Why is the atomic mass found on the periodic table not a whole number?

Answer 1

Two reasons. Firstly, the masses listed in the table are usually averages. Carbon naturally comes mainly in two reasonably common and one considerably less common types: carbon-12 (about 99%), carbon-13 (about 1%), and carbon-14 (tiny amounts). If you choose a bunch of carbon atoms at random and measure their masses, you'll get an average that's a little bit higher than 12 (specifically, something close to 12.011). The same is true for most other elements as well. Secondly, except for carbon-12 (which is the standard and by definition has a mass of 12 amu exactly), the actual masses of atoms aren't whole numbers. The binding energy holding the nucleus together throws the total off slightly from what you would expect, due to energy-mass equivalence (E=mc2).

Answer 2

1. First cause: the atomic weight is the sum of the weights of protons, neutrons ans electrons.; they don't have masses as integers.

2. Second cause: also occurs the so-called mass defect.

Not Atomic Mass for elements; atomic weight is correct.

Answer 3

The Atomic Mass is an average of the different isotopes of that particular atom. For example, Uranium 235, 236, 238 all have different weights, but the mass given on the table is the average of all these weights, including the rarity of each isotope.

Answer 4

Because most chemical elements have more than one radioactively stable isotope; each isotope has its own whole number atomic mass number, but the weighted average of the isotope masses, weighted by their natural abundance level, is not usually a whole number. The latter is what is listed as "atomic weight" on a periodic table.