The atomic weight of boron is reported as 10.81, yet no atom of boron has the mass of 10.81 amu. Which is the best explanation?

a. The measurement of atomic mass is only reliable to two significant figures.

b. The atomic weight is an average of many individual atoms.

c. The atomic weight is an average of many isotopes of the same nuclear composition.

Video transcript

Welcome back everyone in the periodic table. Silicon has an atomic mass listed as 28.08550. And not 28.0 choose the statement that explains why atomic masses in the periodic table are not integral numbers. So when it comes to our element silicon, it's represented by this symbol on the periodic table. And recall that the number at the very top is its atomic number As 14. And right below it, we have its atomic mass as 28.09. Now in the prompt, they give us a more specific one as 28.08550. Which you can see is rounded on the periodic table to 28.09. And the units are atomic mass units am use now when it comes to our atomic number. We want to recall that. This describes our number of protons and number of electrons for our atom when it's neutral and not an isotope or charged. So as well as the atomic mass here, A. K. A. The mass number. We want to recall that this is defined by our number of protons added to our number of neutrons for our atom. And that's why when it specifically comes to isotopes, they're named by integers. For example, if we had an isotope of silicon, it would be hyphen 28 for silicon which is the first isotope we can find or we can find silicon 29 which is another isotope. And again these integers in these isotope names represent the sum of protons and neutrons for that atoms of silicon or that specific atom in general. But in the case of our atomic mass on the periodic table, we want to recognize that this is going to be an average atomic mass of all stable isotopes and that's why it has to be written out as a decimal on our periodic table. And so with this outlined, we can confirm that the only correct choice to complete this example is going to be choice E, which says that the weighted average of the asset hopes is displayed as atomic mass. So E is our final answer for why atomic masses in the periodic table are not integral numbers and are expressed as decimals. Due to the fact that the weighted average of isotopes is displayed as atomic mass. I hope everything I reviewed was clear. If you have any questions, please leave them down below and I'll see everyone in the next practice video.