Why arent the atomic masses of elements whole numbers?

Question

all the atomic masses have a lot of numbers after the decimal

Answer 1

There are frequently small amounts of isotopes with an extra neutron in the nucleus. The number you see is the average of it all.

Answer 2

The atomic number of an element is determined by the number of protons in the nucleus - e.g. oxygen has 8, nitrogen has 7. However, each element can have a varying number of neutrons, which, because they are uncharged, do not alter the chemical properties of the element. E.g., carbon, element 6, has three common isotopes, carbon-12 (6 neutrons), carbon-13 (7 neutrons) and carbon-14 (8 neutrons). Since protons and neutrons weigh about the same, the mass of a carbon atom would be either 12, 13, or 14 depending on which isotope it is. However, most naturally-occuring elements are a MIXTURE of different isotopes. So, to use a trivial example, if you had equal parts carbon-12 and carbon-13 in a sample of carbon, the average atomic mass would be 12.5. The actual atomic mass is determined by the "isotope ratio" of most commonly occurring samples of the element (what fraction of each isotope is present in a typical sample), which is used to determine the "atomic mass" of the element.

Answer 3

The answer depends on which atomic mass you are talking about.

Atomic masses are given as multiples of the atomic mass unit (amu, or dalton). This is currently defined as 1/12 of the mass of the carbon-12 atom.

While it is true that the average atomic mass of a bulk sample of an element is determined by the number and amount of isotopes present, that's not the whole story.

(For example, a sample of carbon contains mostly carbon-12, but it is about 1.1% carbon-13. The average atomic weight of such a sample is 12.0000x0.9889 + 13.0033x0.0011 = 12.0111).

Notice that the exact atomic mass of carbon-13 is NOT 13.0000. How odd is that? Part of the answer is that the amu is defined as a fraction of the mass of a whole nucleus. Another part is that the mass of protons and neutrons bound in a nucleus is always LESS than the mass of the free particles.

A free proton has a mass of 1.00728 amu and a free neutron 1.00866 amu. For carbon-13, with 6 protons and 7 neutrons, you might expect the mass to therefore be 13.1043 amu. It is less because some of the mass of the free particles is converted into nuclear binding energy to hold them together.

The strength of that binding tends to rise with heavier elements, up to iron. After that it begins to decline again. If you fuse together elements lighter than iron, energy is given off. If you fission elements heavier than iron, energy is likewise given off. A graph of these values is known as the curve of binding energy.

Answer 4

Atomic mass of an element is determined by the number of protons and neutrons. You cannot have a fraction of a neutron or fraction of a proton in an nucleus, therefore the atomic masses have to be integer values.

Answer 5

also the mass of the neutron and proton are not exactly the same. a proton is lighter at ~938.272Mev and the neutron ~939.566Mev. and there is also the nuclear binding energy to be taken into account as well. so you still dont get exactly whole number atomic mass ratios even after you have taken the isotope problem into account.

Answer 6

cuz they are a weighted average of the different masses of that elements isotopes