Why does beryllium have an atomic mass of 9.01 instead of 9?

Answer 1

There are two main reasons why an element has an Atomic Mass that is non-integral.

The first reason is that naturally occurring elements often consist of more than one isotope, and the mass given is actually the average mass. A common example is Chlorine, which has two isotopes, 35Cl and 37Cl, in a 3:1 ratio and the average mass ends up at around 35.5.

The other reason is a little more complex. The mass of an atom comprises the sum of the mass of the neutrons, the protons, the electrons and the nuclear binding energy.

For simplicity, let us look at the mass of helium and use the old system where hydrogen has a defined mass of 1.000. You might think that helium (two protons, two neutrons, two electrons) would have a mass of exactly 4.000. However:

1. The mass of a hydrogen atom is the sum of 1 proton + 1 electron. A helium atom has twice this plus two neutrons

2. The mass of a proton, or that of a proton plus an electron, is different from the mass of a neutron.

3. When hydrogen/deuterium/tritium atoms combine to form helium atoms, energy is given out, and enough energy is given out that the equivalence of mass and energy (calculated through E=mc2) results in a significant loss of mass from the final atom.

In combination, these results mean that the mass of one atom is never a simple multiple of another, so with the exception of the atom that we actually define as having a specific mass (as we currently define oxygen to have a mass of 16.0000 amu), no atom will never have an integral mass.

In the case of Beryllium, the mass of 9Be is not exactly 9.0000, plus there are trace amounts of 10Be in naturally occurring samples that take the average mass up to a fraction over 9.00.