Re: What does binding energy means as regards nuclear chemistry?

What does binding energy means as regards nuclear chemistry?

Actual mass of atom < calculated mass attributed to mass defect for binding the nucleus when atom was born. What does birth of atom mean? "Birth of the atom" means the point at which the nucleons forming the atomic nucleus came together, by whatever process.

It is commonly stated that the energy required to hold an atom together is large enough to show up as a small but measureable mass loss (remember E = mc²). A ⁴He nucleus, for example, masses 6.64067´10^-27 kg. The rest masses of the two protons and two neutrons of which it is composed add to 6.69510´10^-27 kg. This "mass defect" shows up as energy, and comes to about 3 TJ (3´10¹² J) per mole.

However, the mass defect doesn't goes into the binding energy of the 4He nucleus: the binding energy is included in the total nuclear mass! The mass defect indicates, instead, that 4He is more stable than two free protons and two free neutrons.

As regards fission, though, binding energy does show up as mass loss. As an example, consider the following fission event:

After the two daughter nucleii had fully decayed (by b^- emission), we would have the following products:

with a total mass of 234.8477 amu or 3.8997´10^-25 kg. However, a nucleus of ²³⁵U masses 235.0439 amu or 3.9030´10^-25 kg.

This entire mass difference (3.3´10^-28 kg) is from the excess binding energy of the ²³⁵U nucleus; it comes to 3´10^-11 J or 18 TJ/mol.

For more information, consult the ABCs of Nuclear Science at Lawrence Berkeley Laboratory. Exact atomic masses were obtained from the CRC Handbook of Chemistry and Physics, 63d Edition.