Why are the atomic masses listed on the periodic table decimals instead of whole numbers?

Answer 1

Because there are more isotopes (same number of protons, but different number of neutrons) of one element. The Atomic Mass in the Periodic Table is sometimes calculated by taking the average atomic mass of the different isotopes. (Taking in account the availability of each isotope in nature)

Answer 2

The masses found on the periodic table are the masses of the most commonly occurring stable isotopes averaged per their relative abundance, and is therefore not a round number.

For instance, one can look to the example of carbon. Its two stable isotopes are 12C and 13C. The relative abundance of 12C is 0.9893; the relative abundance of 13C is 0.0107.

To calculate, one multiplies the the mass of the isotope times its relative abundance, repeats this with each isotope, and then finds the sum.

For carbon, the calculation would look like this:

(12(0.9893))+(13(0.0107))

(11.8716)+(0.1391)

12.0107

12.0107 is typically rounded to 12.011, which is the answer that one will find on most periodic tables.

Answer 3

The atomic mass you see on most periodic tables represents the average atomic mass of the element based on the number and weight of the naturally found isotopes (atoms of the same element with a different number of neutrons) of the element. This is calculated by factoring in the weight of each isotope and the commonness of the isotope.

for example, carbon usually has 12 neutrons, but it has natural isotopes with 12 and 14 neutrons, making the average atomic mass 12.0107

Answer 4

Because their actual measured masses are not exact integers, this is for 2 reasons:

1. few elements are isotopically pure, so the mass on the periodic table is the weighted average of the masses of all isotopes present
2. the mass defect, which causes the masses of the protons and neutrons in the nuclei to be slightly different in different mass nuclei, it is this mass defect that is responsible for radioactivity and nuclear energy

The only atom with an exact integer mass is the isotope carbon-12, which supplies the technical definition for amu: 1 amu is 1/12 the mass of an isolated carbon-12 atom in its ground state. You'll note that on the periodic table, the mass for carbon is somewhere around 12.011; this is because not all carbon is the carbon-12 isotope (the majority of it is, but a small fraction is the stable carbon-13, and an even smaller fraction is the radioactive carbon-14).

There also used to be periodic tables based on oxygen-16 or nitrogen-14 as a reference for the amu.

Answer 5

There are two fundamentally important reasons for the fact that atomic masses are usually not integers. The one with the largest numerical effect is the existence of isotopes. Isotopes are atoms of one element, as determined by a constant atomic number, that have different mass numbers because they have different numbers of neutrons in their nuclei. Most elements have more than one radioactively stable isotope, and the relative natural abundances of these isotopes do not follow any integer-specifying rule. Therefore, the average atomic mass of an element with more than one stable isotope is rarely if ever an integer.

Another fundamentally important factor is one sometimes denoted by the term "mass defect" or "packing fraction". Protons, electrons, and neutrons do not have absolutely constant masses except in isolation from one another, because their mutual presence in atoms is associated with slight variations in effective mass as a result of the existence of the "strong nuclear force" that causes normally repulsive positive electrical charges carried by protons to bind together into an atomic nucleus and all the electrons to remain "outside" the nucleus. The differences due to packing fraction are usually apparent only as a few parts per ten thousand and therefore are less numerically than those caused by existence of isotopes.

Atomic mass for elements is usually known very precisely, so therefore the number goes to many decimal places, which may be rounded at a certain point.

Answer 6

Because this number is an average of all the isotopes of the element. Its also an average of all of the particles of the element.

Answer 7

Few elements form isotopes. Their mass number are expressed in decimals.