Question

fill in the blank question
the atomic mass of an element is not a whole number because the atomic mass is the weighted
blank mass of the isotopes of that element, and isotopes have different
blank

Explanation:

The atomic mass of an element is a weighted average of the masses of its isotopes. Isotopes of an element have different numbers of neutrons, which leads to different atomic masses (or mass numbers, but the actual mass also varies slightly due to binding energy, though for this context, the key is the relative abundance and their masses). So the first blank is "average" (since atomic mass is weighted average of isotope masses) and the second blank is "numbers of neutrons" (or "neutron numbers", as isotopes differ in neutrons, leading to different masses). Wait, re-reading: "the atomic mass is the weighted [blank1] mass of the isotopes... and isotopes have different [blank2]". So blank1: "average" (because atomic mass is weighted average of isotope masses, considering their relative abundances). Blank2: "numbers of neutrons" (or "neutron counts", since isotopes of an element have same number of protons but different neutrons, so different mass numbers and thus different masses). Alternatively, "masses" but no, the reason they have different masses is different neutrons. Wait, the sentence structure: "the atomic mass is the weighted [blank] mass of the isotopes... and isotopes have different [blank]". So first blank: "average" (weighted average). Second blank: "neutron numbers" (or "numbers of neutrons") or "masses" but no, the question is why atomic mass isn't whole number: because it's weighted average of isotopes, and isotopes have different masses (due to different neutrons). Wait, maybe the first blank is "average" and the second is "masses" or "neutron numbers". Let's recall: Atomic mass is the weighted average of the atomic masses (or mass numbers, but more accurately, the actual isotopic masses) of the naturally occurring isotopes of an element, taking into account their relative abundances. Isotopes of an element have the same number of protons but different numbers of neutrons, so they have different atomic masses (mass numbers, or actual masses). So the first blank: "average" (weighted average). The second blank: "numbers of neutrons" (or "neutron counts") or "masses". But the standard fill-in: "The atomic mass of an element is not a whole number because the atomic mass is the weighted average mass of the isotopes of that element, and isotopes have different numbers of neutrons (or "neutron numbers", or "masses" but the core difference is neutrons). Wait, maybe the second blank is "masses" but no, the reason they have different masses is different neutrons. Alternatively, the second blank is "mass numbers" but mass number is protons + neutrons, so different mass numbers (whole numbers) but atomic mass is average, so not whole. But the question is about why atomic mass isn't whole: because it's average of isotopes (which have different masses, and the average can be a decimal) and isotopes have different masses (due to different neutrons). So first blank: "average", second blank: "masses" (or "neutron numbers"). But let's check standard textbook content: Atomic mass is the weighted average of the masses of the isotopes of an element, based on their relative abundances. Isotopes of an element have the same atomic number (protons) but different mass numbers (protons + neutrons), so different numbers of neutrons, hence different masses. So the first blank is "average" and the second is "numbers of neutrons" (or "neutron numbers").

Answer:

First blank: average; Second blank: numbers of neutrons (or neutron numbers, or masses - but more accurately, numbers of neutrons as the reason for different masses)