The atomic mass of hydrogen is 1.00794 Why is this number not rounded off to 1?

Answer 1

because it 's the lightest element in the Periodic Table.

1. The amu is defined based on carbon-12, so all other isotopes of elements are non-integer masses. This is sometimes referred to as the "mass defect".
2. The Atomic Mass of an element is the weighted average of the atomic masses of its natural isotopes. For hydrogen these are hydrogen-1 and hydrogen-2 (deuterium).

Answer 2

It varies depending on which element was used to define the amu.

In my reference from 1959, where the amu was based on oxygen-16, the atomic mass of hydrogen is 1.0083 I believe the modern definition of the amu is based on carbon-12, which results in a different mass for hydrogen. In either case, there would be accuracy issues in rounding to one.

Answer 3

Because it's not exactly 1. This is because hydrogen has different isotopes, the same amount of protons but a different amount of neutrons. 1.00794 is calculated from these masses and the percentages they are found in; the most commonly found, probably around 99% is H-1, followed by H-2, probably <1%, followed by H-3, probably <1%. And we then take an average relative to the abundance of each isotope.

However, even hydrogen-1 does not have a mass of exactly 1, because the units of atomic mass are defined as being 1 / 12th the mass of a carbon-12 atom in the ground state. Because the nuclear binding energy is slightly different for each nucleus, the only atom whose mass really is exactly a whole number is carbon-12.

Answer 4

Because there are three isotopes of hydrogen. These are normal hydrogen, heavy hydrogen, and heavy heavy hydrogen. Each has an atomic mass that is a whole number but the amounts of these isotopes with one or more neutrons are relatively small and the atomic mass of hydrogen in general is the average of all three.

Answer 5

Atomic mass is defined as the average mass of a particular element calculated by accounting for the natural abundance of isotopes of that element. For jydrogen, the isotopes deuterium and tritium are both naturally occurring and are present in small amounts in a hydrogen sample. Since these isotopes are both heavier than hydrogen, the observed mass of hydrogen is slightly larger than 1.

Atomic mass = C1*mass_Hydrogen + C2*mass_Deuterium + C3*mass_Tritium

where C1, C2, C3 represent the relative concentrations of the isotopes.

Also, atomic mass is normalized to carbon, which contains neutrons in addition to protons. Since neutrons and protons don't have exactly the same mass, every atomic mass except for carbon is not going to be integral (in fact, the amu is defined based on a specific isotope of carbon, carbon-12, which by definition has in its ground state a mass of exactly 12 amu. Even other isotopes of carbon have masses that are not quite integers.)

Answer 6

This is not to do with isotopes and relative atomic mass, which is a separate issue. The mass of pure 1H is 1.008 (or thereabouts) and not exactly 1.000 - which is not what you would expect. This is because the atomic mass units scale is based on carbon-12 being exactly 12.00000. However, carbon-12 is composed of 6 protons and 6 neutrons bound together. The binding energy released when the 12C nucleus was formed (from individual protons and neutrons) lowers its combined mass (according to E = mc2). The released energy of binding lowers the mass of the 12C nucleus, hence the original free protons appears to be relatively heavy with a mass of 1.008. If you read about "mass defect" and binding energy this will tell you more about this.

Answer 7

The atomic mass of an atom represents the mass of all the possible isotopes averaged together