Why is it the atomic weight is not whole number?

Answer 1

The atomic weight is the average mass of all of an element's isotopes (isotopes are atoms of the same element with the same number of protons but a different number of neutrons, and protons + neutrons = Atomic Mass.

Each isotope of an element has a whole number of protons and neutrons, but the atomic weight is the average of all the isotopes found, and it is affected by the observed ratios of the different isotopes. If the number was a whole number, then no known isotopes have been found (rare other than some man-made elements).

Short answer: The reason the atomic mass is not a whole number is because it's the weighted average mass of the isotopes of that specific element.

Because the atomic mass is a calculated average of all the Isotopes of an Element with their % of which they occur also calculated in.

The atomic mass of an element is the average mass of all its isotopes, weighted by their abundance. For example, the mass of Carbon is given as 12.011, since Carbon 12 is by far the most abundant, but Carbon 14 also exists, shifting the average mass slightly above 12.

Answer 2

This is not a matter of Math, or Chemistry, but Physics. Nuclear physics mostly. * Almost all the weight of an atom is in the nucleus; around 99.95%. * The nucleus is made of nucleons - protons and (usually) neutrons. * When outside a nucleus a neutron always has the same mass; so does the proton, and the two have masses quite close to each other, but not identical to each other. The neutron is a little heavier. * This at once gives some reason why the At. Wt. is not always a whole number. Hydrogen has just 1 proton. We'll say its AW is 1. Now for Helium; 2 protons and 2 neutrons. Four times as many nucleons, but not 4 times the mass - '''the neutron is heavier than the proton''', so the massof the nucleus is more than 4. * The heavier the element the more neutrons proportionally to protons. Helium is 2 : 2. Uranium is 246 : 92 * There is another effect which is both larger and more complex - too complex to go into a short Wiki Answer. Energy is tied up in binding the nucleons together. This energy also changes the mass, by a larger amount, with the minimum mass per nucleon now not at Hydrogen but at Iron. * Even physicists like to make things simpler if they can. What they have done here is to define the At. Wt. of Carbon 12 as 12 exactly. That is where the a.m.u. is fixed - one twelfth of a C12 atom.

Answer 3

You have to be careful here. Atomic number is a whole number: it's simply the number of protons in each element. You can't have half of a proton in an atom, it just doesn't work. Atomic number/the number of protons has to be a whole number.

Mass number is the number that wouldn't be a whole number. It is the average of the masses (the protons+neutrons) of all of the atoms of the element on Earth. The number of protons in each element would have to stay the same, as that is the defining characteristic of the element. But, because there could be variation in the number of neutrons in the atoms of an element, the mass numbers of different atoms can vary. And because the element's atoms with a certain number of neutrons might be most stable, there may be more of atoms with one mass number than the others- and once you take the average of the mass numbers of all of these element's atoms, you will get a result that isn't a whole number.

Hope this helps.

Answer 4

http://www.school-for-champions.com/science/atomic_weight.htm this explains it well

Answer 5

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