not listed as1.0000, is that there is a contribution for deuterium and tritium. Deuterium has two neutrons, and tritium has three. So deuterium would be 2.0000 and tritium would be 3.0000. But when you add up the percentage contribution of each isotope, hydrogen comes out at 1.0079.

cat lover

Allan is wrong, hydrogen has 1 neutron, 1 proton, deuterium has 2 neutrons 1 proton, tritium has 3 neutrons 1 proton. Boron has 5 protons thereby 11B refers to an isotope of boron with 6 neutrons (5p+6n =11) whereas 12C consist of 6p + 6n therefor it should be heavier. You dont need the exact mass of 11B.

Jose Alejandro

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