Calculating The Coefficients
The coefficients are calculated by fitting to experimentally measured masses of nuclei. Their values can vary depending on how they are fitted to the data. Several examples are as shown below, with units of megaelectronvolts.
| Least-squares fit | Wapstra | Rohlf | |
|---|---|---|---|
| 15.8 | 14.1 | 15.75 | |
| 18.3 | 13 | 17.8 | |
| 0.714 | 0.595 | 0.711 | |
| 23.2 | 19 | 23.7 | |
| 12 | n/a | n/a | |
| (even-even) | n/a | -33.5 | +11.18 |
| (odd-odd) | n/a | +33.5 | -11.18 |
| (even-odd) | n/a | 0 | 0 |
- Wapstra: Atomic Masses of Nuclides, A. H. Wapstra, Springer, 1958
- Rohlf: Modern Physics from a to Z0, James William Rohlf, Wiley, 1994
The semi-empirical mass formula provides a good fit to heavier nuclei, and a poor fit to very light nuclei, especially 4He. This is because the formula does not consider the internal shell structure of the nucleus. For light nuclei, it is usually better to use a model that takes this structure into account.
Read more about this topic: Semi-empirical Mass Formula
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