Explain how the existence of isotopes is related to the atomic masses not being whole numbers?

Answer 1

well, if you didn't have isotopes, the atomic masses would always be whole numbers, because you'd have one and one only number of protons (atomic number) and one and one only number of protons (atomic mass).

but if you've got isotopes, things become a bit more complex.

take, say, Carbon. Standard Carbon is C12, which has 6 protons, and 6 neutrons. Readily available isotope is C13, which has also 6 protons obviously, but 7 neutrons. C13 is roughly 1% of total carbon. So, in round numbers, every 100 atoms you have one extra neutron. So for each atom, on average, you have 0.01 of extra atomic mass.

so the atomic mass of Carbon is not 12.00, but rather 12.01.

or take Iron. 26 protons. Atomic mass is 55.845. Why? Because you've got Fe56, the most common (about 91.7%). But you've also got Fe54 (about 5.8%). And you also have Fe57 (about 2.2%).

Do the math, i.e. 56*.917 + 54*.058 + 57 * 2.2, you get 55.74. Which is not quite 55.845 because i've left Fe58 out.

And i'm not even going to bore you with, say, tin (Sn) which has 10 natural stable isotopes.

anyway, unless you got really lucky (say you'd have Fe55 and Fe57 in equal proportions so the average would be 56.0) you're never going to have a round number for the atomic mass. and this is because of isotopes.

hope this helps

Answer 2

The atomic mass of an isotopically pure sample of an element is very close to a whole number, with the mass-energy difference being the only reason why it's not quite a whole number - hence, oxygen in nature is essentially pure O-16, and as such, its standard atomic mass is very close to an integer, at 15.9994 u. However, chlorine has an atomic mass of 35.45 u - too far away from the nearest integer to be accounted for by the binding energy. Therefore, we must conclude that while the average mass of chlorine atoms appears to be 35.45 u, in fact chlorine in nature is primarily a mixture of different isotopes (in this case Cl-35 and Cl-37); the atomic mass is merely a weighted average (weighted by abundance) of these values. If chlorine were monoisotopic in nature like oxygen, its atomic mass would too be almost an integer.

Answer 3

Lets take carbon for example. There are two naturally stable occurring isotopes. Carbon-12 and carbon-13. So in any given amount of carbon you will have some that have a atomic mass of carbon-12 and some with carbon-13. To be specific it is 98.89% carbon-12 and 1.11% carbon-13. So to get the true average mass we must incorporate both of these.

Answer 4

atomic masses are not a real "mass" indeed but a handy concept. the u.m.a is equivalent to the mass of the hydrogen atom, since mass is measured as a deviation in a magnetic field different atoms give different masses, some times not exactly multiples of the hydrogen atom. Besides that, it was a usual practice to quote the atomic mass of an element as the product of its different isotopes times their relative %. I do not know if such old practice still persists.

Answer 5

oh wait, I understand your question...

you are given a number with a decimal value as the atomic mass. this is because this is an average atomic mass of all the isotopes found in nature with there percentages respectfully.

I thought you were talking about the atomic masses of the isotopes themselves

sorry

Answer 6

the atomic masses are an average of all the isotopes by their percentage of abundance found here on earth.