Why is the atomic mass of an element not equal to its mass number?

Question

anonymous · Answer

Atomic no. is the no. of protons /electrons in a stable atom..
and mass no is the sum of no of protons & no of nuetrons in the nucleus..
hence they are not same..:)

jfraser · Answer

the atomic mass we find on the periodic table is an average of all the isotopes of that particular atom. The mass number of a single atom is the number of protons & neutrons inside a SINGLE atom of that element. Mass number must always be a whole number, since it's describing a count of things. The atomic mass, since it's an average, doesn't have to be a whole number, even though it is often very close to a whole number.